Oogenesis | Embryology

Summary

This video explores the process of oogenesis, detailing the development of female gametes from primordial germ cells to mature ova. It covers the stages of migration, mitosis, and meiosis within the embryo, as well as the differentiation of the gonads into ovaries. The video emphasizes the role of hormones such as follicular stimulating hormone (FSH) and luteinizing hormone (LH) in the maturation of oocytes from birth to puberty, and concludes with the final stages of meiosis during fertilization, ultimately leading to the formation of a zygote.

Takeaways

• 😀 The process of oogenesis begins with primordial germ cells that originate from the epiblast in the bilaminar disk.
• 😀 Primordial germ cells migrate to the yolk sac around week 4, where they start accumulating and eventually become gametes.
• 😀 By the sixth week, primordial germ cells reach the genital ridges, where they differentiate into gonads, forming ovaries for females.
• 😀 Between the fifth month and birth, the germ cells undergo mitosis to form approximately 7 million oocytes, which are then surrounded by epithelial cells.
• 😀 At birth, about 400,000 primary oocytes are present in the ovaries, and these cells are paused in prophase I of meiosis, regulated by oocyte maturation inhibitor (OMI).
• 😀 Between birth and puberty, the follicular cells surrounding the oocytes transform from flat to cuboidal cells and start to produce a glycoprotein membrane called the zona pellucida.
• 😀 By puberty, the number of oocytes drops to about 40,000 primary follicles, and they are still in the primary oocyte phase, surrounded by cuboidal follicular cells.
• 😀 The hormone follicular stimulating hormone (FSH) triggers the transformation of primary follicles into vesicular (antral) follicles, where a fluid-filled sac forms.
• 😀 Only one of the 15-20 primary follicles that undergo this transformation survives and matures due to hormonal feedback that reduces FSH levels.
• 😀 The secondary oocyte formed from the primary oocyte is paused in metaphase II at the time of ovulation, and fertilization is required to complete meiosis and produce a mature ovum.

Q & A

What is the starting point of oogenesis in the embryo?

Oogenesis begins with the primordial germ cells, which originate from the epiblast of the bilaminar disk during early embryonic development.

Where do primordial germ cells migrate to after leaving the yolk sac?

Primordial germ cells migrate to the genital ridges, which will eventually develop into the gonads (ovaries or testes).

What is the role of mitosis in the early stages of oogenesis?

Mitosis is essential for the proliferation of primordial germ cells as they migrate to the genital ridges, increasing their number as they develop into oogonia.

At what stage do germ cells enter meiosis, and what happens during this phase?

Between the fifth month of development and birth, the oogonia enter meiosis, specifically meiosis I, and become primary oocytes. These cells pause at prophase I of meiosis until puberty.

How many primary oocytes are present at birth, and how do they change over time?

At birth, there are approximately 400,000 primary oocytes, which are arrested in prophase I. By puberty, the number decreases to around 40,000.

What happens to the follicular cells surrounding primary oocytes between birth and puberty?

Between birth and puberty, the follicular cells surrounding the primary oocytes change from flat to cuboidal in shape, and they start to produce a glycoprotein membrane called the zona pellucida.

What is the role of follicular stimulating hormone (FSH) in oogenesis?

FSH triggers the transformation of primary follicles into vesicular (antral) follicles. It also stimulates the growth and maturation of these follicles, leading to the development of a secondary oocyte.

What is the significance of the polar body during oogenesis?

The polar body is a small, non-functional cell produced during the division of the primary oocyte into a secondary oocyte. It contains little cytoplasm and degenerates in most cases.

At what point does the secondary oocyte complete meiosis, and what triggers this process?

The secondary oocyte completes meiosis II only after fertilization occurs, which is triggered by the sperm cell's entry into the egg.

What happens to the secondary oocyte if fertilization does not occur?

If fertilization does not occur, the secondary oocyte will not complete meiosis II and will eventually degenerate. The egg is expelled during menstruation.