Abstract
The relative volume of the RPD region of the pituitary gland of Gasterosteus aculeatus form leiurus was greater in animals collected in the spring than in winter. A morphometric analysis of the prolactin cells from spring animals showed only slight changes in cell ultrastructure compared with winter animals. However, in spring the prolactin cells apparently formed and released more secretory granules. Three distinct sites for the release of secretory granules are described. A preliminary study of the response of prolactin cells from spring animals to 70% sea water did not reveal any large scale changes in cell ultrastructure. The significance of this finding is discussed. It is concluded that although the freshwater stickleback cannot be regarded as “physiologically hypophysectomised:” with regard to prolactin secretion in the winter, it still shows a seasonal change in prolactin cell ultrastructure.
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