Session 3 - Gametogenesis Flashcards
What are the differences in production of gametes in oogenesis and spermatogenesis?
Oogenesis:
- very few gametes (~400 in lifetime)
- intermittent production (~1/month)
Spermatogenesis:
- huge number (~200 million/day)
- continuous production
- essentially ‘disposable’ cells
What are the two main functions of meiosis?
Reduce the chromosome number in the gamete to 23 (haploid)
Ensures that every gamete is genetically unique
How many daughter cells are produced by meiosis?
4 daughter cells
In female only one develops into a mature oocyte, others form polar bodies
How does genetic variation arise in meiosis?
Crossing over (exchange of regions of DNA between 2 homologous chromosomes)
Independent assortment (random orientation of each bivalent along the metaphase plate with respect to other bivalents)
Random segregation (random distribution of alleles among the four gametes)
Where does spermatogenesis occur?
Sertoli cells in the seminiferous tubules of the testes
What are spermatogonia?
Male germ cells
Male germ cells divide by MITOSIS to give rise to what cells?
Ad spermatogonium (‘Resting’, reserve stock)
Ap spermatogonium (‘Active’, maintain stock and from puberty onwards produce type B spermatogonia which give rise to primary spermatocytes)
What happens to the spermatocytes after primary spermatocytes have been produced?
Primary spermatocytes divide by meiosis giving rise to secondary spermatocytes and then spermatids (the four daughter cells)
Each primary spermatocyte forms how many spermatids?
4 haploid spermatids
What is spermiogenesis?
The differentiation of spermatids into spermatozoa as they pass from seminiferous tubules to the epididymis.
Are all stages of spermatogenesis visible in a single cross-section of seminiferous tubule?
No, cells tend to appear in groups with the same maturation stages. Each stage follows an orderly sequence along the length of the tubule.
What is the spermatogenic cycle defined as?
The time taken for the same stage of spermatogenesis to reappear in a give segment of seminiferous tubule (~16 days in humans)
What is the spermatogenic wave?
The distance along the seminiferous tubule between the same stage of spermatogenesis.
What is spermiation?
The release of spermatids into the lumen of the seminiferous tubule.
What happens to spermatids as they pass down the seminiferous tubule?
They are remodelled as they pass through the seminiferous tubules, through the rete testis and ductuli efferentes and into the epididymis to finally form spermatozoa.
Describe how the spermatids are transported through the lumen to the epididymis.
Non-motile transport. Transported via Sertoli cell secretions and assisted by peristaltic contractions until they reach the epididymis.
What is semen composed of?
Seminal vesicle secretions (~70%)
Secretions of the prostate (~25%)
Sperm (via the vas deferens) (2-5%)
Bulbourethral gland secretions (<1%)
What do the seminal vesicles secrete into semen?
Amino acids, citrate, fructose, prostaglandins
What does the prostate secrete into semen?
Proteolytic enzymes, zinc
What do the bulbourethral (Cowper’s) glands secrete into semen?
Mucoproteins which help lubricate and neutralise acidic urine in distal urethra.
What is sperm capacitation?
The physiological changes that occur in the final step of maturation of the spermatozoa. Must undergo these changes in order to have the ability to penetrate and fertilise an egg. Occurs in the female genital tract.
What happens during sperm capacitation?
Conditions in female genital tract stimulate:
- removal of glycoproteins and cholesterol from sperm membrane
- activation of sperm signalling pathways
These allow sperm to bind to the zona pellucida of the oocyte and initiate the acrosome reaction.
What must human sperm for in vitro fertilisation first go through so that it is able to fertilise the oocyte?
Must first be incubated in capacitation media
When does maturation of oocytes begin?
Before birth
