Khan Academy - The Ovarian Cycle

Summary

The ovarian cycle is a vital process in the female reproductive system, involving the maturation of eggs in the ovaries, hormone production, and ovulation. The cycle spans 28 days, starting with the development of primordial follicles, which mature into dominant follicles. Ovulation occurs on day 14, where the mature egg is released. The remaining follicle transforms into the corpus luteum, which produces hormones to support pregnancy if fertilization occurs. If not, the corpus luteum degenerates, leading to menstruation. The cycle concludes with menopause, marking the end of ovulation and fertility.

Takeaways

• 😀 The ovarian cycle lasts about 28 days, controlling the maturation of eggs and hormone production, and it is directly tied to the menstrual cycle.
• 😀 The ovaries produce eggs that undergo a maturation process in structures called follicles, which begins with primordial follicles and progresses to secondary oocytes.
• 😀 As follicles develop, granulosa cells increase in number and produce hormones like estrogen, progesterone, and inhibin, which influence the menstrual cycle.
• 😀 Primordial follicles develop into primary follicles, where a new layer called the Zona pellucida forms around the egg, which is nourished by granulosa cells via gap junctions.
• 😀 Pre-antral follicles are characterized by the proliferation of granulosa cells and the formation of the theca layer, which produces androgens converted to estrogen.
• 😀 The antrum, a fluid-filled space inside the follicle, forms as granulosa cells produce fluid, causing the follicle to expand and push against the walls of the ovary.
• 😀 Multiple follicles develop simultaneously, but only the largest (dominant) follicle reaches full maturity and is ovulated, with others undergoing atresia (degeneration).
• 😀 Ovulation occurs on day 14 when the dominant follicle bursts, releasing the egg, which is picked up by the fimbriae and transported to the uterine tube for fertilization.
• 😀 After ovulation, the ruptured follicle transforms into the corpus luteum, which secretes estrogen, progesterone, and inhibin to support the potential pregnancy.
• 😀 If fertilization does not occur, the corpus luteum degenerates through apoptosis around day 25, leading to a drop in hormone levels and the start of a new cycle.
• 😀 Menopause occurs around age 50-51 when the ovaries stop responding to gonadotropins (LH and FSH), leading to the cessation of ovulation and menstruation after many eggs have undergone atresia.

Q & A

What is the ovarian cycle and what role does it play in the reproductive system?

The ovarian cycle is the process by which eggs mature in the ovaries and are released for potential fertilization. It is a key component of the menstrual cycle and is responsible for the development of a secondary oocyte that can be fertilized, leading to pregnancy.

How does the ovarian cycle relate to the menstrual cycle?

The ovarian cycle drives the changes in the ovaries, such as follicle development and ovulation, which are connected to the menstrual cycle. These changes include hormone production and egg maturation, which are essential for menstruation and potential pregnancy.

What are primordial follicles and what is their significance in the ovarian cycle?

Primordial follicles are the initial stage of follicle development in the ovaries. Each contains an immature egg surrounded by granulosa cells. These follicles begin the process of maturation and can potentially develop into eggs capable of ovulation.

What is the role of granulosa cells in follicle development?

Granulosa cells surround the egg in a follicle and help nourish the egg through Gap Junctions. As the follicle matures, these cells also secrete hormones such as estrogen, progesterone, and inhibin, which influence the progression of the ovarian cycle.

What happens during the transition from primordial to primary follicles?

As primordial follicles develop into primary follicles, a new layer called the Zona pellucida forms between the egg and granulosa cells. This separation allows for continued nourishment through Gap Junctions, while the egg remains in meiotic arrest.

What is the significance of the Zona pellucida in follicle maturation?

The Zona pellucida is a layer that forms around the egg, separating it from the surrounding granulosa cells. Despite this separation, nutrients and signals are exchanged through Gap Junctions, allowing the egg to remain nourished and maintained in a state of meiotic arrest.

How do Theca cells contribute to the production of estrogen?

Theca cells, which form around the follicle, have receptors for luteinizing hormone (LH). When LH binds to these cells, they produce androgens, which are then converted into estrogen by the granulosa cells, contributing to rising estrogen levels in the blood.

What is the role of the antrum in follicle development?

The antrum is a fluid-filled space within the follicle that forms as the granulosa cells secrete fluid. The increasing size of the antrum causes the follicle to expand, and this pressure helps push the egg toward ovulation.

What is atresia, and how does it affect egg development?

Atresia is the process by which follicles and the eggs they contain degenerate and die off. This occurs to most of the developing follicles during each ovarian cycle, with only one dominant follicle being ovulated.

What happens to the follicle after ovulation?

After ovulation, the remaining follicle transforms into the corpus luteum. The granulosa cells in the corpus luteum enlarge and produce estrogen, progesterone, and inhibin, which are essential for preparing the uterus for potential implantation of a fertilized egg.