Menstrual Cycle

menstural cycle process

Menstruation (Days 1-5)

Shedding of the uterine lining.

-> If there is no fertilization of egg cell (no sperm cell detected) 

- Since there’s no fertilization, the body’s progesterone and estrogen levels decrease, meaning that the womb stops padding out and starts to degenerate.

- The inner lining of the uterus aka the endometrium will shed out and become our period/menstruation and it will become like a blood-like fluid/structure.

Follicular Phase (Days 1-14)

Production and maturation of follicles.

  • This phase starts on the first day of menstruation and ends in ovulation. (There’s overlap between this phase and menstruation)

  • The menstrual cycle is controlled by the hypothalamus and pituitary gland which are the “controllers” or “masterminds” of reproduction.

  • The hypothalamus (part of the brain) releases GnRH, a hormone to be received by the pituitary gland. (Pituitary Gland: Controls functions of testes and ovaries)

  • This (GnRH) signals the pituitary gland to release 2 hormones: LH and FSH and these hormones will travel down through the nerves to reach the ovaries.

-> These pituitary hormones (LH and FSH) control the maturation of the ovarian follicles.

-> FSH stimulates the production of ovarian follicles, each containing an immature egg (oocyte).

  • Over the course of this phase, the follicles grow and compete for a chance at ovulation. Only one egg fully matures.

  • Increase in FSH levels leads to full maturation of oocytes.

  • The follicles pump out estrogen as they mature. As the egg cells grow, the estrogen levels increase and enter the bloodstream, it acts as “negative feedback” to the pituitary gland, inhibiting the production of FSH within the pituitary gland and telling it to pump more LH instead.

    • Increased estrogen stops the production of FSH (negative feedback to the pituitary gland) and increases the production of LH (positive feedback to the pituitary gland) 

Ovulation (Day 14)

Mature egg moves to the uterus, ready for fertilization.

  • A surge in LH (luteinizing hormone) causes the release of the most matured follicle from an ovary into the fallopian tube to the uterus aka ovulation which makes it ready for fertilization.

  • Once in the uterus, the follicle releases its mature egg (ovum). 


-> With this, a pregnancy can only occur if the egg is fertilized by a sperm cell within the next 24 hours. If no sperm cell by then, pregnancy chances close for that month.

Luteal Phase (Day 15-28)

The follicle becomes luteal corpus. 

  • The empty follicle becomes the luteal corpus. 

  • The luteal corpus releases progesterone (and also estrogen), a hormone that tells the womb’s lining to prepare for a fertilized egg that might embed there and grow. (Thickens the uterus)


-> If pregnant and there is fertilization (sperm cell detected)

- Placenta will release hCG to signal the luteal corpus to continue to produce progesterone to maintain the pregnancy.


-> If there is no fertilization (no sperm cell detected), the luteal corpus regresses and dies.: See “Menstruation”. 

- Progesterone and estrogen send another negative feedback to the pituitary gland, stopping it from making more FSH and LH, this causes the luteal corpus to deteriorate and slow production of ovarian hormones.

Menstruation begins immediately after. (Given there is no fertilization).

menstural cycle hormones

OVARIAN HORMONES

PITUITARY HORMONES

  1. Estrogen

  • Helps egg to be mature and prepares mammary gland to secure breastmilk

  • Released during maturation of egg cells 

3) FSH (Follicle Stimulating Hormone)

  • Stimulates the growth of follicles within the ovary in the follicular phase

  1. Progesterone

  • Thickens the uterine lining and is secreted by the luteal corpus

4) LH (Luteinizing Hormone)

  • Responsible to release the mature egg into the uterus through the fallopian tube

  • Induces ovulation