27

Characteristic Tumors ¹

Edwin D. Murphy

The tumors of the mouse described in this chapter have been selected primarily on the basis of frequency of occurrence among the available inbred strains and the amount of research interest shown in them, Many less frequent types repeatedly appear as incidental findings in tabulations of tumors of untreated mice of inbred strains. Some rare types that have occurred spontaneously in mice at The Jackson Laboratory have been included for completeness. Selected tumors that rarely occur spontaneously but are readily induced have been included, particularly tumors that can be induced by hormonal imbalance. Emphasis has been given to the induced tumors of types important in human pathology, More references have been cited for the less well known tumors than for the common types which have been extensively reviewed.

DEFINITION AND PROPERTIES OF TUMORS

Willis ( 1960) proposed a workable definition for distinguishing true tumors from inflammatory and reparative proliferations, hyperplasias, and malformations with excess of tissue. A tumor "is an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of the normal tissues, and persists in the same excessive manner after cessation of the stimuli which evoked the change." Every pathologist can think of exceptions but these do not invalidate the general applicability of the definition.

Classification of tumors

The commonly used and most useful classification of tumors is histogenetic, that is, the tumors are named according to the tissues from which they arise and of which they consist. In most tumors the neoplastic tissue consists of cells of a single type and, with experience, one can readily classify them. The types of histological differentiation found in tumors appear to be inherent in the parent tissues. Foulds ( 1940) concluded that most adult normal cells have a greater capacity for divergent differentiation than was formerly supposed and that it is unlikely that tumor cells acquire new capacities. The few kinds of tumors in which there is uncertainty regarding the precise tissue of origin require further histopathological research. Meanwhile in these cases we must settle for noncommittal identifying names.

Willis ( 1960) illustrated the application of histogenetic classification to both human and animal tumors. Cloudman ( 1941) presented a histological classification of mouse tumors. Dunham and Stewart ( 1953) gave a classification of transplantable and transmissible animal tumors.

Benign or malignant

In addition to histogenetic classification, it is of practical value in human oncology to attempt to predict the behavior of a tumor from its morphology. The tumors of any given cell type may show a wide range of difference in structure, mode of growth, rate of growth, and danger to the host. Some, called benign, are well differentiated, grow only by expansion with the formation of a capsule, grow slowly, and are dangerous only in terms of position, accidental complication, or excessive hormone production. Others, often less well differentiated, grow rapidly, invade adjacent tissues, spread by metastasis, and unless extirpated at an early stage will kill the host; these are malignant tumors. Between these extremes there may be tumors of intermediate behavior. Thus, the terms benign and malignant are relative and arbitrary.

Criteria for diagnosing malignancy include imperfect differentiation and variation in the size, shape, and staining quality of the cell and the nucleus, invasion of adjacent tissues, and metastasis. In general the degree of malignancy is roughly proportional to the degree to which tumors fail to attain histological differentiation; the most anaplastic tumors are the most malignant. Metastasis depends on the invasion of blood vessels, lymphatics, or serous and other cavities, with the detachment of tumor cells or cell clusters and the establishment of distant secondary deposits. Lymphatic metastasis is less common in mice than in man. When tumors occur in inbred animals, there is available an additional criterion of malignancy: successful transplantation with progressive growth ( Chapter 28).

Stages of development

The fact that tumors develop in a series of stages was first clearly recognized by Rous and Beard ( 1935), in studies on virus-induced skin tumors in rabbits, and by Greene ( 1940), in studies on the development of spontaneous mammary and uterine tumors in rabbits. Foulds ( 1954, 1958) has generalized under the term progression the concept of the development of a tumor by irreversible, qualitative changes in one or more of its characters. The concept includes the early "precancerous" and neoplastic changes and the extended development of neoplastic characters that occurs during serial transplantation. A tumor may change, often abruptly, in growth rate, histological structure, invasiveness, or responsiveness to extraneous stimuli such as hormones or chemotherapeutic agents. These characters tend to be independently variable and subject to independent progression. Progression is independent of the duration or size of a tumor. Some tumors pass through, or bypass, all the theoretically possible developmental stages before they are grossly or even histologically recognizable. Others may become stabilized at any stage for the lifetime of the host. Continued serial transplantation, however, almost inevitably results in continued progression toward an endpoint of rapid growth rate, loss of functional and histological differentiation, and loss of responsiveness to extraneous stimuli.

In mice, Berenblum and Shubik ( 1947) demonstrated stages of initiation and promotion in the induction of skin tumors by a carcinogenic hydrocarbon. Foulds ( 1956) carried out detailed studies on mammary tumors developing from plaques which grow in response to pregnancy and regress after parturition. Many tumors of endocrine glands and their target organs go through a dependent or conditional phase in which removal of the causative stimulus is followed by regression ( Furth, 1953). In such tumors, some cells ultimately become altered if not during residence in the primary host then after transplantation and give rise to autonomous growth, which continues even when the causative stimulus is eliminated.

Genetic factors

Heston ( 1963) stated that the development of inbred strains has constituted probably the greatest advance in all cancer research. Genetic factors are immediately apparent in the different incidences of spontaneous tumor types among inbred strains and in the differing susceptibilities of these strains to the effects of carcinogens. The process of inbreeding fixes genes concerned with susceptibility to tumor formation ( Chapter 2) at the same time, transmitted oncogenic viruses may be carried along as in the case of the mammary tumor agent. Additional environmental factors, bacterial, viral, nutritional, and other, may be perpetuated by laboratory conditions (Chapters 4, 30). In general, the susceptibility of inbred strains and their hybrids to specific tumor induction is related to the frequency of spontaneous tumors of the same type. Potent carcinogens in high doses can induce subcutaneous sarcomas in mice of any strain, but graded doses reveal strain differences. Susceptibility to the development of most tumors appears to be inherited as a multiple-factor character with alternative expression ( Heston, 1963). The character appears when the combined action of the genetic and nongenetic factors surpasses a threshold ( Chapter 9). The nongenetic factors include physical and chemical carcinogens, hormones, nutritional factors, and viruses.

CHARACTERISTIC TUMORS

The most common tumors of inbred strains of mice are mammary tumors (in breeding females), lymphocytic leukemia, primary lung tumor, hepatoma (in males), and reticulum cell sarcoma (in older animals). Andervont and Dunn ( 1962) showed that wild mice maintained under laboratory conditions have a similar predominance of these tumor types, except for lymphocytic leukemia. Mammary tumors occur in high incidence in breeding females of strains C3H, DBA, A, and DD. In virgin females the incidence is high in C3H and DD, intermediate in DBA, and low in A. Tumors of high incidence tend to occur during the latter half of the first year of life and in the early part of the second year. Tumors of low incidence tend to occur during the latter half of the second year and the first half of the third year. Characteristic tumors of inbred strains are listed in Table 27-1. The spectrum of tumors observed in some longer-lived sublines and in wild mice is shown in Table 27-2.

MAMMARY TUMORS

The "mammary tumor of the mouse has probably been the most completely studied of all tumors" ( Dunn, 1959). Accessibility to palpitation, predictable frequency in a number of inbred strains, and ready transplantability have made the mammary tumor an invaluable tool for investigations in genetic, viral, hormonal, chemotherapeutic, nutritional, and other facets of cancer research. The discovery of the milk agent ( Staff, Jackson Memorial Laboratory, 1933) greatly stimulated research on mammary tumors. the agent has the characteristics of a virus and is transmitted by the milk of high mammary tumor strain females to the young. It appears to modify the responsiveness of the mammary gland tissue so that with a favorable genetic constitution and the proper hormonal stimulation, tumors develop at a comparatively early age. A usually much lower incidence of mammary tumors of variable morphology occurs in older mice in the absence of the milk agent ( Table 27-2). Mammary tumors have been induced by estrogenic hormones ( Lacassagne, 1932; Gardner et al., 1959), implantation of pituitary glands ( Mühlbock and Boot, 1959; Heston, 1964), chemical carcinogens ( Andervont and Dunn, 1953), and by polyoma virus ( Dawe et al., 1959).

