5. Oogenesis and the Ovarian Cycle Flashcards
What 4 phases can gametogenesis be divided into? (4 M’s)
Describe week 3 of phase 1.
1) Extraembryonic origin of germ cells and their migration to gonads
2) Increase in germ cell number by mitosis
3) Reduction in chromosome number by meiosis
4) Structural and functional maturation into eggs and sperm
Primordial germ cells leave 10 ectoderm (epiblast) and move to yolk sac wall and collect near exit of allantois. They are now extra-embryonal (lie in endoderm and mesoderm of yolk sac)
Describe week 6 of phase 1.
If the cells in this phase persist at ectopic sites, what might this give rise to?
Describe phase 2.
Primordial germ cells leave yolk sac wall and migrate along the vitelline and dorsal mesentery into gonadal ridge. [pic]
Teratomas.
Once at gonads, primordial germ cells undergo rapid mitotic division, each germ cell produces 2 diploid cells. Number multiplies to millions. From here males and females differ: oogonia undergo intensive mitosis in embyonic ovary from 2-5th month of pregnancy, and lots degenerate after 5 months and continue to menopause.
Describe phase 3.
What are the 4 main events occuring in phase 3?
Meiosis I: reduction division, 2n to 1n. Meiosis II: equatorial division, independant reassortment of maternal and paternal chromosomes
1) DNA replication but still 2n and 2c
2) Pairing of homologous chromosomes and crossing over during prophase producing 4 recombinant chromatids. 2n and 4c
3) Seperation of chromosomes as a result of 1st meiotic division. The 2 daughter cells each have 1 pair of replicated chromosomes. 1n and 2c
4) Separation of chromatids occurs as a result of the second meiotic division. The resulting 4 daughter cells each contain a haploid set of chromosomes (1n) and 1 chromatid (1c)
When do oogonia enter the first meiotic division (prophase)?
When does a primary oocyte complete meiosis 1, and what does it produce?
When does a secondary oocyte begin meiosis II?
From roughly 2,000,000 oocytes at birth, how many are ovulated, and what happens to the rest?
Late foetal period (and enter diplotene in early months after birth - 10 oocyte). They remain at this stage until puberty (when degeneration occurs)
Shortly before ovulation with each menstrual cycle (triggered by LH), producing 2 unequal daughter cells: a secondary oocyte and the first polar body.
After meiosis I, just before ovulation, but arrests at metaphase II and completion is triggered by fertilisation, producing a fertilised egg and a second polar body.
About 400. Rest degenerate in ovary
In the embryo oogonia are naked. What happens to them as meiosis begins?
Contributions from the oocyte and granulosa cells help form what layer?
As the follicle matures, what other layer of cells form around it?
What does this second layer of cells secrete?
They are surrounded by other ovarian cells (granulosa cells) to form a primordial follicle. The cells increase in number to form a primary follicle.
Zona pellucida
Theca cells (prod testosterone -> granulosa cells -> converted to oestrogen by aromatase)
Angiogenic factors to stimulate growth of BVs
[NB pic: cumulus oophorous = granulosa cells]
What stages of development are self-regulated?
What happens after the primary follicle is formed?
Further follicle enlargement and maturation requires LH - what does this do?
Up to primary follicle. Further development requires FSH which stimulates oestrogen production by granulosa cells.
A cavity (antrum) forms and the follicle is now the secondary follicle. Further follicle enlargement and maturation requires activin (granulosa cells) stimulates granulosa cell proliferation.
Stimulates testosterone production from theca cells, enters granulosa cells and converted by aromatase to oestrogen (mainly oestradiol) which stimulates formation of LH receptors on granulosa cells to enable follicle to respond to LH surge. (Where high oestrogen levels suddenly become +ve fb)
The enlarged tertiary follicle/graafian follicle pushes against ovary surface (day 14). What happens next?
What triggers ovulation?
What 2 things cause expulsion of oocyte?
What happens to the oocyte?
Meiosis resumes. Oestrogen from follicle prepares reproductive tract for possible fertilisation. Antrum contains serum components, enzymes (digest follicular wall) and proteoglycans (-ve charge so attract H2O). Volume of antral fluid increases.
LH surge caused by high oestrogen levels.
Intra-follicular pressure (proteoglycans) and theca cell contraction.
Swept into oviduct, peristaltic action carries it to uterus.
What follicles contain primary oocytes?
What contains secondary oocytes?
What is the post-ovulatory follicle called?
Why do oestrogen levels fluctuate widely throughout puberty? How does this affect the endometrium?
When does the first ovulation occur?
What are progesterone levels like throughout puberty?
Primordial, primary, pre-antral, secondray, antral
Tertiary follicle, graafian follicle, ovulated ovum
Corpus luteum
Follicular development that fail to reach ovulation. Endometrium undergoes cycles of proliferation and regression which eventually leads to first mensturation.
When oestrogen feedback mechanism is fully developed - about 5-9m after menarche.
Low - only rise after menarche and 1st ovulation (b/c need corpus luteum)
What are the 3 stages of the ovarian cycle, and how long do they take?
Briefly describe each phase.
Follicular (day 1 - 10), ovulatory (day 11 - 14) and luteal (day 15 - 28)
1) Follicular: each month small numbers of primordial follicules develop. One forms a mature/Graafian follicle which releases the oocyte at ovulation.
2) Ovulatory: mature Graafian follicle bulges from ovary wall, ovulation occurs when the wall ruptures and expels oocyte to peritoneal cavity
3) Luteal: Granulosa cells of mature Graafian follicle divide and form corpus luteum which secretes progesterone to maintain maternal endometrium and allow fertilised oocyte to implant and grow
What hormones does the hormonal control of the ovarian cycle involve?
Briefly describe the involvement of hormones in each of the ovarian cycle:
a) Follicular
b) Ovulatory
c) Luteal
GnRH, FSH, LH, oestrogen, progesterone
a) GnRH stimulates LH and FSH production. LH and FSH stimulate mature follicle to grow. Mature follicle secretes oestrogen. Oestrogen inhibits further LH and FSH secretion (-ve fb) and stimulates growth of maternal uterine endometrium.
b) -ve fb only transient. Oestrogen later stimulates the HP axis causing a burst of LH (and FSH to lesser extent), which stimulates completion of meiosis I and onset of meiosis II in oocyte. May also stimulate synthesis of enzymes involved in bulging ovarian wall and ovulation.
c) after ovulation, granulosa cells of ruptured follicle proliferate and form corpus luteum, which is v. vascularised and secretes progesterone and oestrogens. Progesterone prepares uterine endometrium for implantation
What happens if fertilisation occurs?
Blastocyst implants into maternal endometrium. Developing placenta secretes hCG which means the corpus luteum doesn’t degenerate. Progesterone levels maintained by corpus luteum, which maintains endometrium.
