REPRO: Spermatogenesis Flashcards
Spermatogenesis
Site of sperm storage
formation of haploid sperm from primordial germ cells in the seminiferous tubules of testes
epididymis
Briefly, describe the testes.
- they produce sperm and store it
- they produce hormones that regulate spermatogenesis
- they are well-vascularised and well-innervated
- the normal volume in the testes is approximately 15-25 ml (measured by an orchidometer)
Why do testes lie outside the body cavity?
1.5-2ºC below body temperature for sperm development because overheating reduces sperm count
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 vas deferens.
A testis is 90% seminiferous tubules.
Describe the structure of a cross-section of a seminiferous tubule.
There are spermatogonia on the basement membrane.
The walls of the tubule are lined with Sertoli cells(tall, columnar endothelial 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.
What do the regions between seminiferous tubules contain?
Define Spermatogonia
Leydig cells, blood vessels, lymphatics, interstitial fluid
diploid primary germ cells in the basement membrane of seminiferous tubules which give rise to spermatocytes by mitosis
Leydig cells
A cell that produces testosterone and other androgens and is located between the seminiferous tubules of the testes.
Fate of spermatogonia
a) Commit to becoming sperm and undergo two meiotic divisions (after becoming primary spermatocytes) to become mature sperm
b) Divide by mitosis and replenish themselves
What initiates sperm production?
The function of Sertoli cells?
testosterone
The cell which regulates sperm development
Describe the making of sperm, from spermatogonium to sperm.
Every 16 days, sperm are being produced in the seminiferous tubule.
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 a continuous supply
mature between Sertoli cells, moving slowly towards the central lumen where they finish development, and then to the epididymis for storage
the first step of sperm stages during spermatogenesis
1) Spermatogonia (diploid)
- type Ad spermatogonia divide mitotically, maintaining a population of spermatogonia
- or divide mitotically to produce type Ap spermatogonia, which then differentiate to type B spermatogonia and subsequently primary spermatocytes (which then undergo meiosis)
the second step of sperm stages during spermatogenesis (primary spermatocyte)
2) Primary Spermatocytes (46XY diploid)
- move into the adluminal compartment & duplicate their DNA producing identical sister chromatids
- crossing over between non-sister chromatids
- undergo meiosis I to produce secondary spermatocytes
the last three stages of sperm stages during spermatogenesis
3) Secondary Spermatocytes (23X+23Y)
- haploid number of chromosomes arranged as chromatids
- undergo meiosis II to produce 4 haploid spermatids
4) Spermatids
- round spermatids differentiate into elongated spermatids
5) Spermatozoa
- mature sperm extrude into the lumen of seminiferous tubule
Spermiogenesis
differentiation of spermatids into spermatozoa
Describe steroid production in the testis.
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 convert 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.
more steroids (testosterone) causes more negative feedback
>less FSH/LH released from the pituitary
>Sertoli cells need FSH to survive, therefore there is Sertoli cell death and testicular atrophy
Effect of the parasympathetic nervous system on male gonads
cardiovascular and neural control of an erection
- constricts venous drainage from the penis
- vasodilation of arterial blood flow/corpus cavernosum to the penis
- penis becomes full of blood
- hydrostatic pressure causes the penis to be ERECT
Erections are under parasympathetic control.
Effect of the sympathetic nervous system on male gonads
Under sympathetic control:
- causes smooth muscle contractions for sperm movement into the epididymis, vas deferens, penile urethra for EJACULATION (emission)
Effect of the somatic nervous system on male gonads
some somatic nervous control in synergy with sympathetic nervous control during ejaculation,
*perineal branch of the pudendal nerve from nerve roots S2-S
Expand on some statistics for the ejaculate.
There are 300 million sperm produced per day on average. 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.
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 contribute to the ejaculate.
These secretions comprise 50-70% of the ejaculate
- contains proteins, enzymes, mucus, vitamin C and prostaglandins.
- The high fructose concentrations provide an energy source.
- The high pH protects against the acidic environment of the vagina.
Describe the fluid from the prostate that contributes to the ejaculate.
- 30% of ejaculate
- milky/white fluid with <1% protein content including proteolytic enzymes, prostatic acid phosphatase and prostate-specific antigen involved in liquefaction after initial viscous sperm deposits near the cervix
- high zinc concentration is antibacterial
Structure of sperm
- no cytoplasm
- smallest cell in the body
- only provides DNA
- acrosome (invaginated membrane) containing digestive enzymes
- mitochondrial sheath- contains a lot of mitochondria for energy
- axoneme- microtubule-based cytoskeleton in flagella