Classification

Many classifications of mammary tumors have been proposed. Apolant ( 1906) made the first detailed histological study of mammary tumors of the mouse. His classification was standard for many years. The histological classification of Dunn ( 1959) has the advantage of simplicity and has been extensively applied to the analysis of histological types in a number of inbred strains, with and without the milk agent. Dunn's classification appears in Table 27-3 and an application in Table 27-2.

Gross appearance

Because of the extent of the mammary tissue in the female mouse, from the cervical region to the vulva on the ventral surface and almost to the midline in the back, the tumors may be found at almost any subcutaneous site in the body. In gross appearance, tumors may be round, oval, or coarsely nodular, and well circumscribed. The tumor tissue is usually grayish white and soft and often contains blood-filled cysts and central areas of necrosis.

Adenocarcinoma, Type A

The tumor is composed of uniform small acini, or tubules lined by a single layer of small cuboidal epithelial cells ( Figure 27-1). The tissue appears well differentiated and may show foci of secretory activity. Type A is the characteristic tumor of strain C3H with the mammary tumor agent and together with Type B represents the "typical" adenocarcinomas in mice with the agent.

Adenocarcinoma, Type B

The category represents a diversified group of glandular epithelial patterns, several of which may appear in a single tumor. Areas similar to Type A may be found as well as cysts filled with blood or clear fluid, intracystic papillary projections, irregular cords and tubes, and solid sheets of cells ( Figure 27-2). The amount of stroma may vary.

Adenocarcinoma, Type C

The tumor is composed of multiple cysts of varying size, lined by a single layer of cuboidal epithelial cells, which are closely surrounded by a spindle cell layer ( Figure 27-3). The connective tissue stroma usually appears edematous. Type C has nearly always been found in very old mice that lacked the agent.

Adenoacanthoma

Although foci of stratified squamous epithelium may occur in any type of mammary tumor, the term adenoacanthoma is restricted to tumors in which at least one-fourth of the section shows epidermoid differentiation ( Figure 27-4). The glandular elements of the tumors resemble Types A and B.

Carcinosarcoma

In these tumors there are irregular nests of epithelial cells closely intermixed with spindle cells resembling fibroblasts. Both elements may show numerous mitotic figures. The type occurs frequently among tumors induced by carcinogenic hydrocarbons. On repeated transplantation, purely glandular tumors may undergo a stage of carcinosarcoma and finally become pure sarcomas.

Sarcomas of the subcutaneous tissue must be distinguished from mammary tumors. However, some sarcomas may be derived from the stroma of the mammary gland and may appear in increased number after experimental procedures that increase the incidence of epithelial tumors. Cloudman ( 1941) presented a list of tumors and other lesions, arising in the area of the mammary gland, that may be mistaken for mammary tumors. For the sake of completeness, salivary gland tumors, lymphocytic and reticulum cell neoplasms, and squamous cell carcinoma of the skin should be added. These lesions can be distinguished by histological study.

Miscellaneous mammary tumors

These tumors that fit none of the other categories include a peculiar giant-type cell, epithelial tumors bearing no resemblance to the structure of mammary tissue, and tumors with abundant fibrous stroma resembling the characteristic human "Scirrhous" carcinoma. Organoid tumors composed of ducts and acini radiating from a central area have been described ( Dunn, 1959). The central area is keratinized in the mulluscoid type.

Preneoplastic and early neoplastic change

The preneoplastic and early neoplastic changes in the mammary gland of high-tumor strains have often been studied. The most important precancerous change in high-tumor strains with the milk agent appears to be the "hyperplastic nodule," composed of a localized proliferation of acini. These areas are well demonstrated in whole mounts of the mammary glands ( Chapter 13). The nodules also occur in old female mice without the agent. Foulds ( 1956) has described an early neoplastic "plaque" which grew only during pregnancy and regressed after parturition. The plaques consisted of radially arranged branching tubules. These growths were dependent on the hormonal stimulation of pregnancy.

TUMORS OF THE HEMATOPOIETIC STYSTEM

The classification of tumors of the hematopoietic system in the mouse is adapted from Dunn ( 1954a) and appears in Table 27-4. Only the more common types will be described here. See Dunn's monograph for a description of variants and rarer types.

Lymphocytic neoplasms

The generalized form of lymphocytic neoplasm, lymphocytic leukemia, is the most frequent and most extensively studied form of leukemia in mice. The incidence is high in relatively young mice of high leukemia strains, such as AKR and C58, but the tumor appears sporadically in mice of other strains, usually at a more advanced age. The incidence was low in noninbred mice reviewed by Horn and Stewart ( 1952). Dunn and Andervont ( 1963) found only two lymphocytic neoplasms among 225 wild mice maintained under laboratory conditions. Lymphocytic neoplasms have been induced by a number of viruses ( Moloney, 1960; Lieberman and Kaplan, 1959), by X-irradiation ( Furth and Furth, 1936; Kaplan, 1964), by carcinogenic hydrocarbons ( Law, 1941), and by estrogenic hormones ( Gardner et al., 1959).

A characteristic case shows general enlargement of the lymph nodes, thymus, and spleen. The involved organs are soft and white and often show hemorrhagic areas. The kidney and the liver may be enlarged and pale and contain nodules of soft white tissue. Ascites may occur in advanced cases. Microscopically ( Figure 27-5), the leukemic cell infiltrates the internal organs and may be found in blood smears. Two types of leukemic cells occur. One closely resembles a normal lymphocyte, with a deep basophilic small round nucleus and a thin rim of clear basophilic cytoplasm. In other cases, the cell is larger than the normal adult lymphocyte, with amore vesicular nucleus which may be round or slightly indented and with more abundant cytoplasm.

Reticulum cell neoplasms

The solitary fixed cells of the reticular framework of lymphatic and hematopoietic organs give rise to several distinctive tumors in the mouse. These reticulum cell sarcomas occur in older mice of a variety of inbred strains, particularly in their long-lived hybrids, and are less well known than the tumors of younger mice. Since they involve lymph nodes, spleen, and other organs, in many studies they tend to be lumped grossly with the leukemias. However, they are readily separated histologically.

Reticulum cell neoplasm, Type A. This tumor of unusually well-differentiated reticulum cells has been described in the literature as histiocytoma, monocytoma, reticuloendothelioma, and reticuloendotheliosis. The process may be localized or generalized and tumor cells may be found in the peripheral blood. Gorer ( 1946) reported an incidence of 15 to 20 per cent in strain C57BL mice over 18 months of age (see also Table 27-2). The tumors occur sporadically in old animals of other inbred strains.

At autopsy, enlargement of the liver and ascites are usually found. Involvement of the uterus is common in females. Spleen, lungs, kidneys, thymus, mesenteric, and other lymph nodes may be involved. The tumor tissue is firm, usually white, often shows hemorrhagic foci, and may have a distinctive orange color due to hematoidin. The tumor cell has eosinophilic cytoplasm and a heavily stained basophilic nucleus, and shows great variation in the size and shape of the cell and the nucleus ( Figure 27-6). Differentiation is indicated by erythrophagia and hemosiderin within the tumor cells. Multinucleated cells are frequent and may resemble Langhans-type giant cells. A common variant contains sheets of small spindle cells, with heavily basophilic ovoid nuclei and scanty cytoplasm, resembling fibrosarcoma. Both patterns may appear in the same animal, either intermixed or in separate deposits. An angiomatous pattern is occasionally seen in the tumor in the liver.

