Spermatogenesis Flashcards
What are the roles and structure of the testes?
Produce sperm and store it.
Produce hormones that regulate spermatogenesis.
Lie in scrotum outside body cavity….optimum temperature for sperm production 1.5-2.5oC below the body.
Overheating of testes reduces sperm count.
Well-vascularised, well-innervated.
The testis is 90% seminiferous tubules, site of spermatogenesis.
600m long in each testis! Tubules are tightly coiled.
The epididymis stores the sperm for a while so they can be ejaculated through the vas deferens
Tubules all lead to the epididymis and ultimately vas deferens.
How is sperm made?
The lobules drain into the rete testis, then the epididymis so they can be ejaculated through the vas deferens
If we take a section of the seminiferous tubules we see that the sperm start off on the edge and then by the time they mature they are closer towards the lumen and then they move along the lumen all the way to the rete testis and epididymis
We can also see in the cross section that the spermatogonia on the very outside are actually diploid cells
And so what happens is that a proportion of them at a time commit to meiosis instead of mitosis and at that point they become primary spermatocytes
They undergo meiosis and the two meiotic divisions become haploid and develop into sperm
So they are primary spermatocytes when they enter meiosis one
Then secondary spermatocytes after one when they enter meiosis two
After meiosis two they are fully haploid spermatids and finally develop into spermatozoa
This process is kind of mediated by the Sertoli cells which surround the developing sperm and form a little compartment to secrete scroll factors and adhesion molecules
What would a microscopic cross section of a testis show?
Sertoli cells, tight junctions & adluminal compartment
Primary germ cells or spermatogonia on the basement membrane
Walls of tubule made up of tall columnar endothelial cells Sertoli cells. Tight junctions between these form Adluminal compartment.
Allows specific enclosed environment for spermatogenesis which is filled with secretions from Sertoli cells.
Spaces between the tubules are filled with blood and lymphatic vessels, Leydig cells and interstitial fluid.
Remember the Leydig cells are responsible for secreting androgens and testosterone
What are the 5 stages of the sperm during spermatogenesis?
Spermatogonia:
- Germ cell on basement membrane, capable of mitotic or meiotic division to produce primary spermatocytes or more spermatogonia by mitosis.
- They are diploid.
- The dark ones undergo mitosis
- Some become pale and commit to meiosis
Primary spermatocytes:
- They move into the adluminal compartment and duplicate their DNA to produce sister chromatids which exchange genetic material before entering meiosis I.
- They are 46XY diploid.
Secondary spermatocytes:
- Secondary spermatocyctes have undergone meiosis I to give 23X + 23Y haploid number of chromosomes arranged as sister chromatids.
Spermatids:
- Meiosis II occurs to give 4 haploid spermatids.
- Round spermatid to elongated spermatid differentiation.
Spermatozoa:
- Lose cytoplasm and develop a tail
- Mature sperm extruded into the lumen.
How often does spermatogenesis and its different stages happen?
New cycle every 16 days, entire process takes approximately 74 days.
1. Mitotic proliferation of spermatogonia.
2. Meiosis and development of spermatocytes.
3. Spermiogenesis, elongation, loss of cytoplasm, movement of cellular contents.
Each cell division from a spermatogonium to a spermatid is incomplete - the cells remain connected to one another by cytoplasmic bridges forming a syncytium allowing synchronous development.
Syncytia at various stages of development throughout seminiferous tubule giving continuous supply.
How does the HPG axis contribute to spermatogenesis?
In the male its not cyclical process like in the female, much simpler
Mainly testosterone and a little DHT which feeds back negatively in a steady state
How are steroids produced in the testis?
Leydig cells contain LH receptors and primarily convert cholesterol into androgens. Intra-testicular testosterone levels are 100x those in plasma.
Testosterone cross over to and stimulate Sertoli cell function and thereby control spermatogenesis.
Sertoli cells contain FSH receptors and converts androgens to oestrogen.
FSH establishes a quantitatively normal Sertoli cell population.
Androgen initiates and maintains sperm production.
Interfering with negative feedback
Anabolic steroids are a testosterone based substance so increase negative feedback stopping GnRH and LH and FSH leading to testicular atrophy
What is an erection and how do the different PNSs control it?
Vasodilation of the corpus cavernosum. Partial constriction of the venous return.
Autonomic nervous system causes co-ordinated smooth muscle contractions of vas deferens, glands and urethra.
Parasympathetic control:
- Erection
Sympathetic nervous system control:
- Movement of sperm into epididymis, vas deferens, penile urethra (emission)
Sympathetic and a little somatic nervous system (perineal branch of the pudendal nerve from nerve roots S2–S4)
- Expulsion of the glandular secretions
- Evacuation of urethra.
How does the bulbo-urethral gland contribute to seminal fluid?
Produces a clear viscous secretion high in salt, known as pre-ejaculate. This fluid helps to lubricate the urethra for spermatozoa to pass through, neutralizing traces of acidic urine.
This prevents the sperm from being damaged
How do the seminal vesicles contribute to seminal fluid?
As the sperm come past the seminal vesicles contract and secrete
Secretions comprise 50-70% of the ejaculate.
Contains proteins, enzymes, fructose, mucus, vitamin C and prostaglandins.
High fructose concentrations provide energy source.
High pH protects against acidic environment in vagina.
How does the prostate contribute to the seminal fluid?
Sperm passes prostate
Secretes milky or white fluid roughly 30% of the seminal fluid.
Protein content is less than 1% and includes proteolytic enzymes, prostatic acid phosphatase and prostate-specific antigen which are involved in liquefaction.
High zinc concentration 500–1,000 times that in the blood is antibacterial.
What are oligospermia and aspermia?
Oligospermia is when you have less than 15million per ml
Aspermia is having no sperm
What are the essential components of a single sperm?
The nucleus contains all the DNA
Virtually no cytoplasm
At the front of the sperm there is an invagination of the membrane called the acrosome
The acrosome is full of enzymes so that when the sperm makes contact with the egg the enzymes are released and they cut through the egg’s outer layers and allow some adhesion molecules on the inner acrosomal membrane to bind to the egg
Behind the head of the sperm is the mitochondrial sheath which acts as a sort of engine for the sperm
The tail is made up of an arrangement of 9 pairs of fibres that run longitudinally all the way; its called the axoneme
So you can see the cross section below
The way it works is that the fibres using the energy of the ATP from the mitochondria slide against each other making the tail thrash around
Check diagram to make sure you can label one
