Development and cytodifferentiation of the rabbit pars intermedia. II. Neonatal to adult.

Material from pars intermedia obtained from rabbits ranging from the second week post-partum to the adult stage, and including specimens from pregnant animals, was studied. The rate of cell division became greatly reduced early in postnatal) development. The commonest type of cell (the pars intermedia-glandular cell) becomes increasingly PAS-positive during the early stages of development. Although by 35 days differentiation of all the ACT-type cells is complete, the pars intermedia-glandular cells take as long as 53 days to mature. The epithelioid border of the hypophysial cleft persists throughout life, commonly containing dark cells. A ciliary fringe frequently appears in neonates and persists in pregnancy. Possible functions of such cilia are discussed. Throughout development the fine structure of the vasculature was studied. Secretory granules resembling those within the cells were seen in and around the blood vessels, and the mode of endocrine secretion in the pars intermedia tissue is discussed. The pars intermedia-glandular cells of the pregnant rabbits appeared hyperactive. The functional significance of the mammalian pars intermedia is discussed.