Microscopic examination of a PAS-stained histological section through a Graafian follicle demonstrates a bright reddish-pink, acellular ring around the ovum. Which of the following terms most accurately describes this ring?
The ring described is the zona pellucida, which surrounds the ovum. The zona pellucida is rich in polysaccharides and glycoproteins and consequently stains brightly pink or red with PAS stain. Binding of the sperm cell membrane to the zona pellucida triggers the acrosome reaction, during which acrosomal enzymes are released that digest the zona pellucida, allowing the spermatozoon to contact and fuse with the ovum cell membrane. The follicular cells immediately outside the zona pellucida form the corona radiata . The larger cumulus oophorus is the hill of follicular cells that surrounds the ovum. The theca interna and externa are formed from the connective tissue surrounding the follicle.
Corona radiata
Cumulus oophorus
Theca externa
Zona pellucida