The ultrastructure of the pars intermedia and the subdivision of its glandular cells in the young sheep pituitary.

The ultrastructures of the pituitary pars intermedia of 11 male sheep about 1-13 months of age were observed. The following cells were distinguished in the sheep pars intermedia: pars intermedia glandular cells, marginal cells, interstitial cells, follicular cells, pars distalis-like granular cells, degenerating cells and mast cells. The pars intermedia glandular cells contain a well-developed Golgi apparatus and many mitochondria. The secretory granules had various densities and sizes (100-425 nm in diameter). No image of exocytosis was found. The pars intermedia glandular cells were classified into the following three subtypes: "PI-I, PI-II and PI-III type cells." PI-I type cells contain rather few secretory granules, but their rough endoplasmic reticulum is well developed in parallel arrays. PI-II type cells contain a few secretory granules, and the rough endoplasmic reticulum consists of scattered short strands. PI-III type cells contain abundant secretory granules of high density mixed with a few granules with low density. The rough endoplasmic reticulum is scattered strands. This type of cell was distinguished from granular cells like those of the pars distalis, although the granular cells also contained many dense granules. The marginal cells contacted the glandular cells directly, not being separated by a basal lamina. The degenerating cells were filled with myelinated lysosomes. Mast cells were often found in the interlobular connective tissue.