The mouse has three lacrimal glands. The largest is the Harderian gland which lies behind the eye in the orbit and partially encircles the optic nerve. A small intraorbital lacrimal gland is associated with the eyelid, and a large extraorbital lacrimal gland lies subcutaneously at the base of the ear. Ectopic Harderian glandular tissue occasionally occurs within the glandular parenchyma of the extraorbital lacrimal gland (Fig. 347). A mononuclear infiltrate often appears to be associated with this ectopic tissue.
Ectasia of the Harderian gland is a dilatation of the lumens of the glands with compression of adjacent tissue (Fig. 348). The ectasia appears to be the result of blockage of the duct that drains the mucous secretions to the surface of the eye, but the actual blockage cannot usually be demonstrated microscopically.
Neoplasms of the Harderian gland appear to form a spectrum of tumor types with the majority classified as adenomas and a few which appear to progress from adenomas to adenocarcinomas and even metastasize to the lungs. The adenomas may be so small that they are not visible grossly (Fig. 349; Frith et al., 1980c; Ward, 1983). The tumor cells are usually columnar with finely vacuolated eosinophilic cytoplasm. They most commonly present a papillary pattern, but they may be tubular or cystic in appearance. Harderian gland adenocarcinomas usually are visible at necropsy as large masses involving the orbit and frequently replacing the eye. The neoplastic cells may be well-differentiated (Fig. 350), but may invade locally and metastasize to the lungs (Fig. 351).
Age-related retinal atrophy in the mouse consists of atrophy or loss of the rods, since the mouse does not possess cone cells. The incidence varies from less than 2.0% in the BALB/c strain to 100% in the C3H strain. The lesion may occasionally be unilateral or partial and characterized by atrophy of the layer of rods as well as the outer plexiform and outer nuclear layers (Figs. 353 and 354).