Development of Pituitary Acidophils and their Granulation in Pre- and Postnatal Rats

Development of acidophils was investigated electron microscopically in pituitaries from the rat embryos on the 15th, 17th, 18th and 20th day of gestation and from the neonatal and postnatal rats on the 1st, 3rd, 4th, 6th, 12th and 18th day of birth. Pituitary agranular primordial cells are the stem cells from which acidophils develop. During embryonic life, the male is more advanced in differentiation of primordial cells than the female. Initial granulation begins to take place in “ambiguous cells” (Yoshimura et al., 1970) on the 17th day of gestation as a row of minute granules about 50 mμ along the plasmamembrane. Incidence of ambiguous cells with various shapes is higher in the male than in the female. During embryonic period, only a small number of immature acidophils have developed from ambiguous cells despite spherical enlargement of cell-bodies where dense granules maintain their size within the proximity of 250 mμ. Most of ambiguous cells tend to transform into immature basophils, and even gonadotroph-like basophils have accumulated during the latest pregnancy. On the other hand many immature acidophils emerge directly from primordial cells without the mediation of ambiguous cells. In a certain primordial cells, a row of dense granules as large as 130-200 mμ in diameter suddenly appear along the plasmamembrane on the 18th day of gestation. In equilibrium with the prompt increase in the cell number, their granules rapidly increase in dimension and their maximal size reaches 250 mμ towards the end of fetal period. Postnatal development of acidophils is quite quick as to the population and granulation in both sexes. Numerous immature acidophils, not always spherical but somewhat elongated, develop appositionally from primordial cells. Appearance of a row of large granules of 200-250 mμ along the plasmamebrane of primordial cells seems to be related to morphological representation for the main developmental process of acidophils. Granules in immature acidophils increase in number, their size being persistently less than 200 mμ up to the 6th postnatal day. Enlarged spherical premature acidophils containing numerous granules of 200-300mμ in diameter begin to appear on the 12th day. So-called somatotrophs identical with the fully developed acidophils filled with large granules of about 350 mμ start to appear on the 18th day of age.