L9 + 10 The short but happy life of a sperm

Question 1

Function of the testis

Answer 1

The testis has 2 main products: spermatozoa and hormones

Manufacture of these products occurs in discreet compartments

Production of spermatozoa is complex

A number of measurable parameters may correlate with the function of spermatozoa

Question 2

Compartments of the testis

Answer 2

Seminiferous tubules within which spermatogenesis occurs

Vascularised storm containing Leydig cells

Question 3

Where and what is testosterone synthesised by?

Answer 3

From acetate and cholesterol by Leydig cells

Question 4

How much testosterone is secreted daily?

Answer 4

4-10mg

Question 5

Where is testosterone secreted into?

Answer 5

Principally into blood vessels but also lymph (and lymphatic transport to other structures probably important)

Question 6

What does some testosterone pass through?

Answer 6

Seminiferous tubules (lipid soluble)

Question 7

What is testosterone converted into?

Answer 7

dihydrotestosterone by 5a-reductase in Sertoli cells

Question 8

What are androgens required for?

Answer 8

Spermatogenesis

Question 9

Pituitary control

Answer 9

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

FSH stimulates Sertoli cells and is required for spermatogenesis

Question 10

Seminiferous tubules

Answer 10

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 spermatozoa are in separate adluminal compartment

Question 11

Spermatogenesis in 3 acts

Answer 11

3 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 number of spermatozoa are produced

300-600 per gram of testis per second

Question 12

Spermatogenesis 1 - mitosis

Answer 12

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 divides 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

Question 13

Spermatogenesis 2 - meiosis

Answer 13

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

Question 14

Spermatogenesis 3 - packaging

Answer 14

Cytoplasmic remodelling of spermatid

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
- a small residual body is the dustbin for unwanted cytoplasm, later eaten by sertoli cell

Question 15

Organisation of spermatogenesis

Answer 15

Unlike ovulation, which is regular but infrequent event, spermatogenesis is continuous

Question 16

The spermatogenic cycle

Answer 16

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 4 successive sets of clonal development (at 4 staged of the process) in one place at one time

Question 17

Cycle control

Answer 17

If all spermatogonia were activated on 11th birthday, mature spermatozoa would be produced every 16 days

Result: episodic fertility

If the spermatogonia were activated randomly then continuous production could occur

In fact, small regions seem to be activated together, in wedges around the tubule

Question 18

The spermatogenic wave

Answer 18

If the seminiferous tubules are dissected longitudinally, adjacent synchronised clones of spermatogenesis are seen

Question 19

The final stages of maturation of spermatozoa occur elsewhere

Answer 19

Spermatozoa wash into the rate

Through the vasa efferentia

Into the epididymis where fluid is absorbed and sperm concentrated

In the rete they can twitch: by the cauda epididymis they can swim

The process is dependent on androgen stimulation

Question 20

The components of semen

Answer 20

Spermatozoa mixed with secretions from seminiferous tubules, epididymis etc

Addition of secretions from prostate, seminal vesicles and bulbourethral glands at time of ejaculation

• about 3ml in the male
• half a litre in the boar

Question 21

Cellular components

Answer 21

Spermatozoa

Epithelial cells from tract

Spermatogenic cells

Leucocytes - risk of HIV etc

Question 22

Fluid components

Answer 22

Can’t be essential for fertilisation

Provide a fluid vehicle for spermatozoa

• nutrition (fructose, sorbitol)
• buffer (to protect against vaginal acidity)
• antioxidants (absorb acid, hypotaurine)

Question 23

What does the endocervix do?

Answer 23

Secretes mucus with cyclical variation

Macromolecular network of mucin fibrils? guiding spermatozoa

Oestrogen stimulates watery mucus

Progesterone inhibits secretory activity

Spem can penetrate from day 9, peak at time of ovulation

Question 24

What does the endocervix offer sperm?

Answer 24

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: ‘capacitation’

Question 25

Capacitation

Answer 25

Sperm recovered at ejaculation don’t fertilise ova in vitro immediately

Those from the uterus will have undergone capacitation

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

Question 26

3 properties of cervical mucus

Answer 26

Consistency (watery or viscous)

Spinnbarkeit (means elasticity, stickiness)

Ferning (crystallisation on a glass surface)

Question 27

Testing cervical mucus

Answer 27

These are crude assessments of a complex physiological situation

Detailed testing can follow
e.g. looking at spermatozoa penetrating mucus and assessing their motility

Question 28

Measuring sperm

Answer 28

A number of variables may be evaluated through analysis of semen

These may correlate with fertility with varying degrees; some evidence is controversial

Specimen is obtained by masturbation, collected in a clean container - (condoms often contain spermicide)

Question 29

Volume of sperm

Answer 29

Normal ejaculated volume is 1.5 - 6ml

Volume may be low in retrograde ejaculation, high volume may reflect abstinence or accessory gland inflammation

1.5ml is the cut off(WHO 2010)

Question 30

Concentration and vitality

Answer 30

Sperm concentration, or density, defined as the number of sperm per ml in the total ejaculate

Normal is over 15 million per ml

Vitality: 58% or more live spermatozoa

Question 31

Motility of sperm

Answer 31

Defined as percentage of progressively motile sperm in the ejaculate

Progressively motile means they go somewhere, rather than swim around in circles

WHO uses 32% as the cut off for the lower limit of normal progressive motility

Variation in repeat samples from individuals and poor correlation with fertility

Question 32

Morphology of sperm

Answer 32

Visual assessment of sperm

Greater than 4% normal forms acceptable (WHO 2010)

Other more stringent criteria exist

Question 33

Normozoospermia

Answer 33

normal values

Question 34

Oligozoospermia

Answer 34

low concentration

Question 35

Athenozoospermia

Answer 35

too little motility

Question 36

Teratozoospermia

Answer 36

too many abnormals

Question 37

Oligoasthernoterto-zoospermia

Answer 37

mixture of the three

Question 38

Azoospermia

Answer 38

no spermatozoa

Question 39

Aspermia

Answer 39

no ejaculate