Spermatogenesis

Question 1

How is sperm made and where does it travel- use the structure of the testis to help you answer.

Answer 1

Sperm is produced in the testis and stored in the epididymis. During ejaculation there are contractions in the vas deferens and the epididymis, powering the sperm up and out of the vas deferens. At this stage, the sperm contains the epididymal fluid as well. The sperm and fluid make its way to the vas deferens to the seminal vesicle. Secretions from the seminal vesicle are added to the sperm. This approaches the ejaculatory duct, contains fluid from Cowper’s gland too. Once this is added the ejaculate is ready.

Question 2

What happens to the corpus cavernosum and corpus spongiosum?

Answer 2

The corpus cavernosum is filled with blood when there is an erection
The corpus spongiosum is flexible (located underneath) – It keeps the urethra open to allow for the passage of semen.

Question 3

What is the function of Testes?

Answer 3

Produce sperm and store it

Produce hormones which regulate spermatogenesis

Well-vascularised, well-innervated.

Question 4

Where do the testes lie and why is this important?

Answer 4

They lie in scrotum outside body cavity….optimum temperature for sperm production 1.5-2.5oC below body. Overheating of testes reduces sperm count.

Question 5

What is the normal volume of testes?

Answer 5

15-25ml

Question 6

Testes volume is measured by….

Answer 6

orchidometer

Question 7

Describe the testicular structure

Answer 7

There are lobes containing seminiferous tubules.
About 300 lobes
The tubules are tightly coiled.
The tubules collect in the structure known as the rete testis.
From the rete testis it makes it way to the epididymis.
It is a highly convoluted structure made of three segments:
Head
Body
Tail

The epididymis leads into the vas/ductus deferens

90% of the testes is seminiferous tubules

Average testes contains about 600m of seminiferous tubules

Question 8

Structure of testis

Answer 8

Seminiferous tubules present in the lobes -> rete testes -> epididymis -> vas deferens

Question 9

Describe the structure of the seminiferous tubules

Answer 9

Cross section of seminiferous tubule
Seminiferous epithelium
Outer lining = Basal membrane / basal lamina
Then have germ cells dividing until they make spermatozoa
They divide towards the lumen.
Between each cluster of germ cells you have Sertoli cells
Sertoli cells respond to testosterone and function to regulate the process of spermatogenesis. It also provided substance for spermatogenic cells.

Question 10

Describe the finer structure of Sertoli cells

Answer 10

Area between each Sertoli cells where sperm develop, separated by tight junctions is called adluminal compartment. Sertoli cells maintain very close contact with the sperm and mediate between the endocrine system and the developing sperm, receiving signals and secreting factors that help the sperm to develop (+feedback). From primary spermatocyte, the germ cell is committed to meiosis…no going back.

Question 11

How does the regulatory process take place in the seminiferous tubules?

Answer 11

Via formation of tight junctions
There are tight junctions being formed between the Sertoli cells. The diving germ cells force their way through the junctions. The junctions form a seal around each germ cell.

Question 12

What are the tight junctions?

Answer 12

Exist between Sertoli cells forming blood-testis barrier.
Protects spermatogenic cells from a lot of toxins – most toxins cannot pass through the blood-testis barrier
Open to allow passage of spermatogonia prior to completion of meiosis.
Divides into basal and adluminal compartments.
Protects the spermatogonia from immune attack.
Allows specific enclosed environment for spermatogenesis which is filled with secretions from Sertoli cells.

Question 13

Why are the tight junctions important?

Answer 13

Vasectomy reversal failure and antisperm antibodies. Sperm leak out of the healing vas deferens.

Question 14

What lies outside the basal lamina and what happens there?

Answer 14

Outside the basal lamina is the Leydig cells which produce testosterone which crosses the basal membrane to the Sertoli cells.
The Sertoli cells respond to this to regulate the process of spermatogenesis.

Question 15

What are the five stages of spermatogenesis?

Answer 15

1) Spermatogonia
2) Primary Spermatocyte
3) Secondary Spermatocyte
4) Spermatids
5) Spermatozoa

Question 16

Spermatogonia

Answer 16

There are two populations of spermatogonia: type A and type B
Two types of type A: Type A dark (Ad) and Type A pale (Ap).
Type Ad at any given point of time is capable of undergoing mitotic divisions to replace itself OR it can undergo differentiation to form type Ap.
Type Ap undergoes further differentiation into type B spermatogonia. At this point it is committed to the differentiated pathway for spermatogenesis.
Germ cell on basement membrane
Capable of mitotic or meiotic division to produce primary spermatocytes or more spermatogonia by mitosis
They are diploid.

