Gametogenesis

Question 1

What are spermatognia

Answer 1

• 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

Question 2

Describe how spermatogenesis occurs

Answer 2

• 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

Question 3

Describe the structure of Sertoli cells

Answer 3

• 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

Question 4

Describe spermiogenesis

Answer 4

• 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

Question 5

Explain sperm capacitation

Answer 5

• 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

Question 6

Distuinguish between spermatogenic cycle and wave

Answer 6

• 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

Question 7

State the role of the rete testis and epididymis

Answer 7

• Rete testis - concentrate spermatozoa

- Epididymis - storage tube and where sperm start to mature and learn how to swim

Question 8

State the components of semen

Answer 8

• 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

Question 9

Describe the maturation of oocytes before birth

Answer 9

• 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

Question 10

State the 3 stages of maturation of oocytes at puberty

Answer 10

• Preantral stage
• Antral stage
• Preovulatory stage

Question 11

Describe the preantral stage

Answer 11

• 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

Question 12

Describe the antral stage

Answer 12

• 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

Question 13

Describe the preovulatory stage

Answer 13

• 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

Question 14

Outline oocyte transport

Answer 14

• 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

Question 15

What happens to corpus luteum if no fertilisation occurs

Answer 15

• If fertilisation does not occur, corpus luteum degenerates
• Forms mass of fibrotic scar tissue - corpus albicans
• Progesterone production decreases, precipitating menstrual bleeding

Question 16

What happens to corpus luteum if fertilisation occurs

Answer 16

• Degeneration of corpus luteum prevented by human chorionic gonadotropin (hCG), secreted by developing embryo
  
  • Signal to corpus luteum that fertilisation has occurred
• Corpus luteum continues to grow and forms the corpus luteum of pregnancy (corpus luteum graviditatis)
• Cells continue to secrete progesterone until 4th month
• Secretion of progesterone by placenta then becomes adequate

Question 17

Compare spermatogenesis and oogenesis

Answer 17

• Spermatogenesis produces 200 million sperm per day, oogenesis produces 1 ovum per cycle
• 4 spermatids formed while only one ovum with 3 polar bodies formed
• Spermatogenesis starts at puberty while oogenesis starts in fetus
• Spermatogenesis contious throughout adult life, oogenesis ends at menopause
• Spermatogenesis produces motile gametes, oogenesis has non motile gametes
• Spermatogenesis all stages complete in testes, oogenesis last stage of meiosis 2 occurs in oviduct