Gametogenesis Flashcards
What are the main differences between oogenesis and spermatogenesis?
What are the 2 main functions of meiosis?
- reduce chromosome number in gamete to 23
- ensure every gamete is genetically unique
Explain how a fertilised oocyte goes through meiosis I and meiosis II to give 4 daughter cells
How many daughter cells are made in men vs women?
Men- 4 daughter cells
Women - only 1 as 3 become polar bodies
How does genetic variation arise in meiosis?
- Cross over- DNA exchanged between 2 homologous regions
- Independent assortment- random organisation of bivalent on metaphase plate
- Random segregation - random allele distribution amoung 4 gametes
Where does spermatogenesis occur and in what compartment?
Within semineferous tubules of the testes
- spermatogonia reside in basal compartment
What is the function of tight junctions between the basal and adluminal compartment of the semineferous tubules?
- maintains different cellular environments between 2 compartments
- stops immunological response to sperm
How long are spermatagonia available for?
70 years
Spermatagonia divide by mitosis to give which 2 cell types?
- Ad spermatogonium - resting reserve stock
-
Ap spermatagonium - active stock- maintain stock from puberty onwards
- produce type B spermatogonia to give primary spermatocytes
Which steps make spermatagonia become spermatozoa?
What is the spermatogenic cycle?
Legnth of time it takes for spermatids at the same stage of the cycle to show up again
What is the spermatogenic wave?
The distance between spermatids at the same level of maturation
What is the legnth of the spermatogenic cycle in a human?
~16 days
Explain the process of spermiogenesis
- Spermatids released into the lumen of semineferous tubules= spermiation
- Spermatids remodel as they pass through ST, through rete testes and ductuli efferentes into epididymis
- In epididymis spermatids become spermatozoa
- Until they reach the epididymis they spermatids are non-motile, process helps them become motile
Explain how the structure of sperm relates to its funciton
- Head contains nucleus - genetic info
- Acrosome layer- allows penetration of zona pelliculida
- Tail- provides motility
- Mitochondria- produce ATP to drive flagella tail
What secretions make up semen?
- seminal vesicle secretions (~70%)
- prostate secretions (~25%)
- Sperm (2-5%)
- Bulbourehtral gland secretions (Cowper gland)
What is sperm capacitation?
Final step of maturation occurs in the female genital tract
Glycoproteins and cholesterol are removed to activate sperm signalling pathway ⇒ allows sperm to bind to zona pellucida of oocyte
With regards to capacitation, what is important to remember in in vitro fertilisation?
Sperm must be incubated with capacitation media to mimick environment of female reproductive tract to activate the signicalling pathways of sperm
When does maturation of oocytes begin?
At birth
How does oogenesis start before birth?
- Germ cells arise in gonadal cortex from the yolk sac and differentiate to oogonia
- oogonia proliferate by mitosis
- 3rd month - oogenia arranged in clusters surrounded by flat epithelial cells
- Majority continue to divide by mitosis but some divide by mitosis and then arrest in prophase of meisosis I as primary oocytes
What happens to the number of primary oocytes between before birth?
They drop due to atresia
A mechanism of quality control
What is primordial follicle?
A primary oocyte surrounded by flat follicular cells
How many oocytes remain by puberty?
~40,000
What are the 3 stages that oocytes go through to mature each month?
- Pre-antral
- Antral
- Pre-ovulatory
Describe what happens to the oocyte during the pre-antral stage
Follicular cells of primordial follicle proliferate and change from flat→ cuboidal to produce statified epithelium of granulosa cells
Cell now called the primary follicle
Describe what happens to the oocyte during the antral stage
- Primary follicle develps and fluid spaces appear between granulosa cells
- Collective space called the antrum
- Outer fibrous layer becomes theca externa
- Inner secretory layer becomes theca interna
- Follicle is now a secondary follicle
What hormone do the granulosa cells of the primary and secondary follicle secrete?
Oestrogen
Describe what happens to the oocyte during the pre-ovulatary stage
- Surge in LH induces growth phase
- Follicle completes meiosis 1 → 2 haploid cells of unequal size
- each daughter cells have 23 chromosomes and 46 chromatids
- One cell recieves most cytoplasm, polar bodies recieve very little
- Follicle with cytoplasm enters meiosis II but arrests in metaphase before ovulation
When is Meiosis II completed?
Upon fertilisation
If not fertilised oocyte degrades 24hrs later
What is the name given to the only oocyte chosen to fully mature?
Graafian Follicle
Explain what happens when ovulation occurs?
- FSH and LH stimulate rapid growth of follicle a few days before ovulation
- LH surge increases collegenase activity
- Prostaglandins increase and cause muscular contractions in ovarian wall
- Oocyte breaks out of ovary
What is the corpus luteum? What is it’s function?
Remaining granulosa and theca cells after ovulation ⇒ change to lutein cells
Secrete oestrogen and progesterone to stimulate uterine mucosa to prepare for embryo implant
How long does the corpus luteum remain for after ovulation?
Set time of 14 days if no ferfilisation occurs
How are oocytes transported into fallopian tubes after ovulation?
Sweeping movements of fimbrae and of cilia on epithelial lining of uterine tube
Propels oocyte down fallopian tube
Explain what happens to the corpus luteum if no fertilisation occurs?
Corous luteum degenerates
Forms fibrotic scar tissue called corpus albicans
Progesteron production decreases causing menstrual bleeding
Explain what happens to the corpus luteum if fertilisation occurs?
- Embryo secretes human chorionic gonadotropin which prevents degeneration of corpus luteum
- CL continues to grow and forms corpus luteum of pregnancy → corpus luteum graviditatis
- Secretes progesterone until 4th month of gestation
After 4th month of gestation, what is the main source of progesterone?
The placenta
