Let 4 - Gamatogenesis Flashcards
What do germ cells do after colonising the gonad?
- proliferate by mitosis
- reshuffle genetically and reduce to haploid by meiosis
- cytodifferentiate into mature gametes.
What does the SRY gene do?
it is the sex determining region on the Y chromosome which determines the development of the testes.
Describe the characteristics of oogenesis.
- there are very few gametes - 400 in lifetime.
- intermittent production - once a month.
- around 1 out of the 400 have reproductive potential.
Describe the characteristics of spermatogenesis.
- there are a huge number of sperm produced - around 200 million a day.
- production is continuous.
- the cells are essentially disposable.
What are the main functions of meiosis?
- Reduce the chromosome number in the gamete to 23.
- ensures every gamete is genetically unique.
How does genetic variation arise in meiosis?
- crossing over –> this is the exchange of regions of DNA between two homologous chromosomes —> happens in prophase I.
- Independent assortment –> This is the random orientation of each bivalent along the metaphase plate with respect to other bivalents.
- Random Segregation –> this is. the random distribution of alleles among the four gametes.
What does the head of epididymis do in spermatogenesis?
It is the storage tube for sperm.
What is spermatogonia and how long are they around for?
- they are raw material for spermatogenesis.
- They are available for up to 70 years.
What are produced by the division of spermatogonia?
- They divide by mitosis.
- Ad spermatogonium - this is reserve stock and it is resting.
- Ap spermatogonium - active and maintain stock.
- -> From puberty onwards they produce type B spermatogonia which give rise to primary spermatocytes.
What do primary spermatocytes eventually end up as?
They divide by meiosis giving rise to secondary spermatocytes and then to spermatids.
What does each primary spermatocyte form?
Each primary spermatocyte forms 4 haploid spermatids which differentiate by spermatogenesis into spermatozoa.
What is the spermatogenic cycle defined as?
- It is defined as time taken for reappearance of the same stage within a given segment of tubule.
- approximately 16 days in humans.
- different stages in spermatogenesis are also ordered in space as well as time.
- The distance between the same stage is called the spermatogenic wave.
Describe the process of spermiogenesis?
- spermatids are released into the lumen of seminiferous tubules –> This is known as spermiation.
- They remodel as they pass down the seminiferous tubule , through r2te testis and ductuli efferentes and into the epididymis to finally form spermatozoa.
- Then they are non-motile until they reach the epididymis.
- –> this transport is via Sertoli cell secretions which are assisted by peristaltic contraction.
How is semen released?
- 70% by seminal vesicle secretions such as amino acids, citrate, fructose and prostaglandins.
- 25% by secretions of prostate such as proteolytic enzymes and zinc.
- 2-5% by sperm via the vas deferens.
- < 1% Bulbourethral gland secretions (Cowper gland)
- –> Mucoproteins help to lubricate and neutralise acidic urine in the distal urethra.
What increases the motility of sperm?
When the concentration of zinc falls.
What is the final step of maturation required before sperm can become fertile called?
Sperm Capacitation.
What do the conditions in the female tract stimulate in sperm capacitation?
- removal of glycoproteins and cholesterol from sperm membrane.
- –> this allows sperm to penetrate the acrosome of the ovum so it will be fertile. - Activation of sperm signalling pathways –> this involves atypical soluble adenylyl cyclase and PKA.
- Allow sperm to bind to zona pellucid of oocyte and initiate acrosome reaction.
Describe the number of gametes in a female.
- Before birth all the gametes have already developed.
- Stock declines with age due to atresia.
Describe the process in the maturation of oocytes.
- Germ cells arise from the yolk sac and colonise the gonadal cortex and differentiate into oogonia.
- Oogonia then proliferate rapidly by mitosis.
- By the end of the 3rd month, oogonia are arranged in clusters surrounded by flat epithelial cells.
