Intragranular processing of pro-opiomelanocortin in the intermediate cells of the rat pituitary glands. A quantitative immunocytochemical approach.

Quantitative immunocytochemical studies were done by using the immunogold technique on sections of the intermediate lobe of rat pituitary. Antibodies raised (in rabbits) against the precursor proteins pro-opiomelanocortin (POMC) and ACTH were used. The results clearly indicate that the immature granules are the major site of POMC, as their antigenic density (gold beads/micron2) was almost 3 times as high as that of ACTH. In the mature granules, the antigenic density of ACTH was increased by 2.7-fold compared with the immature granules. Using a computer-assisted method, it was possible to categorize the granules' antigenic density according to their size. Using this approach it was found that the antigenic density of POMC remained constant in all mature granules of varied sizes, whereas the antigenic density of ACTH decreased with increasing granule size. The relationship between granule size, degree of maturation, and antigenic density is discussed.