Gametogenesis Flashcards
What are the 4 phases of gametogenesis?
1) Extraembryonic origin of germ cells –> 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
When do primordial germ cells begin to migrate, where from-to?
3rd week of foetal development; from the primary ectoderm (epiblast) to the yolk sac wall (near exit of allantois)
What is the allantois?
Small sausage-shaped outpouching from the caudal wall of the yolk sac of the early embryo
What happens if primordial germ cells inappropriately migrate?
They may settle at extra-gonadal sites and if they persist at ectopic sites, can give rise to teratomas (tumours).
Where do primordial germ cells migrate from-to at week 6 of foetal development?
From the yolk sac wall, along the vitelline and dorsal mesentery to reach the gonadal ridge
What happens once the primordial germ cells reach the gonads?
Undergo rapid mitotic division to produce two diploid cells (oogonia in women)
When do oogonia undergo intensive mitosis in females during foetal development?
2nd to 5th month of foetal development
When do large numbers of oogonia begin to undergo degeneration?
After 5 months of foetal development
Provide an overview of meiosis to produce haploid oogonia
Meiosis I (reduction division) as it halves the chromosome number (2n to n). Meiosis II (equatorial division) where there is independent re-assortment of maternal and paternal chromosomes (leading to variation) and sister chromatids separate
Describe the process of meiosis
1) Oogonia are diploid (2n) and 2c (two chromatids per chromosome)
2) Pairing of homologous chromosomes and crossing over in prophase of meiosis I –> tetrad (complex of 4 chromatids) –> crossing over form 4 recombinant chromatids –> cell is 2n, 4c
3) Separation of chromosomes in meiosis I –> 1n, 2c
4) Chromatids are separated as a result of meiosis II –> 4 daughter cells –> haploid with one chromatid; 1n, 1c
Describe the time-line of the meiotic stages in a developing female
Foetal period: oogonia enter meiosis I
Birth: oogonia enter prophase I and become primary oocytes
Puberty: Primary oocyte completes meiosis I and begins meiosis II just before ovulation (forms secondary oocyte and 1st polar body)
Conception: Fertilisation triggers completion of meiosis II –> fertilised egg and 2nd polar body
When do oogonia enter the first meiotic divison?
Late foetal development, then enter diplotene stage of prophase I in early months after birth
When do primary oocytes complete the first meiotic division?
Just before ovulation –> forms secondary oocyte and first polar body
When do secondary oocytes enter the second meiotic division?
Just prior to ovulation before arresting at metaphase II
When do the secondary oocytes complete the second meiotic division?
Only if there is fertilisation –> fertilised egg and second polar body
Describe the structure of oogonia
These cells are ‘naked’; not par of any ovarian substructure
Describe the structure of primary oocytes
Surrounded by granulosa cells to form a primary follicle
Describe the structure of primary follicles
Have granulosa cells and a primary oocyte which help to form the zona pellucida
Describe the structure of secondary and Graafian follicles
A layer of theca cells line the follicle (theca interna and externa), have an antrum which contains serum components, enzymes and proteoglycans (enzymes digest follicular wall in ovulation, proteoglycans draw in water to increase volume of antral fluid to help eject oocyte out into oviduct)
When are hormones required for the continuation of oogenesis?
After the primary follicle stage, until then oogenesis is self-regulated
What hormones are required to facilitate the change from primary to secondary follicle?
FSH production (anterior pituitary) which causes oestrogen production in granulosa cells surrounding the primary follicle –> causes production of the antrum
What hormones are required to facilitate the change from secondary to Graafian follicle?
Production of activin (gonads and pituitary) –> stimulates granulosa cell proliferation. LH production –> testosterone production by theca cells –> testosterone enters granulosa cells –> converted to oestrogen (mainly 17β oestradiol) by aromatase enzyme –> stimulates formation of further LH receptors on granulosa cells –> allows follicle to respond to LH surge –> ovulation
What hormone is produced by the Graafian follicle to prepare the reproductive tract for possible fertilisation?
Oestrogen is produced by granulosa cells of the follicle which then prepare the reproductive tract for possible fertilisation
What hormones are required to facilitate ovulation from the Graafian follicle?
LH surge (anterior pituitary) caused by high oestrogen levels –> expulsion of secondary oocyte
When does the formation of sperm begin?
At puberty, not before
Describe spermatogonia
Starting cells of sperm production, kept at the base of the seminiferous tubule by interlocking Sertoli cells, connected by intercellular cytoplasmic bridges
What marks the initiation of spermatogenesis?
Spermatogonia (stem cells) forming type A spermatogonia at regular intervals
How are type B spermatogonia produced?
Type A spermatogonia undergo a limited number of divisions, and at the last division Type B spermatogonia are formed
Describe primary spermatocytes and their migration
Primary spermatoocytes are formed when type B spermatogonia enter meiosis I, and in meiosis I primary spermatocytes become immunologically distinct from the other cells and move through the blood-testis barrier to the interior of the seminiferous tubule
How long does it take primary spermatocytes to complete meiosis I?
24 days (making mRNA for future protein production which is stored until required)
How are secondary spermatocytes formed?
When meiosis I has completed in primary spermatocytes they become secondary spermatocytes and these immediately enter meiosis II
How are spermatids produced?
When secondary spermatocytes finish meiosis II
What is spermiogenesis?
Mturation of spermatids into mature, motile spermatozoa
Describe spermiogenesis
a. Reduction of nuclear size
b. Golgi apparatus condenses crosome
c. Flagellum grows out of centriole
d. Cytoplasm streams away from nucleus
e. Mitochondria form a spiral arrangement around formed flagellum
f. The head becomes partitioned into domains (acrosomal cap and nucleus)
g. Cytoplasm moves to the residual body which is phagocytosed by Sertoli cells
What forms the blood-testis barrier, and what happens if this is compromised?
Sertoli processes in the testis form an immunological barrier and if this is broken then autoimmune infertility can occur
What happens if there is early translation of proteins in primary spermatocytes in meiosis I?
Results in sterility
Where does the increase in germ cell number occur in spermatogenesis?
Mitosis occurs between type B spermatogonia and the primary spermatocyte
Where do the reductions in chromosome number occur in spermatogenesis (meiotic divisions)?
First meiotic division: primary spermatocyte –> forms secondary spermatocyte
Second meiotic division: secondary spermatocyte –> forms spermatid
What is capacitation?
Final stage of sperm cell maturation, occurs in female reproductive tract (requires contact with oviduct secretions) –> physiological reaction occurs –> allows sperm to perform acrosome reaction with secondary oocyte (egg)
Describe the hormonal control of sperm production
GnRH (hypothalamus) –> FSH and LH (anterior pituitary) –> LH stimulates testosterone production by Leydig cells (surrounding tissue of seminiferous tubule) –> produced testosterone and FSH act on Sertoli cells in seminiferous tubule –> secrete androgen binding protein (ABP) and tubular fluid –> ABP binds to testosterone and carries it to fluid region of seminiferous tubule –> stimulates spermatogenesis
What is the role of early theca cells in the primary follicle?
Secrete angiogenic factors to stimulate the growth of blood vessels
