Question

Describe the phases and major steps of the ovarian and uterine cycles. Include:

a. Oocyte and follicle maturation and growth

b. The role of hormones and their subsequent effects

c. Differences between the cycles when fertilization occurs or does not occur

Answer 1

The menstrual cycle of a female is controlled by a group of hormones which controls the ovarian and uterine activities in her body. The oogenesis begins at the 7th gestational week in the womb of female's mother only.

The description of cycles are given below-

OVARIAN CYCLE - The ovarian events taking place during menstrual cycle under the hormonal influence is called as ovarian cycle.

• Follicular phase- First of all , Due to rise in Follicle stimulating hormone several Follicles in the ovary starts to mature due to which theca cells starts to form androstenedione and granulosa cells forms aromatase which converts androstenedione into estrogen which gives negative feedback to hypothalamus. Hence level of FSH falls . This happens during first 10 days of ovulatory cycle.
• At 11 day most of the Follicles die off and only dominant follicle with most FSH receptor matures and it secretes estrogen which gives positive feedback to hypothalamus and more GnRH secretes which increases LH and FSH levels. At day 14 ovulation occurs.
• Ovulatory phase - At this phase the level of luteinising hormone increases which is secreted by the ova released with this estrogen level decreases and progesterone level increases.
• Luteal phase - In this phase level of LH and FSH falls and the ova becomes corpus luteum .This Corpus luteum secretes progesterone simultaneously level of estrogen are maximum and the endometrium thickens.
• When fertilisation doesn't takes place the endometrium sheds due to corpus luteum rupture.

When fertilisation occurs the corpus luteum does not rupture and keeps on secreting progesterone to maintain the endometrium for the implantation of zygote. This causes attainment of pregnancy.

UTERINE CYCLE - The events taking place in uterus under the influence of hormone is called as uterine cycle.

Menstrual phase - The endometrium is thinnest at this phase due to decreased level of progesterone and estrogen by ruptured corpus luteum.

proliferative phase - Rebuilding of functional layer of endometrium under the influence of hormones.

Secretory phase - Right after ovulation due to peak in hormones the endometrium proliferates and the blood supply to it increases.This is essential for sustainable pregnancy.

If pregnancy takes place the endometrium layer remains proliferated and becomes more rich in blood supply for embryo nourishment but if fertilisation doesn't takes place due to fall in hormones the functional layer of endometrium sheds again and menstrual flow begins.