Immunocytochemical and immuno-electron-microscopical study of growth hormone cells in male and female rats of various ages.

Growth hormone (GH) secretory cells were identified by immunogold cytochemistry, and were classified on the basis of the size of secretory granules. Type I cells contained large secretory granules (250-350 nm in diameter). Type II cells contained the large secretory granules and small secretory granules (100-150 nm in diameter). Type III cells contained the small secretory granules. The percentages of each GH cell type changed with aging in male and female rats of the Wistar/Tw strain. Type I cells predominated throughout development; the proportion of type I cells was highest at 6 months of age, and decreased thereafter. The proportion of type II and type III cells decreased from 1 month to 6 months of age, but then increased at 12 and 18 months of age. The pituitary content of GH was highest at 6 months of age, and decreased thereafter. Estrogen and androgen, which are known to affect GH secretion, caused changes in the proportion of each GH cell type. The results suggest that when GH secretion is more active the proportion of type I GH cell increased, and when GH secretion is less active the proportion of type II and type III cells increased. The type III GH cell may therefore be an immature type of GH cell, and the type I cell the mature type of GH cell. Type II cells may be intermediate between type I and III cells.