REPRO: Spermatogenesis Flashcards
Briefly, describe the testes.
- they produce sperm and store it
- they produce hormones which regulate spermatogenesis
- they lie in the scrotum, outside of the body cavity (because the optimum temperature for sperm production is 1.5°C-2.5°C below body temperature)
- overheating of the testes reduces sperm count
- they are well-vascularised and well-innervated
- the normal volume in the testes is approximately 15-25 ml (measured by an orchidometer)
Describe the structure of the tubules in the testes.
The tubules lead to an area on one side called rete. The rete leads to the epididymis and ultimately vas deferens.
A testis is 90% seminiferous tubules, the site of spermatogenesis. They are approximately 600m long in each testis. The tubules are tightly coiled.
Describe the structure of a cross-section of a seminiferous tubule.
There are primary germ cells, or spermatogonia on the basement membrane.
The walls of the tubule are made up of tall, columnar endothelial cells called Sertoli cells. The tight junctions between them form adluminal compartments.
This allows for a specific enclosed environment for spermatogenesis which is filled with secretions from Sertoli cells.
The spaces between the tubules are filled with blood and lymphatic vessels, Leydig cells and interstitial fluid.
What are the sperm stages during spermatogenesis and give a brief description.
Spermatogenesis: Germ cell on basement membrane, capable of mitotic and meiotic division. Meiotic division produces primary spermatocytes (pale) or mitotic division produces more spermatogonia (dark). They are diploid.
Primary spermatocytes: They move into the adluminal compartment and duplicate their DNA to produce sister chromatids which exchange genetic material before entering meiosis 1. They are 46XY diploids.
Secondary spermatocytes: Secondary spermatocytes have undergone meiosis 1 to give 23X + 23Y haploid number of chromosomes arranged as sister chromatids slightly different to each other due to crossing over in meiosis 1.
Spermatids: Meiosis 2 occurs where the sister chromatids separate to give 4 haploid spermatids. Round spermatid to elongated spermatid differentiation.
Spermatozoa: Mature sperm extruded into the lumen.
Spermatogonia –> Primary spermatocytes –> Secondary spermatocytes –> Spermatids –> Spermatozoa
Describe the making of sperm, from spermatogonium to sperm.
Every 16 days, there is an area of the seminiferous tubules that starts producing sperm, thus giving us a wave of sperm being made constantly.
There is a new cycle every 16 days, and the entire process takes approximately 74 days.
- MITOTIC proliferation of spermatogonia.
- MEIOSIS and development of spermatocytes.
- SPERMATOGENESIS, 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 for synchronous development.
There are syncytia at various stages of development throughout the seminiferous tubule giving continuous supply.
How is the Hypothalamic Pituitary Gonadal Axis regulated in the process of spermatogenesis?
Hypothalamus –> GnRH –> Pituitary –> LH + FSH –> Testis –> PRODUCE Testosterone + DHT —-> NF to hypothalamus and pituitary to limit its own production.
Describe steroid production in the testis.
GnRH stimulates pituitary to produce LH and FSH.
Leydig cells contain LH receptors and primarily convert cholesterol into androgens. Intra-testicular testosterone levels are 100x higher than those in the plasma.
Androgens cross over 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, whereas androgen initiates and maintains sperm production.
List some differences between oogonia and spermatogonia.
OOGONIA:
- oogonia are all laid down in the foetus
- they begin meiosis to make the oocyte
- cannot make more oocytes by mitosis
- there is a limited supply
SPERMATOGONIA:
- spermatogonia are laid down in the foetus
- they begin meiosis to make the spermatocyte (or…)
- they divide mitotically to make more spermatogonia
- there is a (theoretically) lifetime supply
What would be the consequence of a male taking anabolic steroids?
The anabolic steroids would alter his HPG axis, providing negative feedback to his pituitary. This would reduce LH and FSH production, leading to testicular atrophy as he loses his sertoli cells.
Describe the cardiovascular and neural control of an erection.
There is vasodilation of the corpus cavernosum, and partial constriction of the venous return to the penis. This causes blood to rush in, causing the erection.
Erections are under parasympathetic control.
Describe the neural control of ejaculation.
The autonomic nervous system causes coordinated smooth muscle contractions of the vas deferens, glands and urethra.
Under parasympathetic control:
- erection
Under sympathetic control:
- movement of sperm into epididymis, vas deferens, penile urethra (emission)
Under somatic control (perineal branch of the pudendal nerve from nerve roots S2-S4):
- expulsion of the glandular secretions and evacuation of the urethra
Pneumonic: Point shoot and score. - Para, sympa, somatic NS.
Expand on some statistics for the ejaculate.
There are 300 million sperm produced per day on average (9mil per hour). There is approximately 120million in average ejaculate. The normal ejaculate volume is 1.5ml - 6ml.
The initial portion of the ejaculate is the most sperm-rich. 99.9% is lost before reaching the ampulla of the uterine tube. Around 120,000 sperm get near to the egg, only one enters.
What is the seminal fluid made of?
The seminal fluid consists of secretions from:
- seminal vesicles
- prostate
- bulbourethral gland combined with epididymal fluid.
Describe the fluid from the bulbourethral gland that contributes to the ejaculate.
It produces a clear, viscous secretion high in salt, known as pre-ejaculate. This fluid helps lubricate the urethra for spermatozoa to pass through, neutralising the traces of acidic urine.
Describe the fluid from the seminal vesicles that contributes to the ejaculate.
These secretions comprise 50-70% of the ejaculate. It contains proteins, enzymes, fructose, mucus, vitamin C and prostaglandins.
The high fructose concentrations provide an energy source. The high pH protects against the acidic environment of the vagina.
