it begins meiosis II but stops at metaphase II. The mature follicle containing the secondary oocyte soon ruptures and releases the oocyte. This is ovulation. At ovulation, the secondary oocyte is expelled into the pelvic cavity and normally is swept into the uterine tube. If sperm are present in the uterine tube and 1 penetrates the secondary oocyte successfully, meiosis II resumes. As meiosis II resumes, the secondary oocyte splits into 2 haploid cells, again of unequal size. The larger cell is the ovum and the second smaller cell is the 2 nd polar body. The nuclei of the sperm and the ovum unite forming a diploid zygote. Therefore, 1 primary oocyte gives rise to a single ovum.
If no sperm are present, the secondary oocytes degenerates.
The remnants of the follicle that released the secondary oocyte develop into the corpus luteum. This produces hormones such as progesterone, estrogen, relaxin and inhibin.
The Mare Reproductive Cycle
Horses are considered seasonal polyestrous animals as their sexual activity is influenced by the season of the year (daylight) nutrition and climate. In North America, mares undergo cyclic sexual activity during the breeding season. Light is the external stimulus for reproduction. Neural input is transferred into an endocrine stimulus in the pineal gland . Melatonin is secreted by the pineal gland and concentrations are inversely proportional to the duration of daylight. Melatonin exerts antagonist effects on the ovaries by depressing Gonadotropin Releasing Hormone (GnRH) released by the hypothalamus and therefore depressing Follicle stimulating hormone (FSH) and Luteinizing hormone (LH).
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