Further studies on the theta cell of the mouse anterior pituitary as revealed by electron microscopy, with special reference to the mode of secretion.

Theta cells reported previously as a new cell type in the anterior pituitary of the mouse were examined with the electron microscope. This type of cell is distinguished by the presence of pleomorphic secretory granules, a characteristic arrangement of the rough surfaced variety of endoplasmic reticulum, a well developed Golgi complex, and an eccentrically located nucleus. The secretory granules are seen at first as small granules of low density within the Golgi vesicles. While they are within the Golgi vesicles they become larger and denser. Simultaneously they move from the proximal to the distal part of the Golgi region and finally emerge from the Golgi area as mature granules in the cytoplasm. Thus, secretory granules are always enveloped by a limiting membrane which originates from the wall of the Golgi vesicle. At the stage of granule-extrusion, the cell membrane fuses with the limiting membrane of the granules and openings in the cell membrane appear at the place of extrusion. The granules then appear to lie within inpocketings of the cell membrane. They lose their density within these inpocketings or within the cytoplasm and occasionally show fragmentation. After complete loss of density, the granules are extruded as amorphous materials to the territory outside of the cell.