oogenesis and the ovarian cycle

Question 1

what is gametogenesis?

Answer 1

the process in which cells undergo meiosis to form gametes

Question 2

what are the 4 phases of gametogenesis?

Answer 2

o The extraembryonic origin of germ cells and their migration to gonads
o Increase in germ cell number by mitosis
o Reduction in chromosome number by meiosis
o Structural and functional maturation into eggs and sperm

Question 3

what are primordial germ cells?

Answer 3

oogonia/spermatogonia

Question 4

what happens at 3 weeks of the developing fetus?

Answer 4

• Primordial germ cells (pgc) leave primary ectoderm (epiblast)  yolk sac wall and collect near the exit of the allantois
• They’re now extra-embryonal, lying in the endoderm and mesoderm of the yolk sac wall

Question 5

what happens at 6 weeks of the developing fetus?

Answer 5

• PGC leave yolk sac wall and migrate along vitelline and dorsal mesentery into the gonadal ridge

Question 6

how do teratomas form?

Answer 6

when some of these cells migrate inappropriately and settle at extragonadal sites

they persist at ectopic sites and may give rise to teratomas

Question 7

where do PGCs undergo rapid mitotic division?

Answer 7

at the gonads

Question 8

describe how the oogonia increase in germ cell number?

Answer 8

o Oogonia undergo intensive mitosis in embryonic ovary from 2nd to 5th month of maternal pregnancy.
o After 5 months, large numbers undergo degeneration. This continues until menopause.

Question 9

describe how the spermatagonia increase in germ cell number?

Answer 9

o Spermatogonia undergo mitosis in early embryonic testes.

o However, spermatogonia undergo periodic waves of mitosis from puberty onwards throughout life.

Question 10

describe the cell divisions in meiosis?

Answer 10

o Meiosis I (equatorial division)- independent re-assortment of maternal and paternal chromosomes.
o 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 4 main events during meiosis?

Answer 11

o DNA replication but still 2n and 2c
o Pairing of homologous chromosomes and crossing over occurs during prophase
o First meiotic division – separation of chromosomes. 2 resulting daughter cells have one of a pair of replicated chromosomes
o Second meiotic division – separation of a chromosome into chromatids

Question 12

what is a tetrad?

Answer 12

complex of homologous chromosome pair consisting of 4 chromatids

Question 13

what does crossing over in a tetrad make?

Answer 13

leads to 4 recombinant chromatids

Question 14

what happens to the oogonia between the fetal period and puberty?

Answer 14

o All oogonia enter the first meiotic division in late fetal period and enter the diplotene stage in early months after birth (1ary oocyte).
o They remain at this stage until puberty

Question 15

what happens to oogonia from puberty to menopause?

Answer 15

o A 1ary oocyte will complete meiosis I shortly before ovulation with each menstrual cycle.
o Two unequal daughter cells will result – a 2ary oocyte (large cell) & the first polar body (small cell)
o Secondary oocyte begins meiosis II and arrests at metaphase II
o Completion of meiosis II is triggered by fertilisation

Question 16

how many chromosomes are found in the secondary oocyte and the polar body?

Answer 16

23 chromosomes in each

Question 17

wha triggers completion of meiosis 2 of a secondary oocyte?

Answer 17

fertilisation

Question 18

what phase does a secondary oocyte become arrested in during meiosis II?

Answer 18

metaphase II

Question 19

how many oocytes do we have at birth?

Answer 19

2,000,000

Question 20

how many primary oocytes survive to puberty?

Answer 20

40,000

Question 21

how many primary oocytes are ovulated?

Answer 21

400

Question 22

what is meant by the oogonia being naked in the embryo?

Answer 22

not part of any substructure of the ovaries

Question 23

how and when is a primordial follicle made?

Answer 23

As meiosis begins, primary oocytes are surrounded by other ovarian cells to form primordial follicle

Question 24

how is a primary follicle made?

Answer 24

ovarian cells surrounding the primary oocyte increase in number

Question 25

what are granulosa cells?

Answer 25

cells surrounding the primary oocyte

Question 26

how is the zona pellucida formed?

Answer 26

contributions from the oocyte and granulosa cells

Question 27

what are theca cells?

Answer 27

layer of cells which form around the primary follicle

Question 28

what do early theca cells secrete and what does this do?

Answer 28

secrete angiogenic factors to stimulate growth of blood vessels

Question 29

how long is the development of the follicle self-regulated?

Answer 29

up to primary follicle stage

Question 30

which hormones are needed for follicle development after the primary follicle stage?

Answer 30

FSH from the pituitary gland

Question 31

what does FSH stimulate?

Answer 31

oestrogen production by granulosa cells

Question 32

what hormones are needed for further enlargement of the secondary follicle?

Answer 32

LH | activin

Question 33

what does LH stimulate?

Answer 33

testosterone production by theca cells

Question 34

how does a follicle respond to a LH surge?

Answer 34

- 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

Question 35

what does activin do?

Answer 35

stimulates granulosa cell proliferation

Question 36

what does the antrum contain and why are these important?

Answer 36

serum components, enzymes and proteoglycans o enzymes are important for digestion of follicular wall o proteoglycans have a negative charge which attracts water molecules

Question 37

describe what happens at the start of menstruation

Answer 37

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

Question 38

when does the first ovulation occur?

Answer 38

- doesnt occur until the oestrogen feedback mechanism is fully developed - occurs 6-9 months after menarche

Question 39

describe progesterone levels over time

Answer 39

o Throughout puberty progesterone levels remain low and will not rise until after menarche and the first ovulation

Question 40

what are the 3 stages of the ovarian cycle and when do they occur?

Answer 40

o Follicular phase – days 1 to 10 o Ovulatory phase – days 11 to 14 o Luteal phase – days 15 to 28

Question 41

what occurs in the follicular phase?

Answer 41

* Each month small numbers (5-12) of primordial follicles are stimulated to grow and develop. * One of these follicles will form a mature or Graafian follicle which releases the oocyte at ovulation.

Question 42

what occurs in the ovulatory phase?

Answer 42

* The mature (Graafian) follicle bulges from wall of the ovary. * Ovulation occurs when the ovary wall ruptures and expels the oocyte into the peritoneal cavity.

Question 43

what occurs in the luteal phase?

Answer 43

* Granulosa cells of mature (Graafian) follicle divide and form corpus luteum. * Corpus luteum secretes hormones that maintain the maternal endometrium. * Allows fertilised oocyte to implant and grow.

Question 44

what hormones are involved in the follicular phase and what do they do?

Answer 44

* 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 45

what hormones are involved in the ovulatory phase and what do they do?

Answer 45

* Negative feedback is only transient. * 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 46

what hormones are involved in the luteal phase and what do they do?

Answer 46

* 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 47

what happens if fertilisation does occur?

Answer 47

* 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.