Physiology Of The Menstrual Cycle

Question 1

Describe the female sex organs

Answer 1

• The female gonad is the ovary which produces the main female sex hormones (oestrogen and progesterone) that underlie the development of the gamete and the uterine wall.
• Closely associated with the ovaries are the fimbriae of the fallopian tubes through which the egg can pass and become fertilised, or, if not fertilised, exit via the uterus, cervix, and vagina.

Question 2

List the sequences of events during oogenesis, emphasising their timing

Answer 2

Oogenesis

• Before birth the oogonia undergo a mitotic division to multiply numbers of potential oocytes.
• The oogonia then commence the first meiotic division before birth to form the primary oocytes.
• All divisions then stop until the female reaches puberty.
• At this point the first meiotic division is completed, giving a haploid cell rich in cytoplasm and nutrients called the secondary oocyte. A second smaller cell is also formed called the first polar body.
• Although the secondary oocyte is haploid with unpaired chromosomes, it still has twice the amount of genetic material of the final gamete.
• After fertilisation meiosis 2 resumes. The secondary oocyte splits into an ovum and a second polar body.
• The nuclei of the sperm cell and the ovum unite, forming a diploid zygote.

Question 3

Name the hormones involved in regulation of the female reproductive cycle and list their actions

Answer 3

• Gonadotropin Releasing Hormone (GnRH) - released at the start of the cycle by the hypothalamus, causing the release of Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH) from the anterior pituitary gland.
• FSH - encourages cell division and oestrogen secretion by the granulosa cells.
• LH - early in the cycle it stimulates androgen production by the theca cells, androgens which then diffuse into the granulosa cells to be converted into oestrogen. Later in the cycle it functions in the formation of the luteal body.
• Oestrogen and progesterone - produced after ovulation by the corpus luteum. The levels of these hormones negatively feedback on the FSH and LH, keeping them at low levels.

Question 4

Draw a diagram showing how gonadotropin and female sex hormone levels change during the female cycle

Answer 4

Refer to physiology of the menstrual cycle word document on Qol

Question 5

Explain how feedback regulation explains these changes in hormone levels

Answer 5

• After ovulation the corpus luteum takes over hormone production, producing oestrogen and progesterone.
• The increasing levels of oestrogen and progesterone causes negative feedback to the hypothalamus which responds by decreasing secretion of GnRH.
• Less FSH and LH are produced by the anterior pituitary due to the lack of GnRH being secreted.
• At around day 22-24 the corpus luteum undergoes atresia, causing a rapid decrease in the levels of oestrogen and progesterone.
• The low levels of oestrogen and progesterone removes the negative feedback effect on the hypothalamus which begins to secrete more GnRH again, causing its level to rise.
• The cycle repeats again.

Question 6

Describe the ovarian and endometrial changes during the female reproductive cycle and explain how these changes can be explained in terms of the relevant hormonal actions

Answer 6

Ovarian changes
- Already covered, see relevant flashcard.

Endometrial changes

• At the beginning of the menstrual cycle the stratum functionalis becomes removed, leaving just the stratum basalis.
• Under the influence of the oestrogen produced in the developing follicles, the uterine lining thickens and proliferates, forming many tubular glands.
• After ovulation the predominant hormone is progesterone which maintains the uterine lining.
• The tubular glands lengthen without a corresponding thickening of the uterine wall and so become “saw-toothed” in appearance.
• The glands become highly vascularised and fill up with secretions rich in glycogen.
• Of there is no fertilisation, then the rapid fall in oestrogen and progesterone levels coming from the atresia of the corpus luteum at about day 24 causes the ischaemia and infarction of the endometrium which subsequently sloughs off and is shed during the menstrual flow.