Spermatogenesis Flashcards
What is spermatogenesis
the production of spermatozoa (mature male gametes) from spermatogonial stem cells.
Describe the male reproductive system
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.
Function of the testes
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
Site of spermatogenesis
Coiled tubules are lined with the seminiferous epithelium. This is the site of spermatogenesis
What does stroma consist of?
Stroma consisting of blood vessels, lymph and Leydig cells. These Leydig cells synthesize and secrete the steroid hormones.
Primordial germ cells
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
What does the germinal epithelium become?
A second set of cells also migrates in: the germinal epithelium which will eventually become Sertoli cells (male) and the Granulosa cells (female)
Give brief overview of spermatogenesis
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
What is the fate of spermatogonial stem cells (SSCs)
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
What direction does development occur in?
Centripetal direction
What is spermiation
Spermiation: fully differentiated sperm released into the lumen
Describe sperm morphology
Acrosome cap; important for fertilisation
Mid-piece contains the mitochondria
Central axoneme made up of bundles of fibres
Tail required for motility
Describe spermiogenesis
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
Chromosome remodelling
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
Sertoli (nurse) cells
see powerpoint…
Describe Sertoli cells
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
When do Spermatogonia become spermatocytes?
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
What is the Adluminal compartment?
Adluminal compartment is an ‘immune-privileged site’ protecting the haploid cells from potential immune rejection
Duration of spermatogenic cycle
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
How do Sertoli cells regulate spermatogenesis
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
Sperm storage and maturation
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
Hormones invovled in control of spermatogenesis
Activin A
Inhibin B
Prolactin
Oestrogen
Oxytocin
Testosterone
Activin A hormonal control of spermatogenesis
Activin A: Positive feedback on pituitary FSH production;
autocrine effects on Sertoli cells; paracrine effects on spermatogenic cells
Inhibin B hormonal control of spermatogenesis
Inhibin B: Stimulatory effect on Leydig cells;
negative feedback on FSH production; acts as Activin antagonist
