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human development > oogenesis and the ovarian cycle > Flashcards

oogenesis and the ovarian cycle Flashcards

1
Q

what is gametogenesis?

A

the process in which cells undergo meiosis to form gametes

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2
Q

what are the 4 phases of gametogenesis?

A

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

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3
Q

what are primordial germ cells?

A

oogonia/spermatogonia

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4
Q

what happens at 3 weeks of the developing fetus?

A
  • 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
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5
Q

what happens at 6 weeks of the developing fetus?

A

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

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6
Q

how do teratomas form?

A

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

they persist at ectopic sites and may give rise to teratomas

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7
Q

where do PGCs undergo rapid mitotic division?

A

at the gonads

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8
Q

describe how the oogonia increase in germ cell number?

A

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.

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9
Q

describe how the spermatagonia increase in germ cell number?

A

o Spermatogonia undergo mitosis in early embryonic testes.

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

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10
Q

describe the cell divisions in meiosis?

A

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.

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11
Q

what are the 4 main events during meiosis?

A

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

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12
Q

what is a tetrad?

A

complex of homologous chromosome pair consisting of 4 chromatids

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13
Q

what does crossing over in a tetrad make?

A

leads to 4 recombinant chromatids

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14
Q

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

A

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

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15
Q

what happens to oogonia from puberty to menopause?

A

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

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16
Q

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

A

23 chromosomes in each

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17
Q

wha triggers completion of meiosis 2 of a secondary oocyte?

A

fertilisation

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18
Q

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

A

metaphase II

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19
Q

how many oocytes do we have at birth?

A

2,000,000

20
Q

how many primary oocytes survive to puberty?

A

40,000

21
Q

how many primary oocytes are ovulated?

A

400

22
Q

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

A

not part of any substructure of the ovaries

23
Q

how and when is a primordial follicle made?

A

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

24
Q

how is a primary follicle made?

A

ovarian cells surrounding the primary oocyte increase in number

25
Q

what are granulosa cells?

A

cells surrounding the primary oocyte

26
Q

how is the zona pellucida formed?

A

contributions from the oocyte and granulosa cells

27
Q

what are theca cells?

A

layer of cells which form around the primary follicle

28
Q

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

A

secrete angiogenic factors to stimulate growth of blood vessels

29
Q

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

A

up to primary follicle stage

30
Q

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

A

FSH from the pituitary gland

31
Q

what does FSH stimulate?

A

oestrogen production by granulosa cells

32
Q

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

A

LH

activin

33
Q

what does LH stimulate?

A

testosterone production by theca cells

34
Q

how does a follicle respond to a LH surge?

A
  • 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
35
Q

what does activin do?

A

stimulates granulosa cell proliferation

36
Q

what does the antrum contain and why are these important?

A

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

37
Q

describe what happens at the start of menstruation

A

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.

38
Q

when does the first ovulation occur?

A
  • doesnt occur until the oestrogen feedback mechanism is fully developed
  • occurs 6-9 months after menarche
39
Q

describe progesterone levels over time

A

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

40
Q

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

A

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

41
Q

what occurs in the follicular phase?

A
  • 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.
42
Q

what occurs in the ovulatory phase?

A
  • 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.
43
Q

what occurs in the luteal phase?

A
  • 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.
44
Q

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

A
  • 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.
45
Q

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

A
  • 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.
46
Q

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

A
  • 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.
47
Q

what happens if fertilisation does occur?

A
  • 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.

human development (20 decks)

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