Transplantation to mice of the same strain is usually successful but growth is slow and may extend for 10 to 12 months. The Type A neoplasm may be compared with the more differentiated types of reticulum cell sarcoma in man, such as the clasmatocytic lymphoma described by Gall and Mallory ( 1942). The tumor induce by the Friend virus ( Friend, 1957) has been derived by Buffet and Furth ( 1959) from the reticulum cell. It resembles some of the leukemic forms of Type A.

Reticulum cell neoplasm, Type B. This multicellular tumor is more common than Type A and may be the most common tumor in older mice of otherwise low-tumor strains and of long-lived hybrids. Jobling ( 1910) and other early investigators used the term Hodgkin's disease; later investigators, Hodgkin's-like. The tumor occurs in 25 per cent of strain C57L/He mice, 18months of age ( Heston, 1963). It is a characteristic tumor of old age in mice of many inbred strains and in wild mice maintained under laboratory conditions ( Dunn and Andervont, 1963). Murphy ( 1963) has reported an incidence of more than 90 per cent at an average age of 13 months in a new inbred strain, SJL/J. Stansly and Soule ( 1962) presented evidence for a filterable agent that can induce Type B reticulum cell neoplasms.

At autopsy the mesenteric node is usually greatly enlarged. Peyer's patches are frequently involved and the tumor appears to spread to the pancreatic and renal nodes. Nodular involvement of the white pulp of the spleen is common and discrete nodules may be found in the liver. Advanced cases may involve kidney, lungs, mediastinal, and peripheral nodes. A minority of cases may be primary in some other node or the spleen. The tumor tissue is firm and white, with little hemorrhage. The process develops slowly; therefore localized cases may readily be found. Transplantation has been successful in a limited percentage of trials.

Microscopically, there is a background of large, pale reticulum cells intermixed with lymphocytes and plasma cells ( Figure 27-7). Tumor giant cells and multinucleated cells resembling foreign-body giant cells and Langhans' giant cells may be found. In strain SJL/J eosinophils may be prominent along with fibrosis ( Figure 27-8), and in several cases tumor giant cells were observed which duplicate the classic features of the Sternberg-Reed cells described in human Hodgkin's disease ( Murphy, 1963).

Granulocytic leukemia

Granulocytic leukemia is rare in comparison with lymphocytic leukemia in the mouse. It can be induced in strain RF mice by ionizing radiation ( Upton, 1961). Graffi ( 1957) described granulocytic leukemias in mice isolated from five different tumors. Grossly, the distribution and appearance of the tumors are usually indistinguishable from those of lymphocytic leukemia, unless the green color of chloroleukemia is present. A high granulocyte count with many undifferentiated cells is found in the peripheral blood. In the tissues, collections of relatively immature granulocytic cells are found ( Figure 27-9). Invasion of the capsule of lymph nodes and infiltration of fatty tissue are helpful in distinguishing this leukemia from extramedullary hematopoiesis.

Plasma cell neoplasm

Rask-Nielsen and Gormsen ( 1956) have reported a low incidence of plasma cell leukemia in several inbred strains. Plasma cell neoplasms have been induced by Plexiglas fragments ( Merwin and Redmon, 1963) and mineral oil ( Potter and Boyce, 1963) introduced into the peritoneal cavity of BALB/c mice. The inducing agents appear to exert their effect by their physical rather than their chemical properties ( Potter and MacCardle, 1964). Transplantable plasma cell tumors secrete a variety of proteins related to γ- and β-immunoglobulins and their subunits in the form of Bence-Jones proteins ( Fahey, 1961).

Dunn ( 1954a) has described both localized and generalized forms of plasma cell neoplasm. The localized type begins in the ileocecal area of old strain C3H mice, and the neoplastic cells extend through all coats of the intestine, through the mesenteric fat, and involve the medullary sinuses of the mesenteric node. One of these tumors has been successfully transplanted and shows heavy infiltration of the gonads and the kidneys. The generalized type ( Figure 27-10) involves lymph nodes, spleen, and perivascular tissue in the kidneys and lungs. The tumor cell has an eccentric nucleus, with a clear area in the cytoplasm, and the Giemsa stain shows the violet-colored cytoplasm of the plasma cell. Russell bodies may occur in the tumor cells. These tumors must be distinguished from plasma cell hyperplasia of lymph nodes (especially frequent in old mice), from inflammatory plasma cell infiltrations, and from other neoplasms.

Mast cell neoplasm

True neoplasms of mast cells are rare in mice. They may be localized and termed mastocytoma or more generalized and termed mast cell leukemia. Dunn ( 1954a) observed several of these tumors at autopsy. They were usually localized masses in lymphatic or connective tissue. Rarely, the neoplasm was widely distributed, involving lymph nodes, spleen, liver, lungs, and kidneys. Microscopically, the neoplastic mast cell is larger than the normal, it is less heavily and uniformly granulated, and the granules generally do not stain so intensely. Transplantable mast cell tumors have been reported ( Dunn and Potter, 1957; Rask-Nielsen and Christensen, 1963). These tumors are of particular interest because they may secrete heparin, histamine, and serotonin.

Stem cell leukemia

In human pathology this term refers to cases of acute leukemia in which the neoplastic cell is undifferentiated. Such tumors are not commonly distinguished in the mouse, but are probably included with the lymphocytic leukemias. Because of the preponderance of lymphocytic over granulocytic and other leukemias in the mouse, the undifferentiated forms are probably related to lymphocytic leukemia.

Misleading nonneoplastic lesions

Pathologists trained in human pathology are frequently misled by interstitial infiltrates of lymphocytes which occur with increasing frequency in the kidney, liver, and other organs of aging mice. The question of leukemic infiltration is often raised. The cell collections are usually perivascular, and on closer examination usually show a mixed population of reticulum cells, lymphocytes, and plasma cells.

A possible error is the misdiagnosis of extramedullary hematopoiesis as granulocytic leukemia. Extramedullary hematopoiesis in the spleen is physiological in the mouse. It requires little stimulus for the liver, lymph modes, and other organs to respond with tumorlike infiltrates, with predominance of the granulocytic series. The spleen may be as large as in many cases of leukemia, but the numerous intermixed megakaryocytes usually rule out granulocytic leukemia. Barnes and Sisman ( 1939) have described and tabulated the important points for differentiating extramedullary hematopoiesis from the rare granulocytic leukemia in the mouse.

Simonds ( 1925) described an enlargement of the mesenteric lymph node in five mice of the Slye stock. The node was enlarged by wide blood-filled spaces, which disrupted the normal histology of the node. The process is not neoplastic but appears to be due to venous congestion. It is frequent in strain C3H and its hybrids and occurs in old C57BL mice.

PULMONARY TUMORS

Spontaneous pulmonary tumors are known to be frequent in only two species, man and mouse ( Stewart, 1959b). Although the characteristic human tumor is bronchogenic in origin, the characteristic tumor of the mouse is alveologenic.

Alveologenic tumors

Alveologenic tumors occur spontaneously in high incidence is strains A, SWR, and BALB/c and can be induced in these and other susceptible strains by a variety of agents including urethan, carcinogenic hydrocarbons, nitrogen mustard, and γ-radiation ( Shimkin, 1955). Even though these tumors have been variously diagnosed as adenoma, papillary cystadenoma, and adenocarcinoma, they may all be morphological variants of a single malignant neoplastic process. The pulmonary tumor of the mouse is a malignant neoplasm, as judged from its lack of encapsulation, local invasiveness, transplantability, and ability to metastasize.

In gross appearance, the tumors are rounded, pearly white nodules, often situated just below the pleura, and projecting slightly. The spontaneous tumors are frequently solitary and usually do not exceed two to four per animal. The induced tumors are almost invariably multiple. Microscopically, most of the tumors present a uniform pattern of closely packed columns of cuboidal columnar cells ( Figure 27-11). The cells are rather uniform in size and shape, with acidophilic cytoplasm and round or oval nuclei. The sparse stroma is composed of mature fibrous tissue. Papillary formation is frequent in larger tumors. Spontaneous and induced tumors are indistinguishable microscopically. The common metastatic tumors in the lungs of mice can usually be suspected from the presence of a primary tumor in another site and distinguished microscopically.

