Gametogenesis Flashcards
1
Q
What are spermatognia
A
- Spermatogonia (male germ cells) are raw material for spermatogenesis
- Divide by mitosis giving rise to Ad and Ap spermatogonium (renew of germ cells)
- Ad spermatogonium - reserve stock
- Ap spermatogonium - maintain stock and from puberty onwards produce type B spermatogonia which give rise to primary spermatocytes
- Ad spermatogonium - reserve stock
2
Q
Describe how spermatogenesis occurs
A
- Spermatogonia form primary spermatocytes
- Primary spermatocytes divide by meiosis giving rise to secondary spermatocytes and then to spermatids
- Each primary spermatocyte forms 4 haploid spermatids which differentiate (spermiogenesis) into spermatozoa
3
Q
Describe the structure of Sertoli cells
A
- Spermatogenesis occurs in gaps of Sertoli cells
- Basal compartment dormant stage before puberty
- Spermatogonia located here - maintain sperm origins
- Tight junctions between Sertoli cells to prevent sperm entering blood stream
- Sperm recognized as foreign and start immune response in the blood
4
Q
Describe spermiogenesis
A
- Spermatids released into lumen of seminiferous tubules - spermiation
- Remodel as they pass down seminiferous tubule, through rete testis and ductuli efferentes and into epididymis to finally form spermatozoa
- Non-motile until they reach epididymis
- Transport via Sertoli cell secretions assisted by peristaltic contractions
5
Q
Explain sperm capacitation
A
- Final maturation step required before sperm become fertile
- Conditions in female genital tract stimulate:
- Removal of glycoproteins and cholesterol from sperm membrane
- Activation of sperm signalling pathways
- Allow sperm to bind to zona pellucida of oocyte and initiate acrosome reaction
- Human sperm for IV fertilisation must first be incubated in capacitation media
- Mimics conditions of vagina to make sperm fertile
6
Q
Distuinguish between spermatogenic cycle and wave
A
- Not all stages in spermatogenesis are visible in a single cross-section of seminiferous tubule
- Cells tend to appear in groups with same maturation stages
- Spermatogenic cycle - time taken for reappearance of the same stage within a given segment of tubule
- ~16 days in human
- Different stages in spermatogenesis are ordered in space as well as time
- Each stage follows in an orderly sequence along the length of the tubule
- Spermatogenic wave - distance between the same stage
7
Q
State the role of the rete testis and epididymis
A
- Rete testis - concentrate spermatozoa
- Epididymis - storage tube and where sperm start to mature and learn how to swim
8
Q
State the components of semen
A
- 2ml per ejaculate
- Seminal vesicle secretions - 70%
- Nutrition such as fructose for sperm
- Alkaline fluid to neutralise acidic environment in vagina
- Secretions of prostate - 25%
- Proteolytic enzymes
- Sperm (via vas deferens) - 2-5%
- 200-500 million per ejaculate
- Bulbourethral gland secretions / Cowper gland - <1%
- Mucoproteins help lubricate and neutralise acidic urine in distal urethra
9
Q
Describe the maturation of oocytes before birth
A
- Germ cells from yolk sac colonise the gonadal cortex and differentiate into oogonia
- Oogonia then proliferate rapidly by mitosis
- By end of 3rd month, oogonia arranged in clusters surrounded by flat epithelial cells
- Majority continue to divide by mitosis but some enter meiosis
- These arrest in prophase of meiosis I and are called primary oocytes
- Max number germ cells reached by mid gestation (~7 million)
- Cell death then begins and many oogonia and primary oocytes degenerate (atresia)
- By 7th month gestation, majority of oogonia have degenerated
- All surviving (~2 million) primary oocytes have now entered meiosis I and are individually surrounded by layer of flat epithelial cells called follicluar cells
- Now called primordial follicle
10
Q
State the 3 stages of maturation of oocytes at puberty
A
- Preantral stage
- Antral stage
- Preovulatory stage
11
Q
Describe the preantral stage
A
- As primordial follicles begin to grow, the surrounding follicular cells change from flat to cuboidal and proliferate to produce a stratified epithelium of granulosa cells
- Granulosa cells secrete layer of glycoprotein on oocyte forming the zona pellucida
12
Q
Describe the antral stage
A
- Fluid filled spaces appear between granulosa cells
- Spaces coalesce to form the antrum
- Several follicles begin to develop with each ovarian cycle, but usually only one reaches maturity
13
Q
Describe the preovulatory stage
A
- Surge in LH induces preovulatory growth phase
- Meiosis I is completed resulting in 2 haploid daughter cells of unequal size
- Each daughter cell has 23 chromosomes and 46 chromatids
- One cell receives most of the cytoplasm and the other becomes a polar body
- Cell then enters meiosis II but arrests in metaphase
- ~3 hours before ovulation
- Meiosis II is only completed if oocyte is fertilised, otherwise cell degenerates
- ~24 hours after ovulation
14
Q
Outline oocyte transport
A
- Shortly before ovulation, fimbriae sweep over surface of ovary
- Uterine tubes begin to contract rhythmically
- Oocyte carried into tube by sweeping movements of fimbriae and by motion of cilia on epithelial lining
- Oocyte then propelled by peristaltic muscular contractions of the tube and by cilia in the mucosa
- If fertilised, oocyte reaches uterine lumen in 3-4 days
15
Q
What happens to corpus luteum if no fertilisation occurs
A
- If fertilisation does not occur, corpus luteum degenerates
- Forms mass of fibrotic scar tissue - corpus albicans
- Progesterone production decreases, precipitating menstrual bleeding
