Follicles that form an antrum during maturation are called antral follicle or Graafian follicle. During the development of the follicle, a glycoprotein polymer capsule called the zona pellucida which is acellular, forms around the oocyte, separating it from the surrounding granulosa cells.
This layer remains with the oocyte after ovulation and contains enzymes that catalyze with sperm to allow penetration.