Menstrual cycle in human beings:
1) The cycle of changes in the female reproductive system that leads to formation of Ovum is called 'menstrual cycle'. It occurs once in 28 to 30 days.
2) During this cycle only one ovum is released and it alternates between the two ovaries in successive months.
3) This cycle starts from the age of 13 to 14 years of age and continues upto 50 to 55 years. Then the production of ova usually stops.
4) The menstrual cycle has two phases. They are 1) Proliferative phase 2) Secretary phase.
Proliferative phase :
5. During the proliferative phase, cells present in the ovarian follicle, fallopian tubes, uterus and vagina increase in their number of mitotic divisions. The ovum is released.
6. The proliferative phase occurs during 12 to 14 days of the cycle and the ovum is released usually on the 14th day of the cycle.
Secretary phase :
7. The second half of the menstrual cycle is called secretary phase which occurs in the next 14 to 16 days. During this phase the ruptured follicle changed into corpus luteum and uterine walls are thickened. If implantation does not occur by 26th day the cells of corpus luteum and the walls of the uterus are detached and expelled out along with some amount of blood and the cycle continues.
8. The menstrual cycle is under the control of hormones secreted by the pituitary gland and ovaries.