L9 - The short but happy life of a sperm Flashcards
Layers of the testes
- The multi-layered tunica covers the testes, It facilitate blood supply to the testes and creates a partition between sperm producing regions of the testes. There are three layers to the tunica, the tunica vasculosa, tunica albuginea and tunica vaginalis
Two main products of the testis
- Spermatozoa
- Hormones
Compartments of the testis
- Seminiferous tubules within which spermatogenesis occurs
- Vascularised stroma containing leydig cells
What is testosterone synthesised from
- Acetate
What is cholesterol produced by
- Leydig cells
How much testosterone secreted daily
4-10 mg
Where does some testosterone also leak into
- Some testosterone passes through to seminiferous tubules (lipid soluble)
What is testosterone converted to
- Converted to dihydrotestosterone by 5a-reductase in sertoli cells
What hormones are required for spermatogenesis
- Androgens
At what stage does androgen production increase and therefore initiates spermatogenesis
- At puberty, androgens rise and spermatogenesis commences
Effect of removal of the pituitary on spermatogenesis
- Removal of pituitary (hypophysectomy) causes testes to shrink and spermatogenesis to arrest
Effect of LH on leydig cells
- LH stimulates leydig cells to produce androgens (which are required for spermatogenesis)
Effect of FSH on sertoli cells
- FSH stimulates sertoli cells and is required for spermatogenesis
What are seminiferous tubules surrounded by
- Myoid cells, then a layer of basement membrane
What type of cells are located within the seminiferous tubules
- Sertoli cells and spermatogenic cells
What forms a physiological barrier between sertoli cells
- Physiological barrier formed by gap and tight junctioned complexes between sertoli cells
- This creates a basal compartment containing spermatogonia, whist spermatocytes, spermatids and spermatazoa are in a separate adluminal compartment
Spermatogenesis summary
- 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
When are germ cells of immature testis activated
- Germ cells of immature testis (prosperatogonia) are reactivated at puberty to undergo rounds of mitosis in the basal compartment of the tubule
What type of cells emerge as a result of activation of prospermatogonia
- From this self regenerating population emerge groups of cells called A1 spermatogonia which undergo a series of divisions to form a clone of cells
What happens after A1 spermatogonia finish dividing
- After the last round of division, the clone divide to form resting primary spermatocytes
What occurs during the formation of 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
Where do the resting primary spermatocytes push through to initiate the meiotic prophase
- Resting primary spermatocytes push through sertoli cells junctions into adluminal compartment
What happens during meiotic prophase
- Paired homologous chromosomes form contacts at pachytene, break, swap segments and rejoin
- Very sensitive to damage at this time
What does the first division of meiosis end with
- 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
Parts of the sperm cell
- Tail for forward propulsion
- Midpiece with mitochondria for energy
- Nucleus with packaged chromosomes
- Cap region forms for sperm-oocyte fusion
- Acrosome forms to penetrate oocyte
What is the small residual body of the dustbin for?
- A small residual body is the dustbin for unwanted cytoplasm, later eaten by sertoli cell
Is spermatogenesis continuous or periodic?
- Unlike ovulation, which is a regular but infrequent event, spermatogenesis is continuous
Features of the spermatogenic cycle
- 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
What is the time for completion of spermatogenesis?
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
What does the spermatogenic wave refer to
- If the seminiferous tubules are dissected longitudinally, adjacent synchronised clones of spermatogenesis are seen
Where do spermatozoa wash into
- Rete testis through the vasa efferentia into the epididymis where fluid is absorbed and sperm is concentrated
Development of sperm movements
- In the rete testis, they can twitch
- By the time they reach the cauda epididymis, they can swim
- This process is dependent on androgen stimulation
Components of semen
- Spermatozoa mixed with secretions from seminiferous tubules, epididymis etc
- Addition of secretions from prostate, seminal vesicles and bulbourethral glands at time of ejaculation
Specific cellular components of semen
- Spermatozoa
- Epithelial cells from tract
- Spermatogenic cells
- Leucocytes - risk of HIV etc
Role of fluid components of semen
- Not essential for fertilisation
- Provide a fluid vehicle for spermatozoa
Nutrition(fructose, sorbitol)
Buffer (to protect against vaginal acidity)
Antioxidants (ascorbic acid, hypotaurine)
What does the endocervix do
- Secretes mucus with cyclical variation
- Macromolecular network of mucin fibrils - guiding spermatozoa?
- Oestrogen stimulates watery mucus
- Progesterone inhibits secretory activity
When can sperm penetrate the endocervix
- Sperm can penetrate from day 9, peak at time of ovulation
What does the endocervix offer sperm
- Receptive to sperm at time of ovulation, interference at other times
- Protection from hostile vagina, and from being phagocytosed
- Supplementation of energy requirements
- Sperm selection by differential motility and morphology
- Short term reservoir within endocervical crypts
- Initiation of the next stage in sperm maturation: ‘capacitiation’
What is capacitation
Sperm recovered at ejaculation don’t fertilise ova in vitro immediately
Those from the uterus will
Have undergone capacitiation
Stripping of glycoprotein from sperm surface which accumulates in the epididymis
Causes hyperactive motility – ‘whiplash’
And make sperm responsive to signals from oocyte where we end our journey
What can cervical mucus be tested for
- Three properties
- Consistency (watery or viscous)
- Spinnbarkeit (means elasticity, stickiness)
- Ferning (crystalisation on a glass surface)
• These are crude assesments of a complex physiological situation
• Detailed testing can follow
Eg Looking at spermatozoa penetrating mucus and assessing their motility
What might a high ejaculation volume reflect
- Abstinence or accessory gland inflammation
What is sperm concentration defined as
- Number of sperm per ml in the total ejaculate
- Normal is over 15 million per ml
- Vitality: 58% or more live spermatozoa
Definition of sperm motility
· Defined as percentage of progressively motile sperm in the ejaculate
· Progressively motile means they go somewhere, rather than swim around in circles
Lower limit cut off for progressive motility
· WHO uses 32% as the cut off for the lower limit of normal for progressive motility
· Variation in repeat samples from individuals and poor correlation with fertility
Def. normozoospermia
- Normal values
Def. oligozoospermia
- Low concentration
Def. asthenozoospermia
- Too little motility
Def. teratozoospermia
- Too many abnormals
Def. oligoasthenoterato-zoospermia
- Mixture of the three
Def. azoospermia
- No spermatozoa
Def. aspermia
- No ejaculate
