Ultrastructural changes in the prostate gland of a seasonally breeding mammal, the grey squirrel (Sciurus carolinensis Gmelin).

Grey squirrels were obtained from the wild every month for two calendar years. The prostate gland was found to be a single lobed structure, subdivided into many lobules each of which was composed of numerous secretory acini with central lumina. Secretion was active throughout the breeding season (January to June), but the gland became atrophic from July to September, and recovered between October and December. Two 'types' of secretory cells were observed in the secretory epithelium throughout the year in both adults and juveniles. During the secretory period 'Type I' cells were characterised by a large nucleus and abundant granular endoplasmic reticulum, ribosomes and secretory granules. The vacuolated 'Type II' cells were more abundant and possessed a smaller nucleus, more substantial Golgi apparatus and numerous secretory vesicles. In the typically atrophic gland almost all organelles associated with secretory activity disappeared, but both types of cells could still be distinguished by their peculiar nuclei and even by their characteristic light and dark cytoplasm in tissue fixed in glutaraldehyde. Recovery of the prostate gland was preceded by a wave of mitotic activity lasting from October to December. However, secretory activity was not resumed until the following January. The two morphological forms were either two functional phases of a single cell type or two distinct populations of secretory cells. Whichever may be the case the prostate gland of the grey squirrel is unique. No other animal has yet been observed to possess secretory cells capable of passing from one morphological and functional phase to the next. If there are two types of secretory cell within this single lobed structure, the organisation of the grey squirrel prostate gland differs from that in the rat and in man where each lobe contains only one single type of secretory cell.