MCAT Menstrual Cycle

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What is the purpose of menstrual cycle?

- Regular natural change in the female reproductive system (in uterus and ovaries) that make pregnancy possible

- Production of oocytes

- Prepare uterus for pregnancy

The menstrual cycle can be described one of the 2 cycles:

- Ovarian cycle

- Uterine cycle

Ovarian cycle

Describes changes that occur in the follicles of the ovary

Uterine Cycle

Describes changes in the endometrial lining of the uterus

Ovarian cycle and uterine cycle happen at the same time. It's just that we separate the 2 so that we can know what happens at each region.

What are the 3 phases of uterine cycle?

Menstruation - proliferative phase - secretory phase

What are the 3 phases of ovarian cycle?

Follicular phase, ovulation, luteal phase

What is a follicle?

- Immature egg (oocyte) with granulosa cells

- Secretes hormones that influence menstrual cycle

- During a normal menstrual cycle, at ovulation, follicle will grow bigger, ruptures and releases a mature egg.

Ovarian Cycle

What happens during follicular phase?

- One follicle will reach maturity, contains ovum

- Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the release of FSH and LH

- FSH increases the proliferation of granulosa cells -> estrogen secreted

- High level of estrogen stimulates LH surge

Ovarian Cycle

What happens during ovulation?

- High estrogen level and LH surge stimulate ovulation

- A mature egg is released from the ovarian follicles into the oviduct (fallopian tube)

Ovarian Cycle

What happens during luteal phase?

- FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum, which produces progesterone

- Increased progesterone = estrogen produced.

- In return, FSH and LH are suppressed by corpus luteum hormones (estrogen and progesterone) -> corpus luteum declines -> decreased progesterone -> menstruation happens.

What happens if fertilization occurs?

The corpus Luteum remains functional.

What happens if fertilization does not occurs?

The corpus luteum will break down, and blood

Uterine Cycle

What happens in menstruation?

When a female sheds the lining of the uterus when the egg has not been fertilized; usually called her "period"

Uterine Cycle

What happens in proliferative phase?

- Ovarian follicles secrete estrogen -> causes the lining of the uterus to proliferate during this time.

- Formation of a new layer of endometrium in the uterus, aka uterus thickens

Uterine Cycle

What happens secretory phase?

- Corresponds to the luteal phase of the ovarian cycle

- The corpus luteum produces progesterone -> endometrium thickens

Birth control

They trick the body into thinking it's pregnant with estrogen and progesterone, so ovulation doesn't occur.

Progesterone vs Estrogens

Progesterone secreted by corpus luteum in response to LH

Estrogen secreted in response to FSH

What hormones are low during menstruation?

estrogen and progesterone