Bronchogenic tumors

Squamous cell carcinomas arising in bronchi have been induced by local application of radioactive substances ( Gates and Warren, 1960) and by a combination of influenza virus and aerosols of hydrocarbons ( Kotin and Wiseley, 1963).

HEPATIC TUMORS

Hepatomas (liver cell carcinoma, hepatocellular carcinoma) occur spontaneously in low incidence in a number of inbred strains, but are common in older males of strains C3H and CBA ( Burns and Schenken, 1940; Gorer, 1940; Andervont, 1950b). Heston et al. ( 1960a) found an incidence of 85 per cent in C3H/He males. Deringer ( 1959) reported 91 per cent in C3HeB males and 58 per cent in virgin females. The tumors are usually solitary but may be multiple. They are usually elevated round or ovoid masses on the surface or margins of the lobes; some may be pedunculated. They may be gray or yellow or the same color as the liver. The histology is usually remarkably uniform and rather closely resembles normal liver ( Figure 27-12). Cords of cells are separated by sinusoids lined by flattened endothelial cells. However, a true lobular architecture is absent and bile ducts occur only at the periphery of the tumor. The size of the hepatoma cells and their nuclei shows a wide range of variation ( Miyagi, 1952). Cytoplasmic hyaline inclusion bodies are frequently observed. The nontumorous portions of the liver appear normal, without evidence of cirrhosis or inflammatory processes. Cholangioma has not been observed in untreated mice.

Metastasis of hepatomas has been described and, although not all spontaneous tumors were successfully transplanted, Andervont and Dunn ( 1952) could not demonstrate a consistent histological difference between those that grew and those that failed to grow. Therefore, the spontaneous tumors of the liver must be considered malignant. It is unlikely that an attempt to distinguish between adenoma and carcinoma would be useful. The incidence of spontaneous hepatoma has been influenced by diet ( Tannenbaum and Silverstone, 1949) and by castration ( Andervont, 1950b). Hepatomas have been induced in the mouse by azo dyes, other chemical carcinogens, radioactive compounds, carbon tetrachloride, chloroform, and urethan ( Heston et al., 1960a). Andervont and Dunn ( 1952) could find no identifiable qualitative histological difference between spontaneous and induced hepatomas.

TUMORS OF THE FEMALE REPRODUCTIVE TRACT

Ovarian tumors

Ovarian tumors occur only sporadically in most inbred strains, although nonneoplastic cysts are common. However, incidences of 34 per cent have been reported in CE females ( Dickie, 1954) and 47 per cent in C3HeB/De virgin females ( Deringer, 1959). The incidence in C3HeB/FeJ is 64 per cent after 19 months of age, and in RIII/J, 60 per cent after 17 months of age (Hummel, 1965, personal communication).

The most common types are granulosa cell tumors and tubular adenomas. A simple classification of spontaneous and induce ovarian tumors includes tubular adenoma, granulosa cell tumor, luteoma, papillary cystadenocarcinoma, and teratoma. Except for the latter two, these types have been induced by X-irradiation ( Furth and Butterworth, 1936), by transplantation of ovaries to the spleen ( Li and Gardner, 1949), by transplantation of ovaries to other sites in strain DBA mice ( Hummel, 1954a), by remotely applied chemical carcinogens ( Howell et al., 1954), and by genic deletion of ova ( Murphy and Russell, 1963).

Tubular adenoma. Downgrowth of the so-called "germinal" epithelium is a common aging change in the ovaries of mice ( Thung, 1961). It may be so extensive that the ovary is replaced by interlacing clefts and tubules lined by cuboidal to columnar epithelium resembling the germinal epithelium ( Figure 27-13). Only arbitrary morphological distinctions can me made between tubular adenomatous change and tubular adenoma, perhaps best at the point where the entire ovary is involved. Bali and Furth ( 1949) found that tubular adenomas could be transplanted and grew slowly, particularly in gonadectomized hosts.

Tubular adenoma is usually a prominent precursor in the formation of granulosa cell tumors and luteomas following genic deletion of ova, X-irradiation, and transplantation to the spleen. Gardner ( 1955) postulated that the germinal epithelium is the source of granulosa cell tumors. Bali and Furth ( 1949) have observed tubules in a tubular adenoma lined partly by germinal epithelium and partly by granulosa cells. At an intermediate stage in ovarian tumorigenesis there is frequently a proliferation of interstitial cells, which can undergo luteinization ( Figure 27-14). The term complex tubular adenoma has been applied to these tumors ( Bali and Furth, 1949). It is possible that some of these lipid-containing tumors have been called luteomas in the literature. Current opinion derives granulosa cell tumors and luteomas from the "interstitial" cells, that in turn are derived from theca cells ( Guthrie, 1957; Mody, 1960). Thung ( 1959) has stressed the tremendous plasticity of the cells of the ovary and has suggested that the various tumors may arise from more than one original cell type.

Granulosa cell tumor. Estrogen-secreting tumors composed of cells resembling the characteristic cells of the membrana granulosa constitute the most common induced type of ovarian tumor in mice. The cells frequently show an elongated nucleus, with densely stippled chromatin especially around the nuclear membrane, and may be organized into sheets, cords, or pseudo-follicles ( Figure 27-15). As in the case of human granulosa cell tumors, there can be so many patterns that the diagnosis is not always certain on morphological grounds alone. Besides estrogen secretion the tumors may also produce hypervolemia by secretion of a substance termed "plethorin" ( Furth and Sobel, 1946). Many of the cells of granulosa cell tumors may undergo a fatty change which is not true luteinization. However, true luteinization may occur and mixed tumors have been described ( Bali and Furth, 1949). The spontaneous tumors of strain CE and its hybrids are frequently granulosa cell in type. They appear to develop from proliferations of peripheral stromal cells, forming cortical plaques and grossly observable mushroom-like caps ( Figure 27-16). Sertoli cell differentiation is common ( Figure 27-17).

Luteoma. Progesterone-secreting tumors composed of cells resembling those of the corpus luteum occur rarely spontaneously but have been induced by X-irradiation, transplantation to the spleen, and genic deletion of ova. The tumors are yellow, and the cells are polygonal, have abundant acidophilic cytoplasm, and are arranged in nodules separated by thin strands of reticular and collagenous fibers ( Figure 27-18). Furth and Sobel ( 1947) have described studies on a transplantable luteoma.

Teratoma or teratocarcinoma. Ovarian teratomas, derived from pluripotent cells, occur very rarely in mice. However, several have been observed in strain C3H mice at The Jackson Laboratory, and Fekete and Ferrigno ( 1952) have reported a transplantable ovarian teratoma that maintained its pleomorphic character through nine transplant generations. A further discussion of this important tumor type is given under the heading Testicular Teratoma.

Other ovarian tumors. Papillary cystadenocarcinomas have been reported ( Cloudman, 1941; Dunn, 1954b). A transplantable mucin-producing tumor has been described by Dunn ( 1954b). Nonspecific tumors of the ovary such as leukemias, reticulum cell sarcomas, hemangioendotheliomas, and fibrosarcomas have been described.

Tumors of the uterine horns

Spontaneous tumors of the uterine horns are infrequent. The sporadic tumors are usually sarcomas, either fibrosarcomas or leiomyosarcomas. Spontaneous adenocarcinomas are rare in most inbred strains, however Dunn (1965, personal communication) has observed a number of transplantable adenocarcinomas is strain BALB/c and some similar tumors in C3H x C57BL hybrids ( Dunn, 1954b; Heston, 1963). Glandular tumors appear to be less readily induced by carcinogens than sarcomas. Fibrosarcomas and leiomyosarcomas have been induced by methylcholanthrene-coated threads ( Murphy, 1961). Rare leiomyomas resembling the common benign human uterine tumor have been reported ( Table 27-2). Endometrial stromal sarcomas occur.

