Constant light exposure induces damage and squamous metaplasia in Harderian glands of albino mice.

Harderian glands from control albino mice kept in a cyclical light/dark environment had tubulo-alveoli comprised of lipid-filled glandular epithelial cells. The porphyrin content of the gland measured 122 microgram/100 mg gland. Constant light exposure for 24 hr caused exopthalmos grossly. Histologically most of the secretory cells were swollen and the lumens of many tubulo-alveoli were obliterated; a few areas of the gland showed damage. The porphyrin content had decreased to 116 microgram/100 mg gland. After 3 days of constant light exposure the tubulo-alveoli were markedly altered. Lipid and cellular debris filled the lumens, and lining cells were highly irregular, ranging in shape from columnar to squamous. The porphyrin content had decreased to 72 micro/100 mg gland and leukocytes and macrophages were abundant. Despite this extensive damage a number of tubulo-alveolar epithelial cells were observed under-going mitosis. After 7 days of constant exposure to light, some tubulo-alveolar epithelial cells had undergone squamous metaplasia, and the porphyrin content had dropped markedly to 50 microgram/100 gland. These pronounced cellular changes are believed to result from a direct effect of light on the gland.