The immature type of pro-opiomelanocortin cell of the rat anterior pituitary as observed by immunogold electron microscopy.

The perinatal development of glandular cells in the rat anterior pituitary producing pro-opiomelanocortin (POMC), the precursor protein of peptide hormones including adrenocorticotropin (ACTH), melanocyte-stimulating hormones (MSH) and endorphins was studied by an immunoelectron microscopic technique using the colloidal gold-antibody method. The POMC cells are classified into the following three types: 1) an immature type, 2) an intermediate type, and 3) a mature type, which correspond to Types III, I and II of the ACTH cells, respectively. The average size of the secretory granules in POMC cells varies widely, measuring 98.6 +/- 8.5 nm in the immature type, 111.4 +/- 6.7 nm in the intermediate type, and 139.3 +/- 23.1 nm in the mature type. The immature type comprises more than 90% of the POMC cells in the late fetal stage, but decreases in number after birth. The intermediate type reaches a peak at 8 days (female) or 33 days (male), while the mature type is that most frequently observed at 45 days, i.e., more than 50-60% of the total POMC cells, when the immature type decreases to about 15%.