Tumors of the cervix and vagina

Spontaneous carcinomas of the cervix and vagina are extremely rare in mice, as in other laboratory animals, and in fact in all animals studied other than man. However, the mouse has become the laboratory animal of choice in the induction of these tumors by estrogenic hormones ( Gardner et al., 1959; Dunn and Green, 1963), chemical carcinogens, and the combination of these agents ( Murphy, 1961). There is one report of a strain of mice in which a high spontaneous incidence of cervical and vaginal carcinoma was observed ( Gardner and Pan, 1948). Unfortunately, the strain was lost because of associated sterility ( Gardner et al., 1959).

Both the rare spontaneous carcinomas and the induced tumors range from well-differentiated squamous cell (epidermoid) carcinomas with extensive keratinization to anaplastic carcinomas with little or no evidence of differentiation. Most of the induced tumors, however, tend to be well differentiated and show much more keratin formation than occurs in human cervical carcinomas. A type of differentiation that occurs in rodents is the formation of mucin by the stratified squamous epithelium of the vagina and cervix. Mucin formation has been demonstrated is some of the tumors induced by methylcholanthrene ( Murphy, 1961). The tumors metastasize to the lungs and are transplantable.

Tumors of the vulva

Papillomas and squamous cell carcinomas of the vulva have been observed in strain 129 females at The Jackson Laboratory.

TUMORS OF THE MALE REPRODUCTIVE TRACT

Interstitial cell tumors of the testis

Sporadically occurring spontaneous interstitial cell tumors have been reported in hybrids of strain A ( Gardner, 1943), and in strains C ( Hooker et al., 1946), BALB/c ( Hummel, 1954b), and RF ( Clifton et al., 1956). Andervont et al. ( 1960) have established the incidence of spontaneous hyperplasias and tumors in BALB/c as probably under 1 per cent. Interstitial cell hypertrophy, hyperplasia, and tumor formation have been reported in a high mammary tumor strain, designated strain H, in which the males also develop mammary tumors ( Athias, 1945; Furtado Dias, 1958). Interstitial cell tumors have been induced by the administration of estrogens, particularly in strains A and BALB/c. Wide differences in susceptibility of inbred strains have been reported by Bonser ( 1944), Gardner et al. ( 1959), and Andervont et al. ( 1960). Experimentally produced cryptorchid testes of BALB/c mice developed a high incidence of interstitial cell hyperplasia and tumor formation ( Huseby, 1958).

The tumors are yellowish brown and consist of masses of large polygonal or irregularly shaped cells with granular or vacuolated cytoplasm and nuclei varying in size ( Figure 27-19). The cells may contain a light brown pigment. Lymphatic metastasis is common. The induced tumors can be transplanted to animals of the same strain if estrogen is present. After serial transplantation the tumors lose their dependency on estrogen. The tumors produce androgen, losing this ability as they become more autonomous ( Gardner et al., 1959).

Testicular teratoma

These tumors, derived from pluripotent cells, are extremely rare in male mice except for strain 129 in which approximately1 per cent develop congenital tumors spontaneously ( Stevens, 1959). It has been possible to raise the incidence as high as 10 per cent by introducing the steel gene (Sl) into strain 129 and by selecting males from second and later litters ( Stevens and Mackensen, 1961). The transplantation of male gonadal ridges from strain 129 fetuses to adult testes has resulted in an 82 per cent incidence of teratoma ( Stevens, 1964).

Grossly, larger tumors are hemorrhagic and the smaller ones appear as solid masses occupying one-fourth to three-fourths of the testicular volume. Usually the masses contain cysts filled with clear or bloody fluid. many of the teratomas contain palpable nodules of bone and cartilage. Histologically ( Figure 27-20), the most common components are nervous tissue, epithelia of various types, cartilage, bone with well-differentiated marrow, muscle, fat, and glandular tissue ( Stevens and Little, 1954). Undifferentiated embryonic cells may be observed. The tumors are transplantable, but only a minority grow progressively. The grafts that merely survive are composed entirely of adult-type tissues. Those that grow progressively may consist purely of embryonal cells, mainly of undifferentiated cells, or principally of differentiated tissues ( Stevens, 1958). Serial section study of testes in 15- to 19-day fetuses has demonstrated the origin of the teratomas within the seminiferous tubules and provided evidence of origin from the primordial germ cells ( Stevens, 1962).

TUMORS OF OTHER ENDOCRINE GLANDS

Adrenal cortical tumors

Adrenal cortical tumors are rare in noninbred mice ( Slye et al., 1921). They are readily induced by gonadectomy in strain CE ( Woolley and Little, 1945a) and in (DBA x CE)F₁ hybrids ( Woolley et al., 1952). They have been similarly induced in strains BALB/c, NH, CBA, C3H, and A ( Frantz and Kirschbaum, 1949). Strain differences were noted in the secretion of androgen, estrogen, or both hormones by the tumors. The response of the adrenal gland to castration is influenced by genetic factors. Strain DBA responds with nodular hyperplasia ( Fekete et al., 1941), while strains C57BR and C57BL show only slight increase in width of the cortex.

Fekete and Little ( 1945) have described the tumors arising in the adrenal cortex of gonadectomized mice of the CE strain. In most of the large tumors the predominant cells were polygonal and diffusely arranged ( Figure 27-21). Rows and cords of cuboidal cells tended to be more prominent at the periphery of the tumors. In some cases a syncytial type of cell resembling Sertoli cells of the testis formed a component of the tumor. Giant cells with yellow pigmented cytoplasm and often with multiple blood nuclei were found in many tumors. Blood vessels were numerous and consisted mainly of capillaries and sinusoids between groups of tumor cells. The larger tumors showed capsular invasion. Stewart et al. ( 1959) presented excellent illustrations of such a tumor and its transplants.

Adrenal medullary tumors

Tumors of the adrenal medulla in mice have been described by Smith et al. ( 1949) and as occasional findings by other investigators. Jones and Woodward ( 1954) reported these tumors in untreated (C3H x I)F₁ virgin females. The tumors closely reproduce the cell type and arrangement of the normal adrenal medulla of the mouse ( Figure 27-22). It is for this reason that the tumors are commonly termed "pheochromocytomas," not because of demonstrated chromaffin staining or endocrine properties.

Pituitary tumors

Spontaneous pituitary tumors have been considered rare in mice. Slye et al. ( 1931) and Gardner et al. ( 1936) have described single cases. Cloudman ( 1941) reported two adenocarcinomas in hybrids between C57BL and C57BR. Furth et al. ( 1960) found an increase of 3.5 per cent in C57L and 1 per cent in LAF₁ hybrids. Pituitary tumors have been observed at The Jackson Laboratory in approximately one-third of retired female breeders of strains C57L/J and C57BR/cdJ (Russell, 1965, personal communication). These tumors are associated with mammary duct hyperplasia and, in more differentiated specimens, the cell type is acidophilic ( Figure 27-23).

Pituitary tumors have been induced in mice by chronic administration of estrogen, ionizing irradiation, goitrogenic drugs, surgical thyroidectomy, radiothyroidectomy, and gonadectomy ( Gorbman, 1956). The tumors induced by gonadectomy in strain CE and its hybrids are associated with adrenal cortical tumors ( Dickie and Woolley, 1949). Subcutaneous isografts of pituitaries develop into chromophobe adenomas ( Mühlbock and Boot, 1959).

The classic cellular classification of pituitary tumors as acidophilic, basophilic, or chromophobic has been of limited value in mice. Most of the tumors have been described as "chromophobe," or lacking in stainable granules. In human pathology this designation usually signifies lack of hormonal function. In mice these tumors show a number of hormonal effects. They are best classified as mammotropic, thyrotropic, adrenotropic, somatotropic, or gonadotropic ( Clifton, 1959). Functional characterization usually requires the study of transplants, since the inductive processes often produce masking hormonal responses in the diagnostic target organs.

