Ovarian Cycle - 1

Summary

This class focuses on the female reproductive system, specifically the ovarian cycle and oogenesis (egg formation). It explains how egg cells develop from primordial germ cells in the fetus and remain arrested in the prophase stage of meiosis I until puberty. With each menstrual cycle, a small number of these eggs mature, but most degenerate. The ovarian cycle involves three main phases: follicular, ovulation, and luteal, with ovulation occurring around day 14. The video also highlights the limited number of eggs females are born with, which decrease over time, making reproductive health crucial for females.

Takeaways

• 😀 Oogenesis is the process of egg (oocyte) formation that starts before puberty and continues into adulthood. It involves several stages including differentiation of primordial germ cells into oocytes.
• 😀 The female reproductive cycle includes two primary cycles: the ovarian cycle and the uterine cycle. The focus of this class is on the ovarian cycle.
• 😀 Oogenesis begins during fetal development when primordial germ cells (stem cells) form around 12-14 million germ cells in the female fetus.
• 😀 Primordial germ cells migrate to the ovaries of the female fetus, where they undergo differentiation into oonia, the precursors to oocytes.
• 😀 The oonia undergo mitosis to proliferate rapidly, forming clusters of cells. These cells are later surrounded by follicular cells, which are somatic ovarian cells.
• 😀 Follicular cells secrete oocyte maturation inhibitor (OMI), a factor that prevents the completion of meiosis, causing oocytes to remain arrested in the prophase of meiosis I.
• 😀 A significant portion of oocytes undergo apoptosis (programmed cell death) during early development, reducing the number of viable oocytes.
• 😀 At birth, a female has about 4 million primordial follicles (arrested oocytes), which decrease in number as the female matures, leaving only around 500,000 viable oocytes at puberty.
• 😀 The ovarian cycle continues after puberty, with only 15-20 oocytes per ovary being recruited for maturation each menstrual cycle, leading to ovulation.
• 😀 A female's egg supply is finite and decreases over time, unlike males who continuously produce sperm throughout their lifetime. This makes female fertility more time-sensitive.
• 😀 By puberty, only about 500,000 primordial follicles remain, and only a small fraction of these will eventually participate in ovulation. Oocytes that survive are those that undergo maturation during the ovarian cycle.

Q & A

What is oogenesis and what does the term mean?

Oogenesis is the process by which female gametes (eggs or ova) are formed. The term comes from 'oo' meaning egg and 'genesis' meaning formation, so it literally means egg formation.

When does oogenesis begin in females?

Oogenesis begins during the embryonic development of a female fetus, well before birth, around the 3rd to 4th week of fetal life.

What are primordial germ cells and what happens to them?

Primordial germ cells are stem-cell-like precursors that appear in the developing female embryo. They migrate to the ovaries and differentiate into oogonia, the early forms of egg cells.

What is the role of follicular cells in oogenesis?

Follicular cells surround the developing oogonia and later oocytes, forming a protective layer. They support the oocytes and secrete a substance called OMI (Oocyte Maturation Inhibitor), which keeps the oocytes in an arrested state during meiosis I.

At what stage of meiosis are primary oocytes arrested?

Primary oocytes are arrested at the prophase I stage of meiosis I, remaining in this state until puberty.

What is a primordial follicle and when is it formed?

A primordial follicle is formed when a primary oocyte is surrounded by a single layer of follicular cells. This structure remains arrested in meiosis I until puberty.

How does the number of germ cells change from fetal development to puberty?

During fetal life, there are about 14 million primordial germ cells, which reduce to about 4 million primary oocytes at birth due to apoptosis. By puberty, only around 500,000 oocytes remain.

Why do females have a limited number of eggs throughout life?

Females are born with a finite number of primordial follicles (about 4 million), and no new oocytes are produced after birth. The number declines over time due to natural degeneration and usage in each menstrual cycle.

How many oocytes typically participate in each ovarian cycle?

In each ovarian cycle, about 15 to 20 oocytes per ovary begin to mature, but usually only one fully matures and is released during ovulation.

What happens during the ovarian cycle leading up to ovulation?

During the ovarian cycle, immature oocytes resume meiosis I and progress to meiosis II, where they are again arrested. Around day 14 of the cycle, ovulation occurs, and the mature ovum is released into the fallopian tube to await fertilization.

How is the female reproductive system different from the male in terms of gamete production?

Unlike males, who continuously produce sperm throughout life, females are born with a fixed number of oocytes that gradually decline, leading to a limited reproductive period.

What causes the reduction in the number of oocytes over time?

The reduction is mainly due to apoptosis, a programmed cell death process that eliminates a large portion of oocytes during fetal development and continues until puberty and adulthood.