Question

A-Describe the phases that are involved in the uterine (menstrual) cycle. Ensure you include associated hormones and the effects of these on the uterus?

B- state at what point in the menstrual cycle fertilization will normally occur.

word count: 300

Answer 1

A) Uterine (menstrual) cycle is the monthly cyclical changes in the female reproductive system under the influence of hormones.
It includes 4 phases:-
- Menstrual phase:- It lasts from day 1 to day 5 of the cycle. Low estrogen and low progesterone levels causes withdrawal of hormonal support to the endometrium of uterus and the endometrium is shed-off resulting in bleeding. Bleeding during menstrual phase of uterine cycle includes dead uterine endometrial tissues and blood from the rupture of uterine endometrial arteries.
- Proliferative phase:- It lasts from day 6 to day 13 of the uterine cycle. During this phase, the follicle stimulating hormone (FSH) released from the anterior pituitary gland causes recruitment of and development of ovarian follicle into mature Graafian follicle. Estrogen is produced by the granulosa cells of developing ovarian follicle. Under the influence of estrogen produced in the ovaries, the uterine endometrium proliferates. There is also development of uterine glands and spiral arteries in the uterus under the influence of estrogen.
- Ovulatory phase:- It occurs around day 14th of the menstrual cycle (in a 28 days menstrual cycle). Around day 14 of the menstrual cycle, estrogen surge occurs which stimulates luteinizing hormone (LH) release and inhibits FSH release from the anterior pituitary gland. This sudden rise in luteinizing hormone (LH) induces ovulation in which the mature Graafian follilce ruptures and releases secondary occyte from the ovary.
- Luteal phase (secretory phase):- It lasts from day 15 to day 28 of the menstrual cycle. The ruptured Graafian follilce gets converted into corpus luteum which secretes progesterone. Under the influence of progesterone produced by corpus luteum along with the help of estrogen, the uterine endometrium further proliferates. The spiral arteries of uterine endometrium becomes highly coiled and the endometrial glands further develops. These changes in the uterine endometrium prepares the uterus for the implantation of the developing embryo.

B) Fertilization is the union of male gamete (sperm) and female gamete (ovum) to from a zygote. Usually, the viability of the ovum released during ovulation is around 24-48 hours and the viability of sperm in the female reproductive tract is between 4-5 days. As ovulation usually occurs around day 14 (between day 12 to day 17) in a 28 day menstrual cycle, the window for fertilization is between day 12 to day 17 when the probability of a viable sperm reaching a viable ovum is highest. As the menstrual cycle and the day of ovulation is highly variable among females, hence the fertilization window can also vary greatly. The highest probability of fertilization to occur is in and around day 14 (when ovulation normally occurs) in a 28 days menstrual (uterine) cycle.