Mammotropic activity has been demonstrated in spontaneous tumors and those induced by estrogens and by irradiation. Transplanted mammotropic tumors cause hyperplasia of all elements of the mammary glands with milk secretion, body growth, and disproportionate increase in weight of the viscera ( Furth et al., 1956). The cell type, although usually described as chromophobic, may show acidophilic granules.

Thyrotropic tumors, induced by procedures which eliminate or suppress thyroid function, arise from the aldehyde fuchsin-positive beta basophils ( Halmi and Gude, 1954). Transplanted tumors cause massive thyroid hyperplasia with formation of dependent thyroid adenomas ( Furth and Clifton, 1957).

Adrenotropic tumors were the first type to be described in irradiated mice ( Furth et al., 1952). They were "chromophobic" by the usual staining methods. The tumors induced by gonadectomy have been described as "basophilic" and postulated to be gonadotropic ( Dickie and Woolley, 1949).

The cranial cavity is often not routinely examined at autopsy; therefore pituitary tumors can be overlooked. Careful removal of the calvarium and brain may be tedious, but it is a simple matter to slice off the upper part of the skull and brain with a razor blade in order to examine the pituitary gland and the brain for tumors.

Thyroid tumors

Spontaneous tumors of the thyroid gland are rare in mice. Slye et al. ( 1926) reported several malignant epithelial and mesodermal tumors. Adenomas and adenocarcinomas are readily induced by goitrogens ( Morris, 1955) and by thyrotropin-secreting pituitary tumors ( Furth, 1954). Figure 27-24 illustrates such a tumor. Adenomatous tumors have been induced by polyoma virus ( Stanton et al., 1959; Dawe et al., 1959).

TUMORS OF SKIN AND SKIN APPENDAGES

Spontaneous tumors of the skin have rarely been reported since creosoted wooden cages have been discarded. A low percentage of papillomas and squamous cell carcinomas occurs in both haired and hairless genotypes of strain HR/De (Deringer, 1951, 1956; Table 27-2). Extensive studies have been carried out on the induction of skin tumors in mice by painting with tars and pure carcinogenic hydrocarbons and by exposure to ultraviolet radiation ( Stewart, 1959a; Boutwell, 1964). Hair-follicle tumors have been induced by polyoma virus ( Dawe et al., 1959).

Papilloma and squamous cell carcinoma

The initial sequence of events during the induction of skin tumors in mice by painting with methylcholanthrene induces: epilation, hyperemia, appearance of areas of ulceration which may heal, and swelling due to edema of the dermis ( Stewart, 1959a). Hyperplasia of the hair follicles in the marginal areas and epithelial hyperplasia with hyperkeratosis occur. Keratinized cysts, which may open on the surface, occur, and broad-based and pedunculated papillomas follow. The epithelial component of the papillomas is hyperplastic with frequent downgrowths of papillae into the stroma, but the cells are well oriented. Foci of carcinomatous change may develop anywhere in the hyperplastic epithelium, not necessarily in the papilloma. The pedunculated papillomas may even regress. In the carcinomatous foci the epithelial cells show increased variability in size, shape, and staining quality, show loss of orientation, frequently form epithelial pearls and keratin, and frequently invade the stroma ( Figure 27-25). As the tumors grow larger and invade more extensively, they eventually ulcerate. They may metastasize to the regional lymph nodes and the lungs. Extremely anaplastic tumors with little evidence of differentiation occur. Forms with spindle-shaped epithelial cells are seen. Spontaneous basal cell tumors have rarely been reported. They have been induced by repeated applications of polyoxyethylene sorbitan monostearate (Tween 60) alone or after a single dose of a carcinogenic hydrocarbon ( Della Porta et al., 1960).

Sebaceous gland tumors

Large round cells with pale foamy cytoplasm and relatively small, pale, oval central nuclei may occur in induced and in spontaneous tumors. The most rapidly growing parts of the tumor may contain small, round, deeply staining cells resembling the small undifferentiated cells of epidermoid carcinomas. Stratified squamous cells may also be found. In the mouse, the preputial and clitoral glands are specialized large sebaceous glands with large alveoli lined by pale sebaceous cells, and ducts lined by stratified squamous epithelium. Tumors of the clitoral and preputial glands have been observed. They may be remarkably organoid in structure. At The Jackson Laboratory a transplantable preputial gland tumor, ESR586, has been established and has maintained a highly differentiated form through over 150 transplant generations ( Figure 27-26). It has been found to contain large amounts of provitamin D and vitamin D. The tumor is a convenient sterol "factory" and has enabled Kandutsch and Russell ( 1960) to find a new pathway in intermediate sterol metabolism.

Melanoma

These highly pigmented tumors have occurred infrequently in strain DBA mice at The Jackson Laboratory ( Cloudman, 1941). The commonest primary site has been on or near the tail, but the ear and other sites have been involved. The tumors are brown to black, smooth and rounded, and visible through the skin of the living animals. The microscopic architecture and cellular detail are heavily masked by the intense pigmentation ( Figure 27-27). The tumor cells may be spindle-shaped or large and oval, and they may be arranged in sheets, whorls, and interlacing bundles. The cytoplasm may be filled with small brown granules of melanin pigment. The tumors metastasize widely, particularly to lymph nodes and lungs. The transplantable Cloudman melanoma, S91, which arose in a DBA mouse, has been extensively studied. An amelanotic variant has been obtained by selection of less pigmented portions during the course of transplantation in albino strain BALB/c mice ( Loustalot et al., 1952).

TUMORS OF THE ALIMENTARY TRACT

Induced tumors

The high incidence of tumors of the alimentary tract in man contrasts with the rarity of such tumors in mice. Spontaneous adenocarcinomas of the stomach, rectum, and colon and squamous cell carcinomas of the upper alimentary tract have only rarely been reported in mice. Adenocarcinomas of the small intestine and squamous cell carcinomas of the forestomach are readily induced by feeding chemical carcinogens ( Stewart, 1953a). Adenocarcinomas of the stomach have been induced by intramural injection of methylcholanthrene ( Stewart et al., 1953). Hyperplastic adenomatous gastritis, which occurs in strain I and DBA, has been mistaken for a malignant process ( Stewart, 1953a).

Salivary gland tumors

Myoepithelioma. A spontaneous tumor of the parotid gland, which closely resembles the myoepithelioma of the parotid gland in man, has been observed repeatedly in strains A and BALB/c and twice in C58 ( Law et al., 1955). The tumors characteristically form large central cysts containing a glairy mucoid substance. The microscopic pattern is that of sheets and cords of cells indistinctly separated into small alveoli by bands of connective tissue ( Figure 27-28). The cells are pleomorphic, a basal rounded cell lying adjacent to the connective tissue stroma. The cells become progressively more flattened and fusiform. Myoglia and fibroglia have been demonstrated in association with the fusiform cells ( Lippincott et al., 1942). A pseudoglandular pattern suggesting acinar structures may appear. Focal keratinization occurs. The tumors, both primary and transplanted, may be associated with a granulocytic leukemoid reaction in the host ( Bateman, 1951). Myoepitheliomas have been found rarely in subcutaneous areas where they may originate from mammary gland elements ( Andervont and Dunn, 1950). Adenocacanthomas of the salivary glands have been induced by carcinogenic hydrocarbons ( Bauer and Byrne, 1950).

Pleomorphic tumors.

