T2 L9 The Short but happy life of a sperm Flashcards
What are the functions of the testes?
Production of spermatozoa and hormones
These occur in discrete compartments
Production of spermatozoa is complex and highly orchestrated process
A number of measurable parameters may correlate with the function of spermatozoa
What are the compartments of the testis?
Seminiferous tubules within which spermatogenesis occurs
Vascularised stroma containing Leydig cells
What hormones are released from the testes?
- Most important hormones are androgens in maintaining reproductive and sexual function. Androgens are required for spermatogenesis.
- Testosterone synthesised from acetate and cholesterol by Leydig cells
How much testosterone is secreted daily in a mature male?
4 – 10 mg testosterone secreted daily
Mainly into blood vessels but also lymph (and lymphatic transport to other structures probably important)
What is testosterone converted to and where does this conversion take place?
Converted to dihydrotestosterone by 5a-reductase
In the Sertoli cells
Where does some of the testosterone end up in?
Seminiferous tubules
How does the pituitary gland control hormone production in the testes?
- LH acts on the Leydig cell to produce testosterone
- FSH acts on the Sertoli cells to start spermatogenesis
NOTE: Testosterone is needed for spermatogenesis to occur.
Describe the structure of the seminiferous tubules
Surrounded by myoid cells (these cells have a slight capacity to contract).
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. Creates a basal compartment containing spermatogonia, whilst spermatocytes, spermatids and spermatazoa are in a separate adluminal compartment
What are the 3 acts of spermatogenesis?
- Mitotic proliferation to produce lots of cells
- Meiotic division to generate genetic diversity
- Cell modelling to package chromosomes for delivery to the oocyte
How much spermatozoa is produced per second?
300 to 600 per gram of testis per second
What is the 1st stage of spermatogenesis? Describe what happens
MITOSIS
Germ cells of immature testis (prospermatogonia) are reactivated at puberty to undergo rounds of mitosis in the basal compartment of the tubule
A1 spermatogonia comes from this population, 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
What is the 2nd stage of spermatogenesis? Describe what happens
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
What is the 3rd stage of spermatogenesis? Describe what happens
CYTOPLASMIC REMODELLING OF SPERMATID
1: Acrosome forms to penetrate oocyte
A small residual body is the dustbin for unwanted cytoplasm, later eaten by sertoli cell
2: Cap region forms for sperm-oocyte fusion
3: Nucleus with packaged chromosomes
4: Midpiece with mitochondria for energy
5: Tail for forward propulsion
How does spermatogenesis compare to ovulation?
Unlike ovulation which is periodical, spermatogenesis is a continuous process
How long does the spermatogenic cycle last for?
Once spermatogenesis 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. Mature spermatozoa is produced every 16 days.
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
Where does a single generation of sperm originate from?
A spermatogonium