Question 17

Primary Spermatocyte

Answer 17

Type B the undergoes further differentiation into a diploid primary spermatocyte and undergoes the first meiotic division.
Cell committed to differentiative pathway
primary spermatocytes are 46XY diploid.
They move into the adluminal compartment and duplicate their DNA to produce sister chromatids which exchange genetic material and enter meiosis I.

Question 18

Secondary Spermatocytes

Answer 18

Following the first meiotic division, two daughter cells are formed, with haploid chromosomes that are arranged as sister chromatids.
Secondary spermatocytes have undergone meiosis I to give 23X + 23Y haploid number of chromosomes arranged as sister chromatids.

Question 19

Spermatids

Answer 19

The secondary spermatocytes undergo a second meiotic division to give 4 truly haploid spermatids.
Meiosis II occurs to give 4 haploid spermatids. Round spermatid to elongated spermatid differentiation.

Question 20

Spermatozoa

Answer 20

Maturation occurs known as spermiogenesis
Removal of extra cytoplasm
Formation of acrosome
Formation of a tail
Mature sperm extruded into the lumen.

Question 21

How long does the process of spermatogenesis take?

Answer 21

New cycle every 16 days, entire process takes approximately 74 days.

Question 22

Define spermiogenesis

Answer 22

Differentiation of spermatids into mature spermatozoa. Acrosome formation, condensation of sperm nuclear chromatin characterised by the replacement of spermatogonial histones with sperm-specific protamines. This results in transcriptional inactivity in spermatozoa

Question 23

Provide a brief outline of spermatogenesis

Answer 23

1. Spermatogonia- Germ cell on basement membrane, cappable of mitotic or meiotic division to produce primary spermatocytes or more spermatogonia by mitosis. They are diploid.
2. PRIMARY SPERMATOCYTE: Cell committed to differentiative pathway, primary spermatocytes are 46XY diploid. They move into the adluminal compartment and duplicate their DNA to produce sister chromatids which exchange genetic material and enter meiosis
3. SECONDARY SPERMATOCYTES: secondary spermatocytes have undergone meiosis I to give 23X+23Y haploid number of chromosomes arranged as sister chromatids.
4. SPERMATIDS: Meiosis II occurs to give 4 haploid spermatids. Round spermatid to elongated spermatid differentiation

Question 24

S

Question 25

What sorts of divisions take place in spermatogenesis?

Answer 25

1. Mitotic proliferation of spermatogonia.
2. Meiosis and development of spermatocytes.
3. Spermiogenesis, elongation, loss of cytoplasm, movement of cellular contents.
   As it progresses, the excess cytoplasm is being eliminated and the tail is being formed as well as the mitochondria.

Movement into lumen controlled by Sertoli cell secretions. Factors produced by sertoli cells are required for development.

Question 26

What controls the HPG axis?

Answer 26

Positive drive of GnRH from the hypothalamus to the pituitary

Question 27

HPG axis in females

Answer 27

It is cyclical
Once FSH and LH act on the ovaries, estrogen and progesterone can act via negative feedback on the hypothalamus or pituitary.
OR in the case of ovulation when estrogen thresholds exceed a certain level, there will be positive feedback acting on the hypothalamus and anterior pituitary.

Question 28

HPG axis in MALES

Answer 28

There is always a tonic level of production of the hormones, it is NOT cyclical.
The feedback is always negative
The androgens and the inhibin will always feedback negatively on the HPG axis.

Question 29

How are androgens produced in the testes- LH?

Answer 29

LH produced by the anterior pituitary acts on the Leydig cells. The Leydig cells will produce testosterone.
Leydig cells contain LH receptors and primarily convert cholesterol into androgens. Intra-testicular testosterone levels are 100x those in plasma.

The testosterone will make its way to the Sertoli cells to regulate the process of spermatogenesis.
Androgens cross over to and stimulate Sertoli cell function and thereby control spermatogenesis.

Question 30

How are androgens produced in the testes- FSH?

