Secretory cycle of the pituitary basophils and its morphological evidence.

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Abstract
Rat pituitary basophils were examined by EM at various times under the following conditions: acute, subacute and chronic administration or perfusion of TRH [thyrotropin-releasing hormone] and LRH [luteinizing hormone-releasing hormone] separately or in combination to normal male and ovariectomized female rats and to thyroidectomized male rats. A sequential, ultrastructural transformation (type I to V) of the basophils was demonstrated which might perform its secretory cycle. The type-I cells represent immature basophils. The II-type cells (correspond to classical TSH-cells) are angular cells in a synthesizing and/or storing phase showing an abundance of secretory granules 100-150 nm in diameter. The II/III-type cells come into 2 categories of cell type, presenting a peripheral vesiculation of the cytoplasm. The III-type cells (correspond to classical LH [lutenizing hormone]-cells) are round in shape and possess a number of vesicular structures and secretory granules of about 200 nm in diameter throughout the cell body. The III/IV-type cells are characterized by numerous round large cisternae which are filled with fine granulated particles. Secretory granules in these cells are 200-250 nm in diameter. The enlarged spherical IV-type cells (correspond to classical FSH [follicle stimulating hormone] cells) are packed with irregularly expanded endoplasmic reticula. Secretory granules are about 250 nm in size. The IV-type cells are presumably at the end stage of granular storage or at the initiation of granular secretion, since there is a transitional cell type (IV/II-type) in which secretory granules are reduced in size (100-150 nm in diameter) and in electron density. Vesicular endoplasmic reticula are infrequently found in the IV/II-type cells, which may be either at the end of the secreting phase or at the subsequent resting phase. The V-type cells appear even in the normal and numerously after repeated or prolonged administrations of TRH or LRH. They may be exhausted basophils. It was not, however, concluded that II-, III- and IV-type cells might correspond to TSH-, LH- and FSH-cells, respectively. Based on a sequential appearance of the cells mentioned above, a hypothesis was proposed: the II-type may transform into the III-type, and subsequently into the IV-type which may come back to the II-type.

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