Oogenesis Flashcards
What is the definition of oogenesis?
Oogenesis is the process of forming and maturing female gametes (oocytes) in the ovaries.
Give an overview of the anatomy of the female reproductive system
Uterus: Supports pregnancy by providing a suitable environment for implantation and fetal development.
Ovaries: Produce oocytes and secrete hormones essential for reproduction.
Ovarian Stroma: Connective tissue containing smooth muscle, stromal cells, and developing follicles.
Uterine (Fallopian) Tubes: Connect ovaries to the uterus, crucial for oocyte transport and fertilization.
What are the functions of the female reproductive system and how does the body enable this?
- Gamete Production: Oocytes develop within follicles.
- Site of Implantation: The uterus prepares for potential pregnancy.
The episodic nature of the menstrual cycle allows this:
- Follicular Phase (First Half): Dominated by oestrogen, leading to oocyte maturation.
- Luteal Phase (Second Half): Progesterone production supports implantation and early pregnancy.
What are some comparisons between Oogenesis and Spermatogenesis?
Gamete Quantity:
Males: ~100 million sperm/day.
Females: ~400 mature oocytes in a lifetime.
Resource Allocation: Each oocyte receives extensive cytoplasmic resources.
Production Pattern:
Sperm production is continuous.
Oogenesis is episodic, occurring only between puberty and menopause.
Longevity:
Males produce sperm throughout life (oldest recorded father: 70s–80s).
Females have a finite supply of oocytes from birth.
What are the general hormone changes in the menstrual cycle?
- Follicular Phase: Rising oestrogen stimulates follicle growth.
- Ovulation: Triggered by a peak in LH.
- Luteal Phase: Increase in progesterone, creating a nutrient-rich uterine lining for implantation.
What are PGCs and Oogonia?
Primordial Germ Cells: Present in fetal development.
Oogonia: Stem cells that expand through mitotic divisions.
What are the stages of Oogenesis?
Pre-birth
- PGCs undergo differentiation, form oogonia which divide through mitotic divisions
- Primary oocytes are formed when these cells enter meiosis I and arrest at prophase I
Post birth
- Upon hormonal stimulation, meiosis resumes.
- The primary oocyte divides into:
- A secondary oocyte (retains cytoplasm).
- A polar body (discards excess genetic material).
- Halving the genetic material symmetrically but the rest of the resources are divided in an asymmetrical way - Ovulation & Fertilization:
The secondary oocyte arrests at metaphase II.
If fertilized, it completes meiosis II, producing another polar body.
What is characteristic of the Primordial follicle?
Small, surrounds oocyte from birth.
What is characteristic of the preantral follicle?
Growth begins from puberty onwards (~400μm in size).
What is characteristic of the antral follicle?
Granulosa cells proliferate and secrete fluid within the layer.
Oocyte is suspended in fluid, linked by cumulus cells (a stalk of granulosa cells)
What happens to the follicle at ovulation?
Oocyte is released, empty follicle transforms into the corpus luteum, a temporary endocrine gland producing progesterone.
What are mural cells?
main body of granulosa cells (not the stalk)
Describe the structure of the follicle
-1. Oocyte: Primary reproductive cell.
- Granulosa Cells:
- Surround oocyte and provide support.
- Function similarly to Sertoli cells in males.
- Secrete oestrogen and glycoproteins to make the zona pelucida which is a protective barrier once the oocyte is released from the follicle, also plays critical roles in fertilisation - Thecal Cells:
- Located outside granulosa layer.
- Produce androgens under LH stimulation, which granulosa cells convert to oestrogens.
What is the importance of the communication between oocyte and granulosa cells and how does this work?
- Granulosa cells are connected to oocyte through cytoplasmic processes
- All of the communication from body needs to come from the granulosa cells as the oocyte has no blood supply
How is follicle development hormonally regulated?
Early Growth:
Influenced by local growth factors.
Without FSH & LH, follicles undergo atresia.
Hormonal Influence:
FSH → Granulosa Cells.
LH → Thecal Cells.
Androgens (from thecal cells) are converted into oestrogens by granulosa cells.
What are the feedback mechanisms in hormone regulation of oogenesis?
Low oestrogen → Negative feedback on HPG axis.
High oestrogen → Positive feedback → LH surge → Ovulation.
Progesterone → Negative feedback, suppressing further ovulation.
What happens in FSH and LH KO mice?
KO of FSH in mice means no preantral
KO of LH mice means no antral
How is it evident that the way cells express specific hormone receptors is important?
During follicular phase, only cells in theca interna bind LH; only granulosa cells bind FSH
What is the positive feedback loop with steroid hormone production?
Producing steroid hormones in reponse to stimulation means the hormones are acting on the somatic cells making it grow more, more hormone production , positive feedback loop etc.
What do Thecal and Granulosa cells produce in response to FSH/LH stimulation?
- Thecal cells produce the androgens androstenedione and testosterone as a result of LH stimulation
- Granulosa cells convert the androgens from thecal cells to oestrogens and oestradiol and oestrone upon FSH stimulation
What is the difference between the follicular stage of the menstrual cycle and the second half?
- Follicular - focus on androgen production in thecal cells and oestrogen production in granulosa cells
- Second half - under stimulation of LH the granulosa cells make large amount of progesterone
What happens for the LH surge to trigger ovulation?
LH Surge Effects:
- Oocyte resumes meiosis I, progressing to metaphase II.
- Cytoplasmic maturation occurs (protein synthesis, microtubule reorganization).
- Follicle ruptures, releasing oocyte into the fallopian tube.
Without an LH surge, the follicle degenerates.
What happens during the luteal phase and with regard to the corpus luteum?
Function:
- Secretes progesterone, preventing new follicle development.
- Basis for many contraceptives (mimicking luteal phase suppression of FSH/LH).
Fate:
- If pregnancy occurs → Maintained by hCG.
- If no pregnancy → Luteolysis (breakdown after 12–14 days) → Menstruation.
Why do FSH and LH stay low in the luteal phase?
We don’t want the development of further follicles when the body is trying to prepare for pregnancy
How is the HPG axis regulated by the steroid hormones?
- Oestrogen Actions:
- Low levels → Negative feedback.
- High levels → Positive feedback (LH surge). - Progesterone:
- Enhances oestrogen’s negative feedback.
Prevents further ovulation after implantation.
- Produced from corpus luteum - Inhibin & Activin:
- Activin → Increases FSH.
- Inhibin → Suppresses FSH secretion.
