Female Reproductive Cycle | Menstrual Cycle

Main Ideas

The lecture explains how ovarian hormones coordinate changes in the uterine lining across the menstrual cycle and how the cycle restarts if fertilization does not occur.

Endometrium and Its Blood Supply

The endometrium is the inner lining of the uterus. It has two layers:

• Stratum functionalis: The layer that thickens and is shed during menstruation.
• Stratum basalis: The deeper layer that remains after menstruation and supports regeneration of the functionalis.

The basalis is supplied by straight arteries, while the functionalis contains spiral (coiled) arteries. The spiral arteries are shed along with the functionalis, contributing to menstrual bleeding.

The Three Phases of the Uterine Cycle

1. Menstrual phase (approximately days 1–5)

   • The stratum functionalis is shed along with blood from its spiral arteries.
   • The stratum basalis remains.

2. Proliferative phase (approximately days 6–14)

   • Estrogen, associated with the follicular phase, promotes rebuilding of the stratum functionalis and growth of its spiral arteries.
   • Estrogen also initiates development of uterine glands, though they are not yet producing secretions.
   • It makes cervical mucus thin, helping sperm travel through the reproductive tract and supporting sperm capacitation.
   • Ovulation is described as occurring around day 14.

3. Secretory phase (approximately days 15–28)

   • Progesterone, produced by the corpus luteum, further thickens the stratum functionalis and supports growth of its spiral arteries.
   • It stimulates the uterine glands to secrete nutrient-rich material containing glycogen, lipids, and proteins.
   • It changes cervical mucus from thin to thick, forming a plug that helps protect the uterus if an embryo is present.

If Fertilization Does Not Occur

Without an implanted embryo producing human chorionic gonadotropin (hCG), the corpus luteum is not maintained and progesterone production falls.

The lecture links the drop in progesterone to abnormal spasms and constriction of the spiral arteries. Their disruption reduces oxygen and nutrient delivery to the functionalis, causing the tissue to become ischemic, die, and shed with blood. This shedding is menstruation and marks the beginning of a new cycle. As progesterone falls, the cervical mucus plug also goes away.

The corpus luteum degenerates into fibrous scar tissue called the corpus albicans.

If Fertilization Occurs

The implanted embryo produces hCG, which helps maintain the corpus luteum and its progesterone production. According to the lecture, the corpus luteum continues this role until the placenta develops, around the 12th week of gestation.

Timing and Cycle Length

The lecture presents 28 days as an average cycle length, with substantial variation between individuals and from month to month. It gives a broad range of about 21–40 days, describing cycles shorter than 21 days or longer than 40 days as potentially pathological.

Speaker and Sources

• Professor Zach Murphy, presenting for Ninja Nerd.
• No other speakers or identifiable sources are featured in the subtitles.