Gonadotropin-releasing hormone (GnRH) is responsible for the release of follicle-stimulating hormone and luteinizing hormone. It is produced by the hypothalamus. The acrosome contains digestive enzymes (hyaluronidase and acrosin). These enzymes break down the outer membrane of the ovum, called as the zona pellucida. The ovarian follicle ripens into the Graafian follicle and produces estrogen, which stimulates the proliferation of the endometrium and the enlargement of the uterine glands. Leydig cells produce testosterone in the presence of the luteinizing hormone. Relaxin is produced by the ovary and the placenta. Relaxin relaxes the intrauterine ligaments, allowing the uterus and pelvis to expand during childbirth (parturition).