Mechanisms of secretory granule transport and exocytosis in anterior pituitary cells.

Mechanisms of secretory granule transport and exocytosis in anterior pituitary cells were studied using various electron microscopic techniques. Some secretory granules were linked with microtubules by short strands. Many thin filaments were associated with all the secretory granules as well as membrane organelles. Adjacent granules were interconnected by these filaments. Some of the granule-associated filaments were proved to be actin filaments. Microtubules and granule-associated filaments must be involved in the secretory granule transport. Secretory granules were not located just under the plasma membrane where abundant actin filaments occupied the subcortical cytoplasm, while a lot of granules were in close contact with the plasma membrane in the region free of the subcortical actin filaments. This indicates that the subcortical actin filaments regulate the granule access to the plasma membrane. Secretory granules just beneath the plasma membrane were linked to it by intervening strands with various length. Annexin II, one of the Ca2+-dependent phospholipid binding proteins, was mainly associated with the plasma and granule membranes, especially localized at their contact sites. Annexin II might be one of the components of the intervening strands, and play an important role in the membrane contact and fusion. Exocytotic process was visualized by the quick-freeze, deep-etch electron microscopy. The initial membrane fusion starts with formation of a cylindrical neck-like structure. The intervening strands still link between the plasma and granule membranes during the membrane fusion.