Osteogensis and the Ovarian cycle Flashcards
(37 cards)
what are the 4 stages of gametogenesis
- The extraembryonic origin of germ cells and their migration to gonads
- Increase in germ cell number by mitosis
- Reduction in chromosome number by meiosis
- Structural and functional maturation into eggs and sperm
what happens in phase 1 of gameotgensis
In week 3
- the primordial germ cells leave the primary ectoderm(epiblast) and move to the yolk sac wall and collect near the exit of the allantois
- They are now extra-embryonal lying in the endoderm and mesoderm of the yolk sac wall
By week 6
- The Primordial Germ Cell leave the yolk sac wall and migrate along the vitelline and the dorsal mesentery into the gonadal ridge.
By week 7
- Testorone is around and it becomes male this is due to TDH testorerone determinating factor, it is coded for by the Y chromosome
- So if you don’t have a Y chromosome you don’t get testosterone factor or testosterone is it becomes a female gonad
where do gametes start of
- they start outside the gonads
what happens if phase I of gametogenesis fails
- some of the cells might migrate inappropriately and settle at extragonadal sites
- they may persist at ectopic sites and give rise to teratomas
what happens in phase two of gametogenesis
- Once at gonads, primordial germ cells undergo rapid mitotic division
- Each germ cell produces two diploid cells (2n)
- primordial germ cells number multiplies exponentially from hundreds to many millions
- then the Male and female proliferation pattern is very different
describe what happens to females during phase two of gametogenesis
- Oogonia undergo intensive mitosis in embryonic ovary from 2nd to 5th month of maternal pregnancy.
- After 5 months, large numbers undergo degeneration. This continues until menopause.
- There are now lots and lots of cells that have a full complement of chromosomes so meiosis now happens as you have to halve the number of chromosomes
describe what happens to males during phase two of gametogensis
- Spermatogonia undergo mitosis in early embryonic testes
- Spermatogonia undergo periodic waves of mitosis form puberty onwards throughout life
what is phase III of gametogensis
meiosis
what does meiosis involve
2 cell divisions and 4 main events
what are the 2 cell divisions in meiosis
1) Meiosis I (equatorial division)- independent re-assortment of maternal and paternal chromosomes.
2) Meiosis II (reduction division) – diploid (2n) to haploid (1n). The diploid number (2n) is restored on fusion of two gametes.
what are the four main events in meiosis
1) DNA replication but still 2n and 2c.
2) Pairing of homologous chromosomes and crossing over occurs during prophase. The complex of the homologous chromosome pair consisting of four chromatids is a tetrad. The result of crossing over is that there are now four recombinant chromatids. Cells now 2n and 4c.
3) Separation of chromosomes as a result of the first meiotic division. The two resulting daughter cells each have one of a pair of replicated chromosomes. Cells now 1n and 2c.
4) Separation of chromatids occurs as a result of the second meiotic division. The resulting
what are the two stages of meiosis in females
- from foetal period to puberty
- from puberty to menopause
what happens to females in the stages between foetal and puberty
- All oogonia enter the first meiotic division in late foetal period and enter the diplotene stage in the early months after birth are the primary oocyte
- They remain at this stage until puberty unless degermation occurs
- It arrests in the first stage of mitotic division until after puberty – the biochemical trigger for the first mitotic division to complete is (trigger of ovulation) LH surge, this ends the first mitotic division as well
what happens to females in the stages between puberty to menopause
- A secondary oocyte will complete in meiosis I shortly before ovulation with each menstrual cycle
- Two unequal daughter cells will result a secondary oocyte and the first polar body (small cell)
- In 2nd meiosis it arrests – completion of 2nd meiosis is completed by fertilisation
what does a polar body have
- Polar body has 23 chromosomes and almost no cytoplasm
when does the secondary oocyte begin and arrest
• Secondary oocyte begins meiosis II and arrests at metaphase II
when does completion of meiosis II happen and what does it result in
- completion is triggered by fertilisation
- • This results in two unequal daughter cells – a fertilised egg & a second polar body
If you are edging towards the age of menopause what are you more likely to get if you get pregnant
- non disjunction in the baby
– proteins don’t come away and you end up with an oocyte with too many chromosomes in, this is a reason why you get an increase in aneuploidy and down syndrome
as you get older…
the number of total oogonia and follicles in a single ovary decrease
what happens in stage 4
Oogensis
name the phase 4 of gametogensis
2,000,000 primary oocytes at birth
40,000 survive to puberty
About 400 are ovulated
The remainder degenerate within the ovary
describe the process of oogenesis
- In the embryo oogonia are naked – they are not part of any substructure of the ovaries
- As meiosis begins, primary oocytes are surrounded by other ovarian cells to form primordial follicle
- These cells increase in number to form a primary follicle
- Contirbutions form the oocyte and grnaulsoa cells help form the zona pellucida
- Follicle matures another layer of cells, called theca cells, form around the follicle
- At this point follicular development is self-regulated it needs FSH to develop
- Early theca cells secrete angiogenic factors to stimulate growth of blood vessels
- FSH will sitmulate oestogen production from the granulosa cells
- A cavity called the antrum is formed
- The follicle is now a secondary follicle
- Further follicle enlargement and maturation requires activin
- Activin (granulosa cells) stimulates granulosa cell proliferation
- Further follicle enlargement and maturation also requires luteinising hormone (LH) (pituitary)
- LH stimulates testosterone production by theca cells
- Testosterone enters granulosa cells where it is converted by aromatase into oestrogen (mainly 17β oestradiol)
- Oestradiol stimulates formation of LH receptors on granulosa cells
- This will enable follicle to respond to LH surge
- The enlarged follicle pushes against the ovary surface
- At this point meiosis resumes.
- Oestrogen from the follicle now prepares the reproductive tract for possible fertilisation
- Antrum contains serum components, enzymes and proteoglycans
- Follicle is now ready for ovulation
- This requires LH surge from pituitary, caused by high oestrogen levels, high oestrogen levels trigger a switch and this becomes a postivie feedback loop and increases LH this goes the other way from the negative feedback where high oestrogen levels decrease the amount of LH
- Expulsion of oocyte due to intra-follicular pressure (proteoglycans) and theca cell contraction
- The oocyte is swept into oviduct
- Peristaltic action carries oocyte to uterus
what are the cells surrounding the primary oocyte called
Cells surrounding the primary oocyte are called granulosa cells
what does the antrum contain
- enzymes are important for digestion of follicular wall
- proteoglycans have a negative charge which attracts water molecules this increases the volume of the antrum
- Thus, volume of antral fluid increases further
