Electron microscopic and cytochemical study of the cortical cytoplasm in the preovulatory human oocytes

Important changes take place in the cortical cytoplasm of human oocytes in connection with their maturation. Contacts between granulosa cell processes and the oocyte are interrupted, the perivitelline space develops, invaded by oocyte long microvilli. Fine flocculent material is deposited in the perivitelline space. As for oocyte cell organelles, changes occur especially in the granular endoplasmic reticulum which, formerly creating a network of cisternae studded with a low number of ribosomes, eventually becomes disintegrated, splitting into individual smooth vesicles scattered together with mitochondria throughout the cytoplasm. The number of Golgi structures drops substantially. Cortical granules mature and become organized into a single layer beneath the cell membrane. Acid mucopolysaccharides, glycoproteins and proteins were demonstrated in their contents. In addition to the above cortical granules, morphologically distinct secretory vesicles are present in the cortical cytoplasm of human preovulatory oocytes. Their number increases markedly during the resumption of meiosis when they release their contents into the perivitelline space. Like the extracellular flocculent material deposited in the perivitelline space, also the secretory vesicles contain glycoproteins and proteins. The extrusion of this glycoprotein material into the perivitelline space coincides temporally with the loosening of microvilli, previously embedded in the zona pellucida material, and with the interruption of oocyte-follicular cells contacts.