Osteogensis and the Ovarian cycle

Question 1

what are the 4 stages of gametogenesis

Answer 1

1. The 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

Question 2

what happens in phase 1 of gameotgensis

Answer 2

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

Question 3

where do gametes start of

Answer 3

• they start outside the gonads

Question 4

what happens if phase I of gametogenesis fails

Answer 4

• some of the cells might migrate inappropriately and settle at extragonadal sites
• they may persist at ectopic sites and give rise to teratomas

Question 5

what happens in phase two of gametogenesis

Answer 5

• 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

Question 6

describe what happens to females during phase two of gametogenesis

Answer 6

• 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

Question 7

describe what happens to males during phase two of gametogensis

Answer 7

• Spermatogonia undergo mitosis in early embryonic testes

- Spermatogonia undergo periodic waves of mitosis form puberty onwards throughout life

Question 8

what is phase III of gametogensis

Answer 8

meiosis

Question 9

what does meiosis involve

Answer 9

2 cell divisions and 4 main events

Question 10

what are the 2 cell divisions in meiosis

Answer 10

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.

Question 11

what are the four main events in meiosis

Answer 11

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

Question 12

what are the two stages of meiosis in females

Answer 12

• from foetal period to puberty

- from puberty to menopause

Question 13

what happens to females in the stages between foetal and puberty

Answer 13

• 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

Question 14

what happens to females in the stages between puberty to menopause

Answer 14

• 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

Question 15

what does a polar body have

Answer 15

• Polar body has 23 chromosomes and almost no cytoplasm

Question 16

when does the secondary oocyte begin and arrest

Answer 16

• Secondary oocyte begins meiosis II and arrests at metaphase II

Question 17

when does completion of meiosis II happen and what does it result in

Answer 17

• completion is triggered by fertilisation

- • This results in two unequal daughter cells – a fertilised egg & a second polar body

Question 18

If you are edging towards the age of menopause what are you more likely to get if you get pregnant

Answer 18

• 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

Question 19

as you get older…

Answer 19

the number of total oogonia and follicles in a single ovary decrease

Question 20

what happens in stage 4

Answer 20

Oogensis

Question 21

name the phase 4 of gametogensis

Answer 21

2,000,000 primary oocytes at birth
40,000 survive to puberty
About 400 are ovulated
The remainder degenerate within the ovary

Question 22

describe the process of oogenesis

Answer 22

• 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

Question 23

what are the cells surrounding the primary oocyte called

Answer 23

Cells surrounding the primary oocyte are called granulosa cells

Question 24

what does the antrum contain

Answer 24

• 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

Question 25

How do you have your first period

Answer 25

• Oestrogen levels fluctuate widely throughout puberty because of follicular development that fail to reach ovulation
• The endometrium is affected by the fluctuating oestrogen levels and undergoes cycles of proliferation and regression which eventually leads to the first menstruation.

Question 26

what is GnRH like before you have your first period

Answer 26

GnRH is pulsatile and is just beginning to get levels of FSH and LH increasing

Question 27

How does your first ovulation happen

Answer 27

• The first ovulation does not occur until the oestrogen feedback mechanism is fully developed. This occurs about 6-9 months after menarche
• Throughout puberty progesterone levels remain low and will not rise until after menarche and the first ovulation

Question 28

what is the menarche

Answer 28

beginning of the menstrual cycle

Question 29

what are the three stages of the ovarian cycle

Answer 29

1. Follicular phase 1-10
2. Ovulatory phase 11-14
3. Luteal phase 15-28

Question 30

what stage is longer when your ovarian cycle is longer

Answer 30

• When people have long cycles the follicular phase is long

- But the luteal phase is nearly always 14 days

Question 31

describe the follicular phase

Answer 31

• Each month small number 5-12 of primordial follicles are sitmulated to grow and develop
• Only One of these follicles will form a mature or Graafian follicle which releases the oocyte at ovulation.

Question 32

describe the ovulatory phase

Answer 32

• The mature follicle bulges from the wall of the ovary

- Ovulation occurs when the ovary wall ruptures and expels the oocyte into the peritoneal cavity

Question 33

describe the luteal phase

Answer 33

• Granulosa cells of mature follicle divide and form the corpus luteum
• Corpus luteum secretes hormones that maintain the maternal endometrium
• Allows fertilised oocyte to implant and grow
• LH receptors are important as hCG binds here from the placenta and this makes sure that progesterone continues to be produced

Question 34

how do the hormones work in the follicular phase of ovulation

Answer 34

• GnRH secreted by the hypothalamus stimulates the anterior pituitary to secrete LH & FSH.
• LH & FSH stimulate the mature follicle to grow
• The mature follicle secretes oestrogen.
• Oestrogen inhibits further LH and FSH secretion by anterior pituitary (-ve feedback).
- Oestrogens stimulate growth of maternal uterine endometrium

Question 35

how does the hormones work in the ovulatory phase of ovulation

Answer 35

• Negative feedback is only temporary
• Oestrogen later stimulates the hypothalamic-pituitary axis resulting in a burst of LH and, to a lesser extent, FSH.
• Burst of LH and FSH stimulates completion of meiosis I & onset of meiosis II in the oocyte.
• May also stimulate synthesis of enzymes involved in bulging of the ovarian wall and ovulation

Question 36

how do the hormones work in the luteal phase of ovulation

Answer 36

• After ovulation.
• The granulosa cells of the ruptured follicle proliferate and form the corpus luteum.
• Corpus luteum is highly vascularised and secretes progesterone and oestrogens.
• Progesterone prepares the uterine endometrium for implantation.

Question 37

what happens if fertilisation does occur

Answer 37

• Blastocyst implants into maternal endometrium.
• Developing placenta secretes human chorionic gonadotrophin (hCG).
• Corpus luteum does not degenerate because of hCG.
• Progesterone levels do not fall because progesterone secretion is maintained by the corpus luteum.
• Progesterone maintains the endometrium