9&10. Testis and spermatogenesis Flashcards
Functions of the testis
The testis has two main products: spermatozoa and hormones
Manufacture of these products occurs in discrete compartments
Production of spermatozoa is complex and highly orchestrated process
A number of measurable parameters may correlate with the function of spermatozoa
Compartments of the testis
Seminiferous tubules within which spermatogenesis occurs
Vascularised stroma containing Leydig cells
Hormones from the testis
Most important hormones are androgens in maintaining reproductive and sexual function
Testosterone synthesised from acetate and cholesterol by Leydig cells
What happens to testosterone?
4 – 10 mg testosterone secreted daily
Principally into blood vessels but also lymph (and lymphatic transport to other structures probably important)
Some testosterone passes through to seminiferous tubules (lipid soluble)
Converted to dihydrotestosterone by 5a-reductase in Sertoli cells
Androgens are required for spermatogenesis
The testis does not function in isolation…
Pituitary control
Production of androgens and spermatozoa related functionally
At puberty, androgens rise and spermatogenesis commences
Removal of pituitary (hypophysectomy) causes testes to shrink and spermatogenesis to arrest
LH stimulates Leydig cells to produce androgens (which are required for spermatogenesis)
FSH stimulates Sertoli cells and is required for spermatogenesis
Seminiferous tubules
Surrounded by myoid cells
Then a layer of basement membrane
Sertoli cells and spermatogenic cells within the tubules
Physiological barrier formed by gap- and tight-junctioned complexes between Sertoli cells
This creates a basal compartment containing spermatogonia, whilst spermatocytes,spermatids and spermatazoa are in a separate adluminal compartment
Spermatogenesis in three acts
Three elements:
- Mitotic proliferation to produce lots of cells
- Meiotic division to generate genetic diversity
- Cell modelling to package chromosomes for delivery to the oocyte
Large numbers of spermatozoa are produced
300 to 600 per gram of testis per second….
Spermatogenesis 1: mitosis
Germ cells of immature testis (prospermatogonia) are reactivated at puberty to undergo rounds of mitosis in the basal compartment of the tubule
From this self regenerating population emerge groups of cells called A1 spermatogonia which undergo a series of divisions to form a clone of cells
Finally after the last round of division, the clone divide to form resting primary spermatocytes
Within this mitotic phase of division, although nuclear division is completed, cytoplasmic division is not, so all of the primary spermatocytes resulting from the division of a spermatogonium are linked by cytoplasmic bridges
Spermatogenesis 2: meiosis
Resting primary spermatocytes push through sertoli cell junctions into adluminal compartment
Enter meiotic prophase
Paired homologous chromosomes form contacts at pachytene, break, swap segments and rejoin
Very sensitive to damage at this time
First division ends with separation of homologous chromosomes to opposites ends of the meiotic spindle, cytoplasm divides forming short-lived secondary spermatocytes
These quickly divide to form haploid spermatids
Spermatogenesis 3: Packaging
Cytoplasmic remodelling of spermatid
5: Tail for forward propulsion
4: Midpiece with mitochondria for energy
3: Nucleus with packaged chromosomes
2: Cap region forms for sperm-oocyte fusion
1: Acrosome forms to penetrate oocyte
A small residual body is the dustbin for unwanted cytoplasm, later eaten by sertoli cell
Organisation of spermatogenesis
Unlike ovulation, which is a regular but infrequent event, spermatogenesis is continuous.
How is this complex process organised in space and time?
The spermatogenic cycle
We considered generation of sperm from a single spermatogonium
Once this process has started, new stem cells at the same location don’t start generation of clones again for a few days
The interval is constant at around 16 days, the process by which the stem cell population controls, or is controlled is unknown
The time for completion of spermatogenesis is 64 days, so there are four successive sets of clonal development (at four separate stages of the process) in one place at one time – and that’s what we see when we look down the microscope
Cycle control
If all spermatogonia were activated on 11th birthday, you would only produce mature spermatozoa every 16 days.
Result: episodic fertility
If the spermatogonia were activated randomly then continuous production could occur
Infact, small regions seem to be activated together, in wedges around the tubule
Spermatogenic wave
Like Mexican wave
If the seminiferous tubules are dissected longitudinally, adjacent synchronised clones of spermatogenesis are seen
Later events in maturation
So far:
Starting with spermatogonia
Mitosis to increase numbers
Meiosis to generate genetic diversity
Maturation to form spermatozoa
Spacial and temporal organization in testis with cycles and waves
Stimulated by androgens and under control of the pituitary
