Oogenesis and Follicular Development

Question 1

Label each part of the female reproductive system.

Answer 1

Question 2

What are the components of the fundamental reproductive unit of the female?

Answer 2

A single ovarian follicle, composed of one germ cell (oocyte), surrounded by endocrine cells.

Question 3

What are the functions of the ovary?

Answer 3

• Oogenesis (the production of gametes)
• Maturation of oocyte
• Expulsion of the mature oocyte (ovulation)
• Secretion of female sex steroid hormones (oestrogen and progesterone) and peptide hormone inhibin

Question 4

Describe the first phase of oogenesis.

Answer 4

• Occurs during fetal life
• Oogonia develop in the embryonic yolk sac 3 weeks post conception
• Migrate to ovary
• Colonise the cortex
• Undergo mitosis
• At 8-10 weeks meiosis begins
• Millions of oocytes degenerate before birth.
  
  • 1-2 million at birth
  • 400,000 around puberty
• Remaining oocytes are arrested in meiotic prophase until last oocytes are ovulated (~50 years).

Question 5

Describe the second phase of oogenesis.

Answer 5

• Second phase occurs at ovulation.
• Meiosis resumes, stimulated by LH.
• The first division of meiosis is completed and the haploid nuclei separate to form 2 cells.
• The cytoplasm is unequally shared, forming:
  
  • a large secondary oocyte
  • a polar body (PB has no further role)
• Meiosis arrests again at metaphase II and the secondary oocyte is ovulated.
• 2nd division of meiosis is only completed in those oocytes that are fertilised.
• Completion of meiosis occurs at the time of fertilisation.

Question 6

How is the primary oocyte formed?

Answer 6

• Oogonia are produced by mitotic division (maximum ~7 million).
• Then, at 8-10 weeks gestation, prophase of 1st meiosis begins - becomes primary oocyte.
  
  • Surrounded by pre-granulosa cells
  • Called the primordial follicle

Question 7

Where do eggs reside in the female reproductive system?

Answer 7

• Eggs exist in ovaries in structues known as follicles.

Question 8

Describe the evolution of a follicle.

Answer 8

• Primordial follicle = single layer of granulosa cells around oocyte.
• Oocyte size increases; multiple layers of granulosa cells.
  • • separation of oocyte from granulosa cells by thick

layer of material (zona pellucida).

• BUT, cytoplamic processes cross the zona pellucida and form gap junctions with oocyte, and nutrients and chemical messengers are passed to oocyte.
• Follicle grows by mitosis of granulosa cells and some differentiate to become theca.
• Antrum begins to form amongst granulosa cells from fluid they secrete.

Question 9

Describe the structure of a primordial follicle.

Identify the structures on the slide.

Answer 9

• Most numerous follicles at any time
• Non-growing (stock pile)
• Oogonium nucleus
• Single layer of follicular cells (granulosa cells)
• Secrete Anti-Müllerian hormone (AMH)
  
  • levels reflect the ovarian follicular reserve and therefore can be measured to assess ovarian ageing

Question 10

Describe the structure of a primary follicle.

Answer 10

• At puberty ≈ 300,000 oocytes
• May experience 450 cycles
• Loses approx. 650 per cycle
• Throughout life, cohorts of small follicles recruited to begin a period of slow growth.
• Follicular (granulosa) cells divide, forming 3 layers around the oocyte.
• Growth is independent of hormones.
• Takes 85 days (3 cycles) to reach three layers of follicular cells.

Question 11

Describe the structure of a secondary follicle.

Answer 11

• Also known as antral follicle, Graafian follicle or preovulatory follicle.
• FSH stimulates rapid development of medium follicles over 14 days; leads either to ovulation or to atresia.
• Zona pellucida develops, enclosing the oocyte and masking its antigens.
• Rapid mitotic division in follicular cells forms many layers.
• Antrum develops and fills with fluid.
• LH activates the theca interna to synthesise androstenedione, the precursor for estradiol 17β synthesis by granulosa cells.

Question 12

Describe the menstrual cycle.

Answer 12

Question 13

Describe the relationship between ovarian and uterine changes during the menstrual cycle.

Answer 13

Question 14

Describe how LH and FSH fluctuate in comparison to how oestrogen and progesterone fluctuate throughout the menstrual cycle.

Answer 14

Question 15

What drives the hormonal cycle?

Answer 15

• Anterior pituitary gonadotrophins
  
  • LH
  • FSH
• Gonadal sex hormones
  
  • Oestrogen
  • Progesterone

Question 16

Describe the chemical mechanisms of ovulation.

Answer 16

• LH surge
  
  • Induces prostaglandin endoperoxide synthase in granulosa cells (sets up pseudoinflammatory response).
• FSH (some LH)
  
  • Stimulates release of plasminogen activator from granulosa cells (converts plasminogen to plasmin).
• Prostaglandins E and F
  • release lysosomal enzymes that digest follicular wall - not completely understood.
• “Stigma”
  
  • Forms on surface of follicle, balloons out, forms vesicle and ruptures - oocyte expelled.
• Process facilitated by intrafollicular pressure and contraction of smooth muscle in theca.

Question 17

Describe the formation of the corpus luteum.

Answer 17

• Mature follicle discharges its antral fluid and egg.
  
  • Collapses around the antrum and undergoes rapid transformation.
• Granulosa cells enlarge, and form gland-like structure (corpus luteum).
• Corpus luteum secretes:
  
  • Oestrogen
  • Progesterone
  • Inhibin
• If there is no egg fertilisation, CL development reaches maximum within approximately 10 days.
  
  • Rapidly degenerates by apoptosis.

Question 18

Compare and contrast spermatogenesis vs. oogenesis.

Answer 18

• In females, mitotic proliferation of oogonia occurs prior to birth.
• In males, spermatogonia proliferate only after puberty.
• In females, meiotic divisions of oocyte produces only one mature ovum.
• In males, meiotic divisions of primary spermaatocyte produces 4 mature spermatozoa.
• In females, second meiotic division is completes only upon fertilisation.
• In males, the products of meiosis (spermatids) undergo substantial differentiation in the maturing process.