Skeletal System

As in mice and hamsters, bones lack Haversian systems. Adult rats, particularly males of some strains, continue to grow, and physeal ossification is not complete until after 1 year of age.

Other Anatomic Features

Adrenals of wild rats are remarkably larger than that of their domesticated cousins. Adrenals of females are larger than those of males. Rats are endowed with prominent brown fat, as in mice. The exorbital lacrimal glands of rats display striking epithelial megalokarya and polykarya, particularly in older males (Fig. 2.4). This increases with age and should not be confused

FIG. 2.4. Exorbital gland from a mature male rat. Note the megalokarya and polykarya of acinar epithelial cells, a normal finding, particularly in old males.

with a disease state. The fur of male albino rats tends to yellow with age.