3 - Gametogenesis and the Ovarian Cycle Flashcards
What are the 4 phases 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 week of development does gametogenesis begin?
Week 3 of development
What happens in phase 1 of gametogenesis?
Begins in week 3 of development
Primary germ cells leaving the primary ectoderm (epiblast) and move to the yolk sac wall and collect near the exit of the allantois
At week 6 the primary germ cells leave the yolk sac and migrate along the vitelline and the dorsal mesentery into the gonadal ridge
At this stage the foetus is neither male or female
At around 7 weeks, testosterone is formed which induces male gonadal development – this only happens when a Y chromosome is present – testosterone determining factor is present
Absence of Y chromosome means no TDF therefore gonads do not become male
Migration of germ cells to extragonadal cells can give rise to teratomas
What happens in phase 2 of gametogenesis?
Phase 2 – increase in germ cells number
Germ cells undergo rapid mitotic division
Each germ cells produces two diploid cells (2n)
Germ cell number multiplies exponentially to produce millions of cells
What is the difference in what happens in males and females in gametogenesis?
Female
Oogonia undergo intensive mitosis in embryonic ovary from the 2nd-5th month of maternal pregnancy
After 5 months, many of these degenerate
This degeneration continues until menopause
Male
Spermatogonia undergo mitosis in the early embryonic testes
Spermatogonia undergo periodic waves of mitosis from puberty onwards throughout life
What occurs during phase 3 of gametogenesis?
Phase 3 – reduction in chromosome number
Meiosis involves 2 cells divisions and 4 main events
Meiosis 1 – diploid [Symbol] haploid
Meiosis 2 – independant reassortment of maternal and paternal chromosome s – this stage allows for genetic diversity
1 = DNA replication
2 = pairing of homologous chromosomes and crossing over
3 = separation of chromosomes in the first meiotic division – two daughter cells with one pair of replicated chromosomes
4 = separation of the chromatids in the second meiotic division – four daughter cells with one haploid set of chromosomes and one chromatid
What is the difference in what happens in phase 3 between males and females?
Females
From foetal period [Symbol] puberty
All oogonia enter the 1st meiotic division in late foetal period and enter the diplotene stage of prophase I in the early months after birth
They remain at this stage until puberty (unless degeneration occurs)
From puberty to menopause the primary oocyte will complete meiosis I shortly before ovulation to produce a secondary oocyte (large cell) and the first polar body
LH surge that causes meiosis I to complete
Secondary oocyte begins meiosis II and arrest at metaphase II
Completion of meiosis II triggered by fertilisation
Results in a fertilised egg and a second polar body
Arrest in meiosis I for eggs that do not fully mature until the woman is close to menopause meaning that the egg is more at risk of developing trisomys such as Down’s syndrome
Males
Begins in the seminiferous tubules of the testes after puberty
Type A spermatogonia are mitotically active throughout reproductive life
They give rise to type B spermatogonia which enter meiosis
Interlocking Sertoli cells keep spermatogonia at the base of the seminiferous epithelium
What happens in phase 4 of gametogenesis - females?
- A primary follicle forms
- Granulosa cells (surrounding the primary oocyte) help to form the zone pellucida
- Layer of theca cells grow and secrete angiogenic factors - stimulate blood vessel growth
- FSH further develops the follicle by stimulating granulosa cells to secrete oestrogen
- Antrum becomes a secondary follicle
- Activin from granulosa cells causes further follicle enlargement
- LH stimulates testosterone from theca cells
- Testosterone enters granulosa cells where it is converted by aromatase into oestrogen
- Oestradiol - stimulates LH receptors on granulosa cells
- Respond in surge of LH for ovulation
- Graafian follicle - meiosis - ovulation - oocyte travels to oviduct - Corpus luteum left behind - degenerates and becomes corpus albicans
What are the 4 stages of sperm maturation?
primary spermatocytes, then secondary spermatocytes, spermatids and mature sperm
Primary spermacytes - 24 days - meiosis 1 secondary spermacytes - immediately enter meiosis II - spermatids
Spermatids -
spermiogenesis to becomes spermatozoa
What is spermiogenesis?
Reduction in nuclear size
Golgi apparatus condenses to form acrosome
Flagellum grows out of centriole
Cytoplasm streams away from nucleus
Mitochondria in spiral arrangement around flagellum
Head partitioned into domains
Cytoplasm moves to residual body which is phagocytosed by Sertoli cells
What is capacitation?
Final step of sperm maturation
Takes place in the female genital tract
Only capacitated sperm are capable of fusing with the oocyte
What are the hormones released to stimulate spermatogenesis?
GnRH stimulates FSH and LH release form the anterior pituitary
LH stimulate testosterone production by Leydig cells
Testosterone and FSH target Sertoli cells which secrete antigens binding protein
ABP binds to testosterone and carries it to the seminiferous tubules where it stimulates spermatogenesis
When does the first ovulation occur?
When the oestrogen feedback mechanisms are fully developed
6-9 months after menarche
What are the phases of the ovarian cycle?
Follicular Phase
Ovulatory Phase
Luteal Phase
What occurs during the follicular phase of the ovarian cycle
Days 1-10
5-12 primoardial follciles are stimulated to grow and develop
Formation of Graafian follicle from one of the maturing primordial follicles
