The short but happy life of a sperm

Question 1

Functions of the testis

Answer 1

Two main products: spermatozoa and hormones

Production of spermatozoa is complex and highly orchestrated process

Question 2

Compartments of the testis

Answer 2

Seminiferous tubules within which spermatogenesis occurs

Vascularised stroma containing leydig cells

Question 3

Hormones from the testis

Answer 3

Most important hormones are androgens in maintaining reproductive and sexual function

Testosterone synthesised from acetate and cholesterol by leydog cells

Question 4

Movement of testosterone

Answer 4

Principally into blood vessels but also lymph

Some passes through to seminiferous tubules and is then converted to dihydrotestosterone by 5a reductase in sertoli cells

Question 5

Pituitary control

Answer 5

Removal of pituitary 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 6

Seminiferous tubules

Answer 6

Surrounded by myoid cells

Then a layer of basement membrane

Sertoli cells and spermatogenesis cells within tubules

Physiological barrier formed by gap and tight junctioned complexes between sertoli cells

Question 7

Spermatogenesis in 3 acts

Answer 7

Mitotic proliferation to produce lots of cells

Meiotic division to generate genetic diversity

Cell modelling to package chromosomes for delivery to the oocyte

Question 8

Spermatogenesis 1- mitosis

Answer 8

Germ cells of immature testis 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 division to form a clone of cells

After the last round of division, the clone divide to form resting primary spermatocytes

Question 9

Spermatogenesis 2- meiosis

Answer 9

Resting primary spermatocytes push through sertoli cell junction into adluminal compartment

Enter meiotic prophase

Paired homologous chromosomes form contacts at pachytene, break, swap segments and rejoin

First division ends with separation of homologous chromosomes to opposite ends of spindle, cytoplasm divides forming short lived secondary spermatocytes

Question 10

Spermatogenesis 3- packaging

Answer 10

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 form to penetrate oocyte

Question 11

The spermatogenic cycle

Answer 11

Once the 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

Time for completion of spermatogenesis is 64 days

Question 12

Cycle control

Answer 12

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

If spermatogonia were activated randomly then continuous production could occur

Question 13

The final stages of maturation of spermatozoa

Answer 13

Spermatozoa wash into the rete 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

Process is dependent on androgen stimulation

Question 14

The components of semen

Answer 14

Spermatozoa mixed with secretions from seminiferous tubules, epididymis etc

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

Question 15

Cellular components of semen

Answer 15

Spermatozoa

Epithelial cells from tract

Spermatogenic cells

Leucocytes- risk of HIV etc

Question 16

Fluid components of semen

Answer 16

Can’t be essential for fertilisation

Provide a fluid vehicle for spermatozoa

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

Question 17

What does the endocervix do?

Answer 17

Secretes mucus with cyclical variation

Macromolecular network of mucin fibrils guiding spermatozoa

Oestrogen stimulates watery mucus

Progesterone inhibits secretory activity

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

Question 18

What does the endocervix offer sperm?

Answer 18

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 19

Capacitation

Answer 19

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

Question 20

Three properties of cervical mucus

Answer 20

Consistency (watery of viscous)

Spinnbarkeit (elasticity, stickiness)

Ferning (crystalisation on a glass surface)

Question 21

Testing cervical mucus

Answer 21

Crude assessments of a complex physiological situation

Detailed testing can follow

Question 22

Testing semen

Answer 22

Normal ejaculated volume is 1.5-6ml

1.5ml is the cut off

Question 23

Semen concentration and vitality

Answer 23

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

Question 24

Motility

Answer 24

Percentage of progressively motile sperm in the ejaculate

Progressively motile means they go somewhere rather than swim around

32% cut off for the lower limit of normal

Variation in repeat sample from individuals and poor correlation with fertility