- Majority continue to divide by mitosis but some enter meiosis
- –> The ones that enter meiosis are arrested in prophase of meiosis I until puberty and are called primary oocytes.
When is the max number of germ cells is women reached?
By mid gestation.
Describe the maturation of oocytes before birth.
- After the max number of germ cells is reached by mid gestation, cell death begins and many oogonia and primary oocytes degenerate
- -> atresia. - By the 7th month of gestation, the majority of oogonia have degenerated.
- All surviving primary oocytes have now entered meiosis I and are individually surrounded by a layer of flat epithelial cells called follicular cells.
- They are now called primordial follicle.
What are the stages the oocytes go through in puberty each month?
- preantral
- Antral
- Preovulatory
How is the zona pellucida of the follicule formed?
- Granulosa cells secrete a layer of glycoprotein on oocyte forming the bona pellucida
What happens as the primordial follicles begin to grow?
-The surrounding follicular cells change from flat to cuboidal and proliferate to produce a stratified epithelium of granulosa cells.
When does the antral stage occur?
This happens as development continues.
- fluid filled spaces appear between granulosa cells, which then coalesce to form the Antrum.
What are the granulosa cells surrounding the oocyte called?
Cumulus oophorus.
What happens in the preovulatory stage?
- There is a surge in LH which induces preovulatory growth phase.
- meiosis I is now completed resulting in 2 haploid daughter cells go unequal size.
- Each daughter cell now has 23 chromosomes and 46 chromatids.
- One cell receives most of the cytoplasm, the other (first polar body) receives none.
- the cell then enters meiosis II but arrests in metaphase around 2 hours before ovulation.
- Meiosis II is only completed if the oocyte is fertilised, otherwise the cell degenerates at around 24 hours after ovulation.
Describe what happens in ovulation.
- FSH and LH stimulate the rapid growth of follicle several days before ovulation occurs.
- The mature follicle is now around 2.5cm in diameter and called Graafian.
- LH surge increases collagenase activity.
- Prostaglandins increases the response to LH and cause local muscular contractions in the ovarian wall.
- LH surge increases collagenase activity.
- The oocyte is then extruded and breaks free from the ovary.
How is the corpus luteum formed?
- Remaining granulosa and theca interna cells become vascularised.
- There is a Deepish yellowish pigment and change into lutein cells which form the corpus luteum.
What does the corpus luteum do?
- secretes oestrogens and progesterone.
2. stimulates uterine mucosa to enter secretory stage in preparation for embryo implantation.
How long does the corpus luteum last?
It dies after 14 days if no fertilisation occurs.
How is the oocyte transported?
- shortly before ovulation, fimbriae sweep over the surface of the ovary.
- The uterine tube begins to contract rhythmically.
- Oocyte is then carried into tube by sweeping movements of the fimbriae and by motion of cilia on epithelial lining.
- The oocyte are then propelled by peristaltic muscular contractions of the tube and by cilia in the mucosa
- If fertilised, the oocyte reaches the uterine lumen in about 3 to 4 days.
Describe the release of the hormones in the ovarian cycle.
- under the influence of hypothalamic GnRH, anterior pituitary gland releases FSH and LH.
- Follicles are stimulated to grow by FSH and to mature by FSH and LH.
- Ovulation occurs on LH surge
- LH also promotes the development of the corpus luteum.
Compare spermatogenesis with oogenesis.
Spermatogenesis
- around 200 million sperm per day.
- 4 spermatids are formed with no polar body formation and equal division of cytoplasm.
- Starts at puberty.
- Continues throughout adult life.
- The gametes are motile.
- All the stages are complete in the testes.
Oogenesis
1. usually 1 ovum per 28 day menstrual cycle.
2. one ovum with unequal division of cytoplasm and 3 polar bodies formed.
3. starts in the foetus.
4. ends at menopause.
5 The gametes are non-motile.
6. The last stage of meiosis 2 occurs in the oviduct.
