Ovarian Function Flashcards
The Oocyte contains…
- Haploid polar body (discarded partner of the nucleus post meiosis)
- Cytoplasm - contains food reserves for developing zygote
- Chromosomes - ready for 2nd meiotic division post-fertilisation
- Cell surface membrane
- Zona pellucida - protective jelly-like layer
Oocyte journey in the female tract
1) Ovary - develops follicles as it matures
2) Ampulla - where oocyte is released during ovulation and can be fertilized by sperm
- If fertilised will travel down, from zygote to blastocyst
3) Uterus - where zygote implants into uterine wall
Fertilisation + the Zona Pellucida
- Many sperm try reaching the egg but only one will penetrate through ZP to get to the egg
- Membranes depolarise to not allow any more sperm in
- All but the nucleus of sperm is digested
Requirements for fertility
- Production of normal sperm and egg cells
- Time constraint - sperm + egg must both be in ampulla
- Sperm must be able to penetrate + fertilise an egg - due to sperm quality
- Normal pregnancy
All under hormonal control
Oogenesis - Similarities and Differences with Spermatogensis
Steps of Gametogenesis are the same in both sexes
Timing + Outcome differ…
- O starts in the embryo
- O is not continuous (2 meiotic stops)
- Each diploid cell develops only 1 haploid cell instead of 4
- O continues after puberty + stops at menopause
How oogenesis starts pre-birth
Primordial germ cell migrates to genital ridge during early development to form diploid cells - Oogonium
During embryogenesis, becomes primary oocyte arrested at Prophase I of Meiosis I
Oogenesis during + post- puberty
- Ovulation begins in puberty where meiosis I resumes forming a secondary oocyte arrested at Metaphase II
- First polar body formed
The egg cell post-fertilisation
- Meiosis II resumes forming ovum haploid cell + second polar body
- Cell contains female + male pronuclei after fertilised by sperm
- Pronuclear fusion
- Zygote formed
Adult Oogenesis In the Ovary - Folliculogenesis
Process of developing follicles that support + contain the oocytes
- Primordial follicles
- Primary follicles containing oocytes
- Secondary follicles surrounded by cells that grow and fluid is introduced into central follicle
- Fluid builds up and ovulation occurs when the ovum pops out
- Remains of follicle - corpus - matures and degenerates
Oocyte is accompanied by somatic cells - Granulosa cells
- Nurture germ cells
- Surrounds the oocyte
- Produces hormones: oestradiol from androgens indirectly by making aromatase + progesterone post-ovulation
Oocyte is accompanied by somatic cells - Theca cells
- Produce hormones: androgen + progesterone post-ovulation
- Surrounds granulosa cells
- Compose the basement membrane of follicle
What is the follicle?
The unit of the oocyte, granulosa cells and theca cells
- Oocytes develop follicles and can’t exist without each other
Gonadotropin dependent cyclic recruitment
Stimulated by hormones - driven by gonadotropin
- FSH develops follicles to antral ones
- The numbers whittle down by one a month - atresia
Female Germ Cell Numbers - Embryo/Foetus
- Primordial germ cells = 170
- By mid-pregnancy = 7 000 000
- There are a limited number of mitotic divisions
Female Germ Cell Numbers - Birth
Born with ~ 2 000 000
Female Germ Cell Numbers - Puberty
400 000
Female Germ Cell Numbers - Menopause
Occurs when low ovarian reserve
Only 400 to 500 will be ovulated
Most oocytes undergo atresia
Primordial follicle
- Primary oocyte surrounded by a single layer of flattened granulosa
Primary (Preantral) follicle
Primary oocyte
- Granulosa becomes cuboidal - thickens
- Theca cell layer forms and zona pellucida
Secondary (Antral/Graafian) follicle
- Primary oocyte
- Granulosa proliferste + becomes 3-6 layers deep
- Theca cells forms two distinct layers - interna + externa
- 15-20 follicles recruited a month
Tertiary (Pre-ovulatory) Follicle
- Secondary oocyte
- Granulosa secrete follicular fluid
- Ooocyte surrounded by corona radiata + on stalk of cumulus oophorus
- Only 1 dominant follicle per cycle
Hormonal Control of Oogenesis (Folliculogenesis) - Puberty
- Re-initiation of oogenesis
- Initiation folliculogenesis
- Caused by initiation of pulsatile release of gonadotropin-releasing hormone (GnRH) from hypothalamus
Female Hypothalamic Pituitary Gonadal Axis
- Pituitary gland synthesises and releases hormones to the ovaries
- The ovaries make hormones that could have +/- influence on the pituitary
Hypothalamus hormonal action
Pulsative release of GnRH (gonadotropin-releasing hormone) which acts on the anterior pituitary
Anterior Pituitary Gonadotrophins - FSH
Follicle-stimulating hormone
- Acts on the ovary (granulosa cells)
- Stimulates recruitment and development of follicles (15-20 per cycle)
Anterior Pituitary Gonadotrophins - LH
Luteinizing hormone
- Acts on ovary (theca cells)
- Stimulates follicle maturation, ovulation and development of the corpus luteum
Anterior Pituitary Gonadotrophins - FSH + LH
Stimulate secretion of oestradiol and ovulation
Oestradiol Production - ‘Two Cell Hypothesis’
LH stimulates theca cells to produce androgens
FSH stimulates the granulosa cells (aromatase) to convert the androgen to oestradiol
Ovarian Hormones - Oestradiol
- Growth of body and sex organs at puberty
- Development of secondary sexual characteristics
- Reproduction
- Follicle maturation
- Preparation of the endometrium
for pregnancy - Thinning of cervical mucus
