6. Spermatogenesis and Fertilisation

Question 1

What phases of gametogenesis are the same for males and females?

What phases are different? How?

Where are spermatogonia kept?

Answer 1

Phase 1 - 2: origin and migration, mitosis. Spermatogonia undergo mitosis in early embryonic testes, and periodic waves of mitosis from puberty onwards throughout life.

Phase 3: meiosis. Phase 4: structural and functional maturation. Faster than oogenesis, begins in seminiferious tubules of testes after puberty. Type A spermatogonia are mitotically active throughout reproductive life, give rise to Type B spermatogonia which enter meiosis.

Base of seminiferious epithelium by interlocking sertoli cells, connected by intercellular cytoplasmic bridges

Question 2

Label 1-8 in the seminiferious tubule.

Answer 2

1. Basal lamina (membrane) (not recognizable)
2. Myofibroblast
3. Fibrocyte
4. Sertoli cell
5. Spermatogonia
6. Various stages of the germ cells during spermatogenesis
7. Spermatozoon
8. Lumen

Question 3

What do the developing germ cells become before mature sperm?

What seperates the blood from the testes? What would happen if these were not seperated?

How long do primary spermatocytes take to complete meiosis I and why?

When do secondary spermatocytes enter meiosis II?

Answer 3

Spermatogonia at periphery of seminiferious tubule -> advance to lumen during which time they become primary spermatocytes (type B spermatogonia entering meiosis) -> secondary spermatocytes -> spermatids -> mature sperm

Sertoli processes form blood-testes barrier, move through it when become primary spermatocytes. If barrier broken, autoimmune infertility can occur.

24 days - make and store mRNA for later protein production. (Early translation = sterility). After meiosis I completed -> secondary spermatocytes.

A few hours after completing meiosis I, produces spermatids.

Question 4

What is spermiogenesis? Describe what happens.

Can the ‘finished product’ fertililse an egg? Why?

Answer 4

Spermatids undergo it to become spermatozoa. Involves: reduction in nuclear size, golgi apparatus condenses -> acrosome, flagellum grows out of centriole, cytoplasm streams away from nucleus, mitochondria in spiral arrangement around flagellum, head partitioned into domains, cytoplasm moves to residual body which is phagocytosed by Sertoli cells

Not yet. Capacitation: final step of sperm maturation in female genital tract, requires contact with oviduct secretions. Only capacitated sperm capable of acrosome reaction and fusion with oocyte.

Question 5

What hormones are involved in sperm production and what are their roles?

Answer 5

GnRH: FSH and LH release. LH stimulates testosterone production by Leydig cells (surrounding seminiferious tubules). Testosterone and FSH target Sertoli cells which secrete androgen binding protein (ABP) and tubular fluid. ABP binds to testosterone and carries it to area of seminiferous tubule where it stimulates spermatogenesis.

Question 6

Describe sperm deposition and transport into uterus.

How does cervix mucus change throughout the menstural cycle?

What happens to the oocyte once ovulation has occured?

Answer 6

Sperm deposited at cervical os whose ciliated surface helps sperm towards the cervical canal. Transport into uterus via sperm’s propulsion and fluid currents caused by uterine cilia (2-7h for 14cm). During this capacitation happens (removal of sperm glycoprotein coat = hyperactive and sensitive to signals).

Changes from sperm hostile to sperm friendly at ovulation.

Makes its way into oviduct

Question 7

Brieflt describe how an egg is fertilised.

What is the acrosome reaction?

Answer 7

Sperm finds and recognises egg, has acrosome reaction to penetrate extracellular layer where its membrane fuses with egg cell membrane and triggers Ca²⁺ wave in egg. Polyspermy blocked by cortical vesicle release from egg. Fertilisation cone forms around sperm head. Movement and fusion of pronuclei (assisted by microtubules from centrosome associated with male pronucleus).

Acrosome membrane fuses with overlying plasma membrane, enzymes released allowing passage through zona pellucida e.g. hyaluronidase (dissolves intercellular matrix between cumulus cells, and other enzymes dissolve zona pellucida). Sperm finish its journey between the ZP and oocyte membrane, sperm enveloped, fusion occurs. Large increase in intracellular Ca²⁺ = wave across egg.

Question 8

How do the sperm and egg find each other?

How do the cells fuse with each other in a spp specific way?

How is the number of cells fusing restricted?

Answer 8

Sperm attracted to egg via chemotaxis (only in mature eggs and sperm)

Mammals: glycoprotein ZP3 in zona pellucida binds to β1,4 galactosyl transferase receptor on the sperm plasma membrane. Capacitated sperm are spp specific in their binding to ZP3, which trippers change in Ca²⁺ and pH in sperm and acrosome reaction

Production of second messengers on sperm/egg binding triggers polyspermy blocking. Slow block (release of Ca²⁺ wave triggers cortical granule release and activation of cell division). Zona pellucida hardens and stops further sperm entry, sperm receptors removed

Question 9

What does the Ca²⁺ wave cause?

TThe fertilised ovum/zygote starts to travel down the oviduct and starts to divide. Describe the divisions.

What is ‘hatching’?

Answer 9

Slow block to polyspermy, formation of hyaline layer, stimulation of protein synthesis, DNA replication and cytoplasmic movements of morphogenetic material.

Undergoes cleavage to form 2 cells. At 16-32 cell stage there is polarisation of 2 cell populations: outer cells form one population (trophoblast precursors) and inner cells are puriblast cells. This is called a MORULA. 32-64 cell stage (4-5d), polarised cells start differentiating into different cell types: blastocyst.

Around day 5 the blastocyst frees itself from zona pellucida via series of expansion-contraction cycles and enzymes that dissolve ZP.

It attaches to uterine wall around day 7-9.