The corona radiata surround an ovum or unfertilized egg cell, and consist of two or three strata (layers) of follicular cells. They are attached to the outer protective layer of the ovum, the zona pellucida, and their main purpose in many animals is to supply vital proteins to the cell. They are formed by follicle cells adhering to the oocyte before it leaves the ovarian follicle, and originate from the squamous granulosa cells present at the primordial stage of follicular development. The corona radiata is formed when the granulosa cells enlarge and become cuboidal, which occurs during the transition from the primordial to primary stage. These cuboidal granulosa cells, also known as the granulosa radiata, form more layers throughout the maturation process, and remain attached to the zona pellucida after the ovulation of the Graafian follicle. For fertilization to occur, sperm cells rely on hyaluronidase (an enzyme found in the acrosome of spermatozoa) to disperse the corona radiata from the zona pellucida of the secondary (ovulated) follicle, thus permitting entry into the perivitelline space and allowing contact between the sperm cell and the nucleus of the oocyte.