Ultrastructural and immunocytochemical studies on the cytoskeleton in the anterior pituitary of rats, with special regard to the relationship between actin filaments and secretory granules.

As previously reported, in anterior pituitary cells of the rat, secretory granules are linked with adjacent granules, cytoorganelles, microtubules, and plasma membrane by thin filaments, 4-10 nm in diameter. The quick-freeze, deep-etching method revealed that some of the filaments linking adjacent secretory granules show 5 nm-spaced striations on their surface which are known to be characteristic of actin. Immunocytochemistry showed that actin is localized in the cytoplasm beneath the plasma membrane, and around or between secretory granules. The heavy meromyosin decoration method demonstrated that actin filaments are mainly located in the cytoplasm beneath the plasma membrane, while some actin filaments are connected with the limiting membrane of the secretory granules. The actin filaments associated with the secretory granules are considered to be involved in the intracellular transport of the granules, while those localized in the peripheral cytoplasmic matrix might control the approach of the secretory granules to the plasma membrane and their release.