Development of cortical granules and the cortical reaction in rat and Hamster eggs

The origin of cortical granules is described in hamster and rat oocytes by electron microscopy. They occur first near vesicular elements of small Golgi complexes. Most of these Golgi complexes lie peripherally, but in the rat, there are a few also near the germinal vesicle. The granules arise at the concave surfaces of the Golgi complexes and apparently migrate to the cell membrane where they become aligned in close association to it. Cortical granules are found in the cytoplasm of the first polar body but not in the second. The cortical reaction, initiated by sperm attachment, consists of a gradual fusion of the membranes surrounding cortical granules with that of the egg plasma membrane and eversion of their contents into the perivitelline space. Occasionally, in the rat several granules fuse laterally to form larger cortical “caverns.” Such caverns have a single opening to the perivitelline space. The origin and extrusion of cortical granules can be compared usefully with the synthesis and merocrine‐type secretion of proteinaceous granules or mucoid droplets.