Menstrual Cycle- Reproduction

Menstrual cycles are characteristic of primates and do not occur in other vertebrate groups. The length of the cycle is highly variable, though 28 days is generally regarded as typical for human female. The cycle in the chimpanzee rehires about 35 days. Both estrous and menstrual cycles are regulated by the same interplay of pituitary add ovarian hormones, and the effects of the ovarian hormones on the reproductive tract are comparable in most respects. The chief differences between the two types of cycles are;

1. The presence of a menstrual phase in primates, and
2. The spreading of sexual receptivity throughout the cycle, rather than the limitation of it to a definite period as found in nonprimates.

During the menstrual phase the superficial layers of the endometrium are sloughed rupturing spiral arteries, resulting in bleeding. This type of bleeding does not occur in nonprimates. Spiral arteries are absent from the uteri of estrous mammals but are present in primates with the exception of the New World monkeys. New World monkey menstruate, but the of blood is greatly reduced. The menstrual phase lasting four to seven days is regarded as the beginning of the primate cycle. This arrangement is sanctioned because menstruation is the easiest period of the cycle to recognise and because it corresponds with the formation of new follicles in the ovaries. However, if the uterus alone is considered, menstruation represents the terminal event with subsidence of the corpus luteum and a consequent deficiency of ovarian hormones. The endometrium cannot maintain itself and hence regresses and the surface disintegrates.