oogenesis and the ovarian cycle Flashcards
what is gametogenesis?
the process in which cells undergo meiosis to form gametes
what are the 4 phases of gametogenesis?
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
what are primordial germ cells?
oogonia/spermatogonia
what happens at 3 weeks of the developing fetus?
- 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
what happens at 6 weeks of the developing fetus?
• PGC leave yolk sac wall and migrate along vitelline and dorsal mesentery into the gonadal ridge
how do teratomas form?
when some of these cells migrate inappropriately and settle at extragonadal sites
they persist at ectopic sites and may give rise to teratomas
where do PGCs undergo rapid mitotic division?
at the gonads
describe how the oogonia increase in germ cell number?
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.
describe how the spermatagonia increase in germ cell number?
o Spermatogonia undergo mitosis in early embryonic testes.
o However, spermatogonia undergo periodic waves of mitosis from puberty onwards throughout life.
describe the cell divisions in meiosis?
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.
what are the 4 main events during meiosis?
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
what is a tetrad?
complex of homologous chromosome pair consisting of 4 chromatids
what does crossing over in a tetrad make?
leads to 4 recombinant chromatids
what happens to the oogonia between the fetal period and puberty?
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
what happens to oogonia from puberty to menopause?
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
how many chromosomes are found in the secondary oocyte and the polar body?
23 chromosomes in each
wha triggers completion of meiosis 2 of a secondary oocyte?
fertilisation
what phase does a secondary oocyte become arrested in during meiosis II?
metaphase II
how many oocytes do we have at birth?
2,000,000
how many primary oocytes survive to puberty?
40,000
how many primary oocytes are ovulated?
400
what is meant by the oogonia being naked in the embryo?
not part of any substructure of the ovaries
how and when is a primordial follicle made?
As meiosis begins, primary oocytes are surrounded by other ovarian cells to form primordial follicle
how is a primary follicle made?
ovarian cells surrounding the primary oocyte increase in number
what are granulosa cells?
cells surrounding the primary oocyte
how is the zona pellucida formed?
contributions from the oocyte and granulosa cells
what are theca cells?
layer of cells which form around the primary follicle
what do early theca cells secrete and what does this do?
secrete angiogenic factors to stimulate growth of blood vessels
how long is the development of the follicle self-regulated?
up to primary follicle stage
which hormones are needed for follicle development after the primary follicle stage?
FSH from the pituitary gland
what does FSH stimulate?
oestrogen production by granulosa cells
what hormones are needed for further enlargement of the secondary follicle?
LH
activin
what does LH stimulate?
testosterone production by theca cells
how does a follicle respond to a LH surge?
- 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
what does activin do?
stimulates granulosa cell proliferation
what does the antrum contain and why are these important?
serum components, enzymes and proteoglycans
o enzymes are important for digestion of follicular wall
o proteoglycans have a negative charge which attracts water molecules
describe what happens at the start of menstruation
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.
when does the first ovulation occur?
- doesnt occur until the oestrogen feedback mechanism is fully developed
- occurs 6-9 months after menarche
describe progesterone levels over time
o Throughout puberty progesterone levels remain low and will not rise until after menarche and the first ovulation
what are the 3 stages of the ovarian cycle and when do they occur?
o Follicular phase – days 1 to 10
o Ovulatory phase – days 11 to 14
o Luteal phase – days 15 to 28
what occurs in the follicular phase?
- 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.
what occurs in the ovulatory phase?
- 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.
what occurs in the luteal phase?
- 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.
what hormones are involved in the follicular phase and what do they do?
- 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.
what hormones are involved in the ovulatory phase and what do they do?
- 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.
what hormones are involved in the luteal phase and what do they do?
- 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.
what happens if fertilisation does occur?
- 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.
