Menstrual Cycle
It is the reproductive cycle in female primates e.g. monkeys, apes, and human beings. The first menstruation begins at puberty and is called menarche. Menstrual cycles cease around 50 years of age; that is termed menopause. In human females, menstruation is repeated at an average interval of about 28/29 days, and the cycle of events starting from one menstruation to the next one is called the menstrual cycle. It has four phases;
- Menstrual / Bleeding Phase (1st -5th Day): This phase lasts for 3-5 days. The menstrual flow results due to the breakdown of the endometrial lining of the uterus and its blood vessels which releases blood, mucus, and tissue fluid pass out and are called menstruation. It only occurs if the released ovum is not fertilized which results in lowered levels of LH causing degeneration of the corpus luteum, and a drop in progesterone levels.
- Proliferative / Follicular Phase (6th -13th Day): It lasts for about 8-12 days. During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of the uterus regenerates through proliferation. These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones. The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase and stimulates follicular development as well as the secretion of estrogens by the growing follicles.
- Ovulatory Phase: (14th day): The shortest phase in the ovarian cycle in which LH and FSH attain a peak level. Rapid secretion of LH (LH surge) ruptures the Graafian follicle to release the ovum (ovulation).
- Secretory / Luteal Phase (15th -28th day): It lasts for about 14 days. In this phase after the ovulation, the Graafian follicle is transformed into a yellow glandular mass called Corpus luteum that secretes a large amount of progesterone which is essential for the growth and maintenance of the endometrium making the uterus ready for implantation (of the fertilized ovum) and other events of pregnancy. If fertilization does not occur, the corpus luteum degenerates. This causes the disintegration of the endometrium leading to menstruation, and marking a new cycle.
NCERT Notes on Human Reproduction Class 12 Chapter 2
NCERT Notes of Class 12 Chapter 2 Human Reproduction: Human reproduction is the biological process by which a new individual offspring is produced from one or two parent organisms. The Human Reproduction process involves the fusion of gametes, which are specialized cells that carry genetic information from each parent, resulting in the formation of a zygote.