Oogenesis - the ovarian cycle (Part 2)

Summary

This lesson delves into the ovarian cycle, detailing the maturation stages of a follicle from primordial to corpus luteum. It simplifies the process by focusing on the cycle's physical changes, avoiding hormonal regulation for now. The journey from early primary follicles to the selection of a dominant follicle and ovulation at day 14 is outlined. Post-ovulation, the follicle transforms into the corpus luteum, crucial for embryo support if fertilization occurs, or degenerates into corpus albicans if it doesn't, concluding the 28-day cycle.

Takeaways

• 🔬 The ovarian cycle begins with the maturation of primordial follicles, which start as 15 to 20 and only one typically reaches ovulation.
• 🌀 The follicles that do not ovulate undergo a process called atresia, turning into atretic follicles.
• 🌱 Surviving primordial follicles become early primary follicles, where the follicular cells change from squamous to cuboidal cells.
• 🌿 As primary follicles grow, they form granulosa cells and secrete glycoproteins creating the zona pellucida, crucial for fertilization.
• 🍽️ Granulosa cells extend finger-like processes to feed the oocyte, overcoming the barrier posed by the zona pellucida.
• 🔴 Stromal cells organize around the follicle to form the theca follicle layer, which is essential for nutrient supply and hormone secretion.
• 💧 The granulosa cells proliferate and secrete fluids into the antrum, defining the secondary follicle stage.
• 🥚 The cumulus oophorus is formed as the granulosa cells mound over the oocyte within the antrum.
• 🌱 Theca interna differentiates and is highly vascularized to supply nutrients and plays a role in hormone secretion.
• 🥚 The secondary oocyte completes meiosis one and enters meiosis two but is arrested at metaphase two.
• 🌟 The dominant follicle, or Graafian follicle, is the one from which the ovum will originate, pushing against the ovary's surface before ovulation.
• 💥 At ovulation, the follicle ruptures, and the oocyte, with cumulus oophorus and corona radiata, is ejected from the ovary.
• 🌀 Post-ovulation, the ruptured follicle's cells transform into lutein cells forming the corpus luteum, which is essential for embryo maintenance.
• 🌱 The fate of the corpus luteum depends on fertilization; if it occurs, it persists, otherwise, it degenerates into corpus albicans.

Q & A

What is the first stage of follicular development in the ovarian cycle?

The first stage of follicular development in the ovarian cycle begins with 15 to 20 primordial follicles starting maturation, but only one oocyte typically reaches ovulation.

What happens to follicles that do not reach ovulation?

Follicles that do not reach ovulation degenerate through a process called atresia, turning them into atretic follicles.

How do the follicular cells transform during the early primary follicle stage?

During the early primary follicle stage, the follicular cells change from squamous cells to cuboidal cells.

What is the role of the zona pellucida?

The zona pellucida is a glycoprotein layer formed between the granulosa cells and the oocyte. It plays an important role in fertilization and separates the oocyte from the surrounding cells.

How do granulosa cells nourish the oocyte despite the zona pellucida barrier?

Granulosa cells send finger-like processes through the zona pellucida to feed the oocyte, allowing nutrient transfer.

What are the roles of the theca interna and theca externa?

The theca interna is highly vascularized and plays a key role in hormone secretion, while the theca externa has the ability to contract, which helps during ovulation.

What is the difference between a primary oocyte and a secondary oocyte?

A primary oocyte is arrested in meiosis I, but after completing meiosis I and starting meiosis II, it becomes a secondary oocyte, which is arrested again at metaphase II until fertilization.

What is the purpose of a polar body formed during oocyte division?

The polar body receives minimal cytoplasm during oocyte division and serves no function; it eventually degenerates and disappears.

What is a Graafian follicle, and when does it form?

A Graafian follicle forms after the oocyte completes meiosis I and before ovulation. It is the dominant follicle that contains the secondary oocyte.

What happens to the corpus luteum if fertilization does not occur?

If fertilization does not occur, the corpus luteum degenerates and becomes the corpus albicans, which serves no function and eventually disappears.