Gross ( 1953) and Stewart ( 1955) described the induction of a new type of salivary gland tumor following inoculation of newborn mice with cell-free filtrates of leukemic mouse tissues. The tumors are most prominent in the parotid and exorbital lacrymal glands ( Law et al., 1955). They are usually bilateral. They are multinodular, the nodules varying from grayish white to pearly white, often resembling a bunch of grapes. When they become cystic, the contents are serous rather than mucoid. Histologically, both an epithelial and a mesenchymal of fibroblastic component can usually be distinguished ( Figure 27-29). The epithelial component consists of small tubules or minute cysts lined by a cuboidal epithelium. The connective tissue component can range from a poorly differentiated mesenchymal type of tissue, with basophilic fusiform cells separated by a loose network of fibers, to a well-differentiated tissue composed of fibroblasts with eosinophilic cytoplasm and intercellular collagen. The tumors are clearly multicentric in origin, undoubtedly reflecting their induction by the polyoma virus. On transplantation the mesenchymal component frequently outgrows the epithelial component ( Law et al., 1955).

Similar tumors occur in the submaxillary, sublingual, and accessory salivary glands of the oropharynx, and the submucosal glands of the nasal passages and trachea ( Dawe et al., 1959). Polyoma virus also induces tumors of thymus, bone, thyroid gland, hair follicles, mammary glands, subcutaneous connective tissue, renal medulla, adrenal medulla, and other sites ( Dawe et al., 1959). Almost all of these tumors differ in many respects from the characteristic tumors of these tissues in the mouse ( Dawe, 1960), except for the subcutaneous sarcomas ( Law et al., 1955).

Intramandibular tumors

Carcinomas of the alveolar socket associated with exogenous hairs have been reported in nearly 1 per cent of old mice related to strain O20 ( Van Rijssel and Mühlbock, 1955). Experimental introduction of nylon threads, whisker hairs, and stainless steel wire into the alveolar socket induced many more of these tumors ( Hollander and van Rijssel, 1963).

TUMORS OF THE MESODERMAL TISSUES

"A sarcoma is a malignant tumor arising from any nonepithelial mesodermal tissue—fibrous, mucoid, fatty, osseus, cartilaginous, synovial, lymphoid, hemopoietic, vascular, muscular or meningeal. The simplest nomenclature specifies each form of sarcoma by an appropriate prefix, fibro-, myxo-, lipo-, osteo-, etc." ( Willis, 1960). There are benign forms for each of these tumors, but they are rarely reported for the mouse. The classification of some of these tumors requires special stains such as Mallory's phosphotungstic acid hematoxylin.

Fibrosarcoma

Fibrosarcoma is often used as a term for any tumor composed of spindle-shaped cells whose properties have not been further characterized. Under these circumstances the noncommittal term sarcoma or the descriptive term spindle cell sarcoma should be used. Statistically, the guess is more often right than wrong, but a number of more specialized mesodermal tumors are missed.

Fibrosarcomas occur spontaneously in the subcutaneous connective tissue and also in the internal organs. Dunn et al. ( 1956) have reported 106 subcutaneous sarcomas, probably derived from fibroblasts, among 4049 female mice of strains C3H, C3Hf, C57BL, and the F₁ and backcross hybrids. None of the tumors occurred in C57BL or C57BL backcross females. Fibrosarcomas have been induced by a number of subcutaneously injected carcinogens ( Stewart, 1953b), by long-term tissue culture ( Sanford et al., 1950), and by subcutaneously implanted plastic films ( Oppenheimer et al., 1959).

Fibrosarcomas are usually smooth, rounded, white, and often firm in texture. Microscopically, they are composed of elongated spindle-shaped cells arranged in bundles running in different directions ( Figure 27-30). The cytoplasm is pale, acidophilic, and usually scanty. Collagen fibers are present and fine, branching fibroglial fibrils can be demonstrated by the phosphotungstic acid hematoxylin stain. Reticulum fibers usually form a network embracing single cells. In poorly differentiated tumors, the spindle-shaped cells may be in the minority. The predominant cell is large and polyhedral. Multinucleated tumor giant cells may be formed.

Leiomyosarcoma

Tumors of smooth muscle cells occur spontaneously in the uterus and have been induced by chemical carcinogens in this organ ( Murphy, 1961) and in the alimentary tract ( Saxén and Stewart, 1952). The tumors are composed of interlacing bundles of rather large spindle-shaped cells with abundant acidophilic cytoplasm ( Figure 27-31). Stroma is scanty and collagenous material minimal. Characteristic coarse, short, myoglial fibrils can be demonstrated by the phosphotungstic acid hematoxylin stain. Giant cells with one to several large nuclei may be present.

Rhabdomyosarcoma

Tumors of striated muscle have been observed in several strains of mice at The Jackson Laboratory. Stewart et al. ( 1959) gave a well-illustrated account of transplantable rhabdomyosarcoma H6668 that arose spontaneously in a BALB/c mouse. The most striking cell type is a large cell which may be round, sometimes oval, racquet-shaped, or straplike ( Figure 27-32). The nuclei are large, generally round, and centrally located. Giant cells with multiple nuclei are common. The cytoplasm is abundant and acidophilic. A number of the cells may contain longitudinal myofibrils which, in a few cells, may be arranged to show cross striations. Cross striations are not demonstrable in all tumors. There is also a small tumor cell which may be round, oval, or spindle-shaped.

Granular myoblastoma

This tumor of disputed histogenesis has been considered peculiar to man. It has been induced in the uterine cervix of mice by estrogen treatment ( Murphy, 1961; Dunn and Green, 1963). The tumors are composed of large round cells packed with faintly eosinophilic granules of varying size. Dunn and Green ( 1965) have reported a transplantable tumor in strain C3H.

Liposarcoma

Malignant tumors of fatty tissue are extremely rare in the mouse. A transplantable liposarcoma has been established at The Jackson Laboratory in strain WB/Re mice.

Osteogenic sarcoma

Malignant tumors usually originating in bone and forming sporadically in mice. Most of the spontaneous osteogenic sarcomas reported fro inbred strains have occurred in sublines of C3H and its hybrids ( Dunn and Andervont, 1963; Hilberg, 1954). The incidence is below 1 per cent. Females are affected more frequently than males. Pybus and Miller ( 1938) derived sublines of the Simpson stock that developed a high incidence of bone tumors. The group of tumors showed the wide range of differentiation of the osteoblast: osseus, fibrous, cartilaginous, and osteoclastic. The most common type consisted principally of cancellous bone and osteoid tissue and ranged in structure from benign-appearing osteomas to osteogenic sarcomas with no sharp dividing line ( Pybus and Miller, 1940). Next most common were fibrosarcomatous tumors. Several tumors were classified as giant-celled tumor, chondro-osteosarcoma, and osteoma of the compact type. Unfortunately, descendants of these mice no longer develop bone tumors.

Spontaneous osteogenic sarcomas have been transplanted but rarely maintain their bone-forming property for more than several generations ( Stewart et al., 1959; Hilberg, 1956). Transplantable chondrosarcomas have been described ( Ehrlich, 1906; Swarm, 1963). Osteogenic sarcomas have been induced in mice by X-irradiation and by bone-seeking radioactive substances ( Glucksmann et al., 1957; Finkel et al., 1964), chemical carcinogens ( Brunschwig and Bissell, 1938), and by polyoma virus. The majority of bone tumors induced by polyoma virus lacked the usual histological and cytological criteria of malignancy ( Dawe et al., 1959), but metastasizing tumors have been described ( Sjögren and Ringertz, 1962; Stanton et al., 1959).

An osteogenic sarcoma may be bony-hard or may be composed of softer tissue that is gritty when cut. Microscopically, the tumor can usually be identified by trabeculae of osteoid or partially ossified tissue ( Figure 27-33). More cellular portions are composed of interlacing bundles of spindle-shaped cells, resembling fibrosarcoma. In the trabeculae, cells are isolated in a matrix of hyaline material and may be rounded, resembling osteocytes, or may be spindle-shaped. The tumor cells may be palisaded along the borders of osteoid tissue, in the pattern of osteoblasts. There may be multinucleated giant cells having the appearance of osteoclasts. Foci of cartilage may be found.

