70 Oogenesis and follicular development Flashcards
What makes up the fundamental female reproductive unit?
Single ovarian follicle, composed of one germ cell (oocyte), surrounded by endocrine cells
What is menarche?
- Beginning of menstrual cycles
* Average: 11-13 y
What determines age at puberty?
- Genetics
- Nutrition
- Geographic location
- Exposure to light
- Body composition, fat deposition
- Exercise
What controls the menstrual cycle?
Gonadotrophins and gonadal hormones
What are the 3 phases of the ovarian cycle?
- Follicular phase:
• Av. 15 days (range 9-23 days) - Ovulatory phase:
• 1-3 days (and culminates with ovulation) - Luteal phase:
• 13 days - (is less variable than follicular)
What are the 3 phases in endometrial cycle?
- Menstruation
- Proliferative
- Secretory
What are 4 ovarian functions?
- OOGENESIS - production of gametes during the foetal period
- MATURATION of oocyte
- EXPULSION of the mature oocyte (ovulation)
- SECRETION of female sex steroid hormones (oestrogen + progesterone) and peptide hormone inhibin
Development of oocyte and ovarian follicle?
- Primordial follicle:
• Oocyte
• Granulosa cells - Primary follicle:
• Fully grown oocyte
• Zona pellucida - Prenatal follicle:
• Early theca - Early antral follicle:
• Fluid in antrum
• Theca - Mature follicle: (1.5cm):
• Cumulus oophorous
What is oogenesis?
- Steps/ process a developing egg (oocyte) goes through to differentiate into a mature egg (ovum)
- Involves completion of meiosis which occurs at the time of fertilisation
Progression of oogonia to primordial follicle?
- Oogonia produced by mitotic division (max # ~ 7 million)
- 8-10 wks of gestation, prophase of 1st meiosis stars - becomes primary oocyte
- Surrounded by pre-granulosa cells: called primordial follicle
What is mitosis?
Process of cell division that results in 2 genetically identical daughter cells developing from a single parent cell
What is meiosis?
• Division of a germ cell involving 2 fissions of the nucleus and giving rise to 4 gametes, or sex cells, each possessing 1/2 the no. of chromosome of the OG cell
Describe the 1st phase of oogenesis that occurs during foetal 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 around birth
• 400,000 around puberty - Remaining oocytes are arrested in meiotic prophase until last oocytes are ovulates (up to ~ 50 years)
Rate of cytogenetic abnormalities increases with maternal age. What are 2 common cytogenetic abnormalities?
- Aneuploidy
2. Down Syndrome
Describe the 2nd phase of oogenesis at ovulation?
Meiosis resumes (stimulated by LH):
• 1st division of meiosis is completed and the haploid nuclei separate to form 2 cells
• Cytoplasm is unequally shared forming - a large secondary oocyte and 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
Oocytes summary?
Female germ cells: • 1 st observed in the embryonic yolk sac • Then migrate to developing ovaries • Proliferate • Reach max. numbers before birth
Spermatogenesis vs oogenesis?
- Proliferation:
• Females - mitotic proliferation of oogonia occurs prior birth
• Males - spermatogonia proliferate only after puberty - Meiotic divisions:
• Females - meiotic divisions of oocyte produces only 1 mature ovum
• Males - meiotic divisions of primary spermatocyte produces 4 mature spermatozoa - Second meiotic division:
• Females - 2nd meiotic division only completed upon fertilisation
• Males - products of meiosis (spermatids) undergo substantial differentiation in the maturing process
Female - Oogenesis: • Continuous? • Temp? • Begins? • Stem cells? • Mobile gametes?
- Discontinuous (periods of arrest)
- Normal body temp
- Meiosis begins before birth (initial stimulus is not steroidal)
- Results in finite numbers of oocytes (ovary has no stem cells)
- Results in immobile gametes
Spermatogenesis: • Continuous? • Temp? • Begins? • Stem cells? • Mobile gametes?
- Continuous
- Lower temp required
- Meiosis begins at puberty (indirectly dependent on progesterone)
- Results in infinite numbers of sperm (testis has a stem cell population)
- Results in motile gametes
What are follicles?
Eggs exist in ovaries in structures known as follicles
Follicle development?
- 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 cytoplasmic 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 from amongst granulosa cells from fluid they secrete
Features of small follicles (primordial)
- Most numerous follicles at any time
- Non-growing (stock pile)
- Oogonium nucleus
- Single layer of follicular cells (granulosa cells)
What hormone is secreted by primordial follicles?
Use?
- Secrete anti-Müllerian hormone (AMH)
* Levels reflect the ovarian follicular reserve and therefore can be measure to assess ovarian ageing
Medium follicles (primary)
• At puberty she has ≈ _____ oocytes
• She may experience ___ cycles
• She loses approx. ___ per cycle
- At puberty she has ≈ 300,000 oocytes
- She may experience 450 cycles
- She loses approx. 650 per cycle
What are the stages of medium follicle (primary) development?
- Throughout life, cohorts of small follicles recruited to gain 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 3 layers of follicular cells
Other names for large follicles?
- Secondary
- Antral
- Graafian
- Preovulatory
Stages of development of large follicles?
- FSH stimulates rapid development of medium follicles over 14 days: leads to either ovulation or atresia
• Zona pellucida develops and 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 internal to synthesis androstenedione- precursor for estradiol 17-β synthesis by granulosa cells
- Initial growth is _________ of hormones and takes ___ days
- Final growth is _________ on FSH and takes ______ days
- Initial growth is independent of hormones and takes 85 days
- Final growth is dependent on FSH and takes 10-14 days
Process of corpus luteum formation?
- Mature follicle discharges its antral fluid and egg
• Collapses around antrum and undergoes rapid transformation - Granulosa cells enlarge and form gland-like structure - CORPUS LUTEUM
- Corpus luteum secretes:
• Oestrogen, progesterone, inhibit - If no egg fertilisation, CL developments reaches max. within ~ 10 days:
• Rapidly degenerates by apoptosis
What does corpus luteum secrete?
- Oestrogen
- Progesterone
- Inhibin
What happens in follicular phase (Days 1-14)?
Bleeding starts (Day 1): • Uterine bleeding (Days 1-5/6) • Multiple follicles develop (Days 1-7) • One follicle becomes dominant (Day 7) • Dominant follicle matures (Days 7-14)
What happens in luteal phase (Days 14-28)
Ovulation starts (Day 14): • Corpus luteum functions (Days 14-25) • Corpus luteum degenerates (Days 25-28)
