Spermatogenesis

Question 1

What is spermatogenesis

Answer 1

the production of spermatozoa (mature male gametes) from spermatogonial stem cells.

Question 2

Describe the male reproductive system

Answer 2

Testes - Houses seminiferous tubules; site of spermatogenesis.

Epididymis – Sperm storage and maturation.

Vas deferens - Transport of sperm from epididymis to urethra during ejaculation.

Seminal vesicle - Produces a mucus secretion which aids the mobility of sperm.

Prostate gland - Produces an alkaline secretion that neutralises acidity of any urine in the urethra and aids the mobility of sperm.

Urethra - Tube that carries urine and sperm out of the body.

Question 3

Function of the testes

Answer 3

To produce androgens and other hormones for sexual differentiation and secondary sexual characteristics

To produce spermatozoa for sexual reproduction: occurs in the seminiferous tubules with maturation in the epididymis

Question 4

Site of spermatogenesis

Answer 4

Coiled tubules are lined with the seminiferous epithelium. This is the site of spermatogenesis

Question 5

What does stroma consist of?

Answer 5

Stroma consisting of blood vessels, lymph and Leydig cells. These Leydig cells synthesize and secrete the steroid hormones.

Question 6

Primordial germ cells

Answer 6

Primordial germ cells (PGCs) are the gamete precursors. They are first identifiable at about three weeks gestation

The PGC population expands by mitosis and migrates to the genital ridge primordium by six weeks gestation

Question 7

What does the germinal epithelium become?

Answer 7

A second set of cells also migrates in: the germinal epithelium which will eventually become Sertoli cells (male) and the Granulosa cells (female)

Question 8

Give brief overview of spermatogenesis

Answer 8

Begins at puberty; about 100 million produced per day

Involves mitosis and meiosis

Final differentiation step is called spermiogenesis

Generates four mature spermatozoa – identical in size but not genetically

Question 9

What is the fate of spermatogonial stem cells (SSCs)

Answer 9

SSCs self-regenerating pool undergo rounds of mitosis

At intervals, groups of morphologically distinct cells emerge: type A spermatogonia

Produce a clone of 16 cells which enter further rounds of mitosis with some differentiation in between

This produces type B spermatogonia which will go on to become primary spermatocytes

Divide or differentiate? Growth factors

Question 10

What direction does development occur in?

Answer 10

Centripetal direction

Question 11

What is spermiation

Answer 11

Spermiation: fully differentiated sperm released into the lumen

Question 12

Describe sperm morphology

Answer 12

Acrosome cap; important for fertilisation

Mid-piece contains the mitochondria

Central axoneme made up of bundles of fibres

Tail required for motility

Question 13

Describe spermiogenesis

Answer 13

Golgi apparatus forms the acrosome cap

One of the centrioles of the spermatid elongates to become the tail

Nucleus changes shape to fit into the sperm head

The remaining cytoplasm and organelles (residual body) are removed by Sertoli cells via phagocytosis

See diagram on powerpoint

Question 14

Chromosome remodelling

Answer 14

X and Y chromosome transcription stops before meiotic divisions

Autosomal transcriptional activity ceases later, during spermiogenesis

Massive repackaging of the DNA: histones are replaced by protamines; tightly compressed chromatin with no gene expression

Question 15

Sertoli (nurse) cells

Answer 15

see powerpoint…

Question 16

Describe Sertoli cells

Answer 16

Maintain close contact with each other via ‘tight junctional complexes’. Form a barrier to macromolecules called the blood-testis barrier. This separates testes into basal and adluminal compartments

Question 17

When do Spermatogonia become spermatocytes?

Answer 17

Spermatogonia become spermatocytes once they enter meiosis and move away from the basement membrane. They cross into the adluminal compartment where they receive all their nutrients from Sertoli cells

Question 18

What is the Adluminal compartment?

Answer 18

Adluminal compartment is an ‘immune-privileged site’ protecting the haploid cells from potential immune rejection

Question 19

Duration of spermatogenic cycle

Answer 19

At any given time there are four clones of developing sperm cells (within a region of seminiferous epithelium) at different stages of maturation. So every section of seminiferous tubule produces sperm every 16 days.

There are pools of cells at different points in the testes entering the cycle at different times to ensure a constant production of sperm

Question 20

How do Sertoli cells regulate spermatogenesis

Answer 20

There is a set lag time between development of SSC ‘families’

Transverse section through tubule will reveal populations at four distinct stages

For constant production of mature sperm, different sections of tubule must be at different stages; wave of seminiferous epithelium

High degree of spatial and temporal organisation of spermatogenesis; gap junctions between adjacent Sertoli cells provide means for communication

Communication seems to go both ways: rat SSCs transplanted into mouse seminiferous tubules imposed rat timing on spermatogenesis

Question 21

Sperm storage and maturation

Answer 21

90% fluid is absorbed in vasa efferentia; dependent on oestrogen

Passage through epididymis takes 5-11 days; sperm acquire potential to swim and to fertilise oocyte; dependent on androgens

Mature sperm are stored in the tail end of epididymis ready for ejaculation via the vas deferens

Question 22

Hormones invovled in control of spermatogenesis

Answer 22

Activin A
Inhibin B
Prolactin
Oestrogen
Oxytocin
Testosterone

Question 23

Activin A hormonal control of spermatogenesis

Answer 23

Activin A: Positive feedback on pituitary FSH production;
autocrine effects on Sertoli cells; paracrine effects on spermatogenic cells

Question 24

Inhibin B hormonal control of spermatogenesis

Answer 24

Inhibin B: Stimulatory effect on Leydig cells;
negative feedback on FSH production; acts as Activin antagonist

Question 25

Prolactin

Answer 25

Prolactin: Enhances LH stimulation of Leydig cells

Question 26

Oestrogen

Answer 26

Oestrogen: Required for fluid absorption in testis

Question 27

Oxytocin

Answer 27

Oxytocin: Stimulates seminiferous tubule motility

Question 28

Steriod conversion in testes

Answer 28

Some Testosterone and Androstenedione from Leydig cells enter the Sertoli cells

They may be converted to the more potent Dihydrotestosterone
or to the oestrogens Oestrone and Oestradiol 17β

These hormones act to stimulate the Sertoli cells but are also secreted into the blood or the tubular fluid

Question 29

Testosterone action on Sertoli cells

Answer 29

Testosterone acts on Sertoli cells where it is essential for: maintenance of the blood-testis barrier; sertoli-spermatid adhesion; spermatid elongation; spermiation; production of testicular fluid

Question 30

What is testosterone essential for?

Answer 30

Testosterone is also essential for the development of male secondary sex characteristics

Question 31

Where does testosterone travel to?

Answer 31

Testosterone travels to the tubule lumen, binds to androgen binding proteins (ABP) secreted by the Sertoli cells. Testosterone-ABP travels to and stimulates the ducts of testes

Question 32

Testosterone acts autocrinologically on?

Answer 32

Testosterone acts autocrinologically on Leydig cells in negative feedback loop

Question 33

What is Testosterone converted to in Sertoli cells?

Answer 33

Testosterone is converted within the Sertoli cells to dihydrotestosterone and oestrogen which further stimulate Sertoli cells and support spermatogenesis

Question 34

Self assessment questions

Answer 34

During spermatogenesis, which cell type are the first to become haploid?

What is the blood-testis barrier and why does it exist?

What are the key hormones acting in the male reproductive system and what are their main roles?