Hemangioendothelioma

Malignant tumors of the vascular endothelium occur in low frequency in many inbred strains and in wild mice ( Table 27-2). Deringer ( 1962b) found an incidence of 24 per cent in strain HR/De. Hemangioendotheliomas have been induced by carcinogenic hydrocarbons, ultraviolet radiation, 4-o-tolylazo-o-toluidine ( Andervont, 1950a), and urethan ( Deringer, 1962b). The spontaneous tumors occur in various sites, such as subcutaneous tissues, liver, spleen, ovaries, and mesentery.

The tumors form extremely vascular soft red masses. A collagenous fiber capsule and its extensions separate the masses into coarse and fine nodules. These fibers also form the supporting stroma for the neoplastic cells and blood vessels. The tumors are composed predominantly of blood vascular channels and sheets of neoplastic cells ( Figure 27-34). The tumor cells may be flat, round, polygonal, or spindle-shaped. Many of the tumor cells lining blood vessels resemble hyperplastic endothelial cells. More benign-appearing forms resembling cavernous hemangioma occur in the liver. Stewart et al. ( 1959) described and illustrated transplantable hemangioendothelioma H6221, which arose in the epididymus of a BALB/c mouse at The Jackson Laboratory.

LESS COMMON SITES OF SPONTANEOUS TUMORS

Brain and spinal cord

Spontaneous tumors of the brain and spinal cord are extremely rare in mice. The widely used transplantable tumor C1300 arose in the region of the spinal cord in an A/J mouse. Gorer ( 1947) identified it as a "round cell tumor" and added that it might possibly be a neuroblastoma. In noninbred mice, Horn and Stewart ( 1952) found reports of an ependymoma and an endothelioma of the brain and a "spindle cell" sarcoma of the spinal cord. Cloudman ( 1941) reported a medulloblastoma and a glioma in C57BL females. Stewart et al. ( 1950) reported two cases of primary tumors involving spinal nerve roots and meninges in strain NHO mice. Dickie (1965, personal communication) found a transplantable meningeal sarcoma in backcross DED2F₁ x DBA/2WyDi. A glioblastoma multiforme has been reported by Andervont et al. ( 1958) in a (BALB/c x C3H)F₁ mouse.

Direct implantation of carcinogenic hydrocarbons in the brain has induced glioblastoma multiforme, medulloblastoma, medulloepithelioma, astrocytoma, oligodendroglioma, spongioblastoma polare, ependymoma, pinealoma, and meningeal sarcoma ( Stewart, 1953c; Peers, 1940; Zimmerman and Arnold, 1941).

Kidney

Rare adenomas and adenocarcinomas of the renal cortex have been reported by Tyzzer ( 1909), Haaland ( 1911), Slye et al. ( 1921), and Cloudman ( 1941). Figure 27-35 illustrates and adenocarcinoma. Claude ( 1958) reported the occurrence of bilateral renal adenocarcinomas in over 40 per cent of adults of a subline of BALB/c. Papillary cystadenomas have been induced by X-irradiation ( Berdjis, 1959; Rosen and Cole, 1962). Berdjis ( 1959) illustrated a clear cell adenocarcinoma (hypernephroma). Stevenson and von Haam ( 1962) reported that methylcholanthrene induced a renal cell adenocarcinoma and a number of tumors derived from the transitional epithelium of the pelvis. Transitional cell papillomas and carcinomas of the renal pelvis have been reported by Cloudman ( 1941).

Bladder

Spontaneous tumors of the urinary bladder are extremely rare. Cloudman ( 1941) mentioned papillomas and one transitional cell carcinoma. Heston and Deringer ( 1952) recorded a papilloma in strain C3Hf. Papillomas and transitional cell carcinomas have been induced by 2-acetylaminofluorene ( Armstrong and Bonser, 1944), other aromatic amines ( Bonser et al., 1956), and directly applied methylcholanthrene ( Jull, 1951). Bonser and Jull ( 1956) described the histogenesis of the induced tumors.

Pancreas

Cloudman ( 1941) reported three adenocarcinomas and two islet cell tumors. Hueper ( 1936) reported a case of islet adenoma. Additional islet cell tumors have been recorded ( Table 27-2). A high incidence of ö-cell hyperplasias and tumors has been reported in (C3Hf x I)F₁ hybrids ( Jones, 1964).

Harderian gland

The Harderian gland is a retro-orbital lachrymal gland ( Chapter 13). Spontaneous tumors are commonly reported as incidental findings in various inbred strains and hybrids ( Table 27-2). Harderian gland tumors have been induced by X-irradiation ( Furth et al., 1960) and by urethan ( Tannenbaum and Silverstone, 1958). Large tumors cause protrusion of the eye. Microscopically, they are usually papillary and may be cystic or have solid adenomatous areas. Many of the cells closely resemble those typical of the normal gland, with foamy cytoplasm and basally located nuclei ( Figure 27-36). Harderian gland tumors may be invasive, metastasize to regional lymph nodes and lung, and be transplantable ( Upton et al., 1960).

DEVELOPMENT OF NEW TUMOR TYPES

Manipulation of environmental factors

By refinement of techniques, most human tumor types can be duplicated in the mouse by direct application of chemical carcinogens to the comparable tissues. The small size of the mouse is no great hindrance. An increasing variety of tumor types is being produced by hormonal imbalance, irradiation, and oncogenic viruses. Combinations of genetic and environmental factors can be highly selective in the production of specific tumors.

Manipulation of genetic factors

More physiological models of human tumors can be expected from the development of new inbred strains and their hybrids and from the introduction of mutant genes into existing strains. The development of the SJL/J strain revealed a high incidence of reticulum cell sarcoma including close replicas of human Hodgkin's disease ( Murphy, 1963). It has been shown that alleles at the W locus which limit the migration of primordial germ cells produce ovarian tumor adenomas in C57BL/6J mice ( Russell and Fekete, 1958). By placing these genes on a hybrid background, it was possible to induce the characteristic range of ovarian tumors ( Murphy and Russell, 1963). Dickie ( 1954) reported a wide variety of tumors in F₁ hybrids and backcross generations, involving strains CE, DBA, and DE, that were not characteristic of the parent strains. Adenocarcinomas of the uterus closely resembling the common human tumor have been found in C3H x C57BL hybrids ( Heston, 1963; Dunn, 1954b). Spontaneous carcinomas of the cervix occurred in high incidence in the PM stock ( Gardner and Pan, 1948), which has been lost because of sterility factors. These animals were derived from the stock in which Pybus and Miller ( 1938, 1940) described a high incidence of osteogenic sarcomas.

It is likely that many other analogues of important human tumor types can be developed by hybridizing our present strains. Selection of underlying genetic factors by the use of proper doses of carcinogens applied to segregating generations may be as effective as Snell's use of transplanted tumors to isolate the genetic factors concerned with tissue transplantation ( Chapter 24).

SUMMARY

The characteristic spontaneous tumors of the mouse are described and illustrated. The five common tumors of the major inbred strains are mammary tumors, lymphocytic leukemias, primary lung tumors, hepatomas, and reticulum cell sarcomas. Ovarian tumors are frequent in several inbred strains, and hemangioendotheliomas in one. Additional spontaneous and some induced tumors, characteristic of the mouse, are described. The definition, classification, and biological properties of tumors are discussed briefly. Possibilities of manipulation of environmental and genetic factors in the development of experimental models for human tumor types are presented.

¹The writing of this chapter was supported in part by Public Health Service Grant CA 05985 from the National Cancer Institute, by a Fulbright award, and by a guest professorship at Universität Frankfurt, Institut für Humangenetik und Vergleichende Erbpathologie.

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See also PubMed.

Zimmerman, H.M., and H. Arnold. 1941. Experimental brain tumors. I. Tumors produced with methylcholanthrene. Cancer Res. 1: 919-938.