Spermatogenesis

Question 1

Describe the testes

Answer 1

• they produce sperm and store it in the epididymis
• they produce hormones which regulate spermatogenesis
• they lie in the scrotum, outside of the body cavity (because the optimum temp for sperm production is 1.5-2.5 degrees below body temp)
• overheating of the testes reduces sperm count
• well-vascularised and well-innervated
• normal volume is 15-25ml (measured by orchidometer)

Question 2

Describe the structure of the tubules in the testes

Answer 2

A testis is 90% seminiferous tubules, the site of spermatogenesis.

They are approximately 600m long in each testis. The tubules are tightly coiled.

They lead to the epididymis, and ultimately the vas deferens

Question 3

Describe the structure of a cross-section of a seminiferous tubule.

Answer 3

There are primary germ cells, or spermatogonia on the basement membrane. A proportion of them at any given moment can commit to becoming sperm and undergo meiosis, or they can undergo mitosis and replicate themselves.

The walls of the tubule are made up of tall, columnar endothelial cells called Sertoli cells. The tight junctions between them form adluminal compartments.
This allows for a specific enclosed environment for spermatogenesis which is filled with secretions from Sertoli cells.

The spaces between the tubules are filled with blood and lymphatic vessels, Leydig cells and interstitial fluid.

Question 4

Summarise the sperm stages during spermatogenesis

Answer 4

Spermatogonia: they are diploid. They are germ cells on the basement membrane, capable of mitotic or meiotic division to produce primary spermatocytes or more spermatogonia by mitosis.

Primary spermatocytes: they are 46XY diploid. They move into the adluminal compartment and duplicate their DNA to produce sister chromatids which exchange genetic material before entering meiosis I.

Secondary spermatocytes: secondary spermatocytes have undergone meiosis I to give 23X + 23Y haploid number of chromosomes arranged as sister chromatids

Spermatids: Meiosis II occurs to give 4 haploid spermatids. Round spermatid to elongated spermatid differentiation

Spermatozoa: mature sperm extruded into the lumen

Question 5

List some differences between oogonia and spermatogonia

Answer 5

OOGONIA:

• all laid down in the foetus
• begin meiosis to make the oocyte (arrest in meiosis I)
• cannot make more oocytes by mitosis
• there is a limited supply

SPERMATOGONIA:

• spermatogonia are laid down in the foetus
• they begin meiosis to make the spermatocyte after puberty
• they divide mitotically to make more spermatogonia
• there is a lifetime supply

Question 6

Describe steroid production in the testis

Answer 6

The pituitary produces FSH and LH. The LH binds to the Leydig cells, which are outside the seminiferous tubules. They then produce testosterone. Some of the testosterone goes around the body and produces secondary effect e.g. beard/deep voice, and some acts on Sertoli cells.

Sertoli cells secrete and nurture the developing sperm. They also contain FSH receptors. The FSH causes a quantitatively normal Sertoli cell population, so without FSH Sertoli cells would die.

in both cases, the testosterone produced by the Leydig cells feedbacks negatively on the hypothalamus and pituitary

Question 7

What would be the consequence of a male taking anabolic steroids?

Answer 7

The anabolic steroids would alter his HPG axis, providing negative feedback to his pituitary.

This would cause a drop in LH and FSH production, leading to testicular atrophy.

Question 8

Describe the physiology behind an erection

Answer 8

The autonomic nervous (parasympathetic) system causes constriction of the venous return from the penis, and dilation of the arterial blood flow into the penis. The penis fills with blood and as a result causes a hydrostatic pressure, thus causing the penis to become erect.

Question 9

Describe the neural control of ejaculation

Answer 9

Once we have an erection, at the moment of ejaculation, the sympathetic nervous system causes a series of smooth muscle contractions in the epididymis, vas deferens, and various glands that move sperm into the urethra.

Expulsion of the sperm from the urethra is partly under sympathetic control, but there is a bit of somatic nervous system control in that too.

Question 10

What does the seminal fluid consist of?

Answer 10

• seminal vesicles
• prostate
• bulbourethral gland combined with epididymal fluid

Question 11

What does the bulbourethral gland do?

Answer 11

It produces a clear, viscous secretion high in salt, which is known as a pre-ejaculate.

This fluid helps lubricate the urethra for spermatozoa to pass through, neutralising the traces of acidic urine

Question 12

Describe the fluid from the seminal vesicles that contributes to the ejaculate

Answer 12

These secretions comprise 50-70% of the ejaculate. It contains proteins, enzymes, fructose, mucus, vitamin C and prostaglandins.
The high fructose concentrations provide an energy source. The high pH protects against the acidic environment of the vagina.

Question 13

Describe the fluid from the prostate that contributes to the ejaculate

Answer 13

It secretes a milky or white fluid, roughly about 30% of the seminal fluid.
It’s protein content is less than 1%, and it includes proteolytic enzymes, prostatic acid phosphatase and prostate-specific antigen which are involved in liquefaction. The high zinc concentration is 500-1000 times more than in the blood (antibacterial).

Question 14

How does an orchidometer work?

Answer 14

Put the orchidometer in your pocket, and palpate the testes with one hand and feel the orchidometer with the other until the appropriate size bead is matched

Question 15

What are spermatogonia?

Answer 15

Dipoloid cells which can reproduce themselves by mitosis

Question 16

Summarise the process of spermatogenesis

Answer 16

There is a new cycle every 16 days, so the whole process takes approximately 74 days.

1) Mitotic proliferation of spermatogonia
2) Meiosis and development of spermatocytes
3) Spermiogenesis, elongation, loss of cytoplasm, movement of cellular contents

Each cell division from a spermatogonium to a spermatid is incomplete - the cells remain connected to one another by cytoplasmic bridges, forming a syncytium allowing synchronous development.

Question 17

Describe why the structure of the sperm is useful for its function

Answer 17

If you look at the sperm, it has virtually no cytoplasm. All it has is DNA. It is the smallest cell in the body, and the fastest. All the molecular machinery, the cell division, all the mitochondria, are all from the mother in the egg.

The sperm doesn’t provide any mtDNA, any ER, or any cellular machinery, just DNA. The ribosomes, the mtDNA, the cytoplasm, is all in the egg, which is the biggest cell in the body. In front of the DNA, there is a bag of enzymes. Invagination of the membrane contains enzymes which can cut through the outer layers of the egg to get in. As the sperm approaches the egg, the bag on the top of the head ruptures, and the sperm cuts through the outer layers of the egg. This is called the acrosome. Behind the head of the sperm there is a mitochondrial sheath which contains lots of mitochondria. The connecting piece severs and only the head enters the egg, or if the tail does get in the egg the egg destroys the tail and just takes the DNA, which is why we have no mtDNA from men. The tail piece propels the sperm along. It contains the axoneme, which runs all the way down the sperm.