Pituitary intracisternal granule formation during the estrus cycle of the rat.

An ultrastructural study of the anterior pituitaries of cycling rats reveals changes in the ultrastructure of the gonadotrophs during the afternoon of proestrus when the release of follicle stimulating hormone (FSH) and luteinizing hormone (LH) is high. The rough endoplasmic reticulum cisternae become greatly dilated, forming extensive channels which are continuous with the perivascular space and provide an increased surface for the release of hormones. Granules and cytoplasmic islands of granules are seen in these spaces. Such changes are not observed during diestrus and early proestrus when the release of FSH/LH is low. The data indicate that intracisternal granule formation is an important mechanism by which anterior pituitary cells respond to increased hormonal requirements.