Answer 30

FSH acts directly on the Sertoli cells
It regulates Sertoli cell population/number and also acts on Sertoli cells to produce androgen binding protein (ABP). This ABP binds to testosterone and acts to concentrate the testosterone within the seminiferous epithelium, to aid with the regulation of spermatogenesis.
Sertoli cells contain FSH receptors and convert androgens to oestrogen.

FSH establishes a quantitatively normal Sertoli cell population, whereas androgen initiates and maintains sperm production.

Question 31

Why is Inhibin B important?

Answer 31

Thought to be male version of AMH

Question 32

Inhibin production in the testis

Answer 32

InhibinB is produced primarily by Sertoli cells in response to FSH

Inhibin B reduces FSH production by the anterior pituitary.

Germ cells appear to be required for Inhibin B production.

FSH and Inhibin B in combination have been correlated with testicular volume and spermatogenic activity but this hasn’t proved clinically useful as yet.

Semen analysis remains the gold standard for assessing male fertility

Question 33

Cytoskeleton structure of the sperm cell
Divided into 2 main regions:

Answer 33

Head
- contains nucleus
- contains an acrosomal region too
Tail/flagellum

Question 34

The tail is divided into three distinct regions:

Answer 34

The middle piece
Houses the mitochondria – energy source
Houses ion channels – responsible for the intracellular calcium signalling, that takes pace when the sperm is in the female reproductive tract.
Principal piece
End piece

Question 35

How much sperm is produced?

Answer 35

300 million sperm produced per day on average.
3,500 per second so 9 million during this lecture
approximately 120 million in average ejaculate

Question 36

Normal ejaculation volume?

Answer 36

Normal ejaculate volume is 1.5ml - 6ml.
Spermatozoa account for 1-5% of an ejaculate – only small volum

Question 37

What part is the most sperm rich?

Answer 37

Initial portion of the ejaculate is most sperm rich.
99.9% lost before reaching ampulla of the uterine tube
around 120,000 sperm get near to egg, only one enters

Question 38

Seminal fluid consists of secretions from:

Answer 38

seminal vesicles, prostate, bulbo-urethral gland (cowpers gland) combined with epididymal fluid

Question 39

What is the role of seminal fluid?

Answer 39

Transport of sperm through the male reproductive tract.
When sperm is in the male reproductive tract, it is non motile. So seminal fluid acts as a transport medium

Coagulation of the ejaculate and creating a sperm deposit in the vagina.
So is initially ejaculated as a coagulant which will later liquify.
Gives the sperm better chance of making its way through the cervix

Creation of a neutral to slightly alkaline buffered milieu in the vagina to protect spermatozoa from the acidic vaginal milieu (environment).

Activation and augmenting the motility of the sperm cells.

Coating the sperm cells with capacitation inhibitors.
Capacitation is the process that sperm cells have to go through in order to be able to fertilise an egg.

Supplying nutrients for the sperm cells.

Question 40

What does a normal morphology look like?

Answer 40

Oval head
Large acrosomal region (2/3 if head)
Nucleus
Mid piece
Tail

Question 41

What does condensed acrosome look like?

Answer 41

Condensed acrosome
Small acrosomal region
Large nuclear region

Question 42

Other morphologies?

Answer 42

Small head
Large head
Double headed
Double tailed

Question 43

Abnormal Middle piece-causes?

Answer 43

As a result of cytoplasmic droplets. During spermiogenesis when the extra cytoplasm is being removed, if there is a fault in this process you have some excess cytoplasm by the mid piece in the form of cytoplasmic droplets.

Question 44

Other morphological changes taken into account?

Answer 44

A change is that we now count 4 categories of motility (3 in 2010)
In 1999 used to count 4: rapidly, slowly, non progressive and non motile
Has seen that there is clinical value in seeing the progression taking place.

Another change is using the values as limits to make clinical decisions in fertility treatments.

Question 45

SPERM SUMMARY

Answer 45

Sperm production is continuous from puberty.
Process is long and complex with many errors.
Objective is to produce high numbers.
Process driven by FSH and testosterone.
Result is: Sperm highly specialised for function.

Question 46

30 year old man with Sertoli cell only syndrome.
What is the likely effect on spermatogenesis

Answer 46

It would not occur
Significant reduction in testosterone levels.
Only Sertoli cells – they will respond to FSH
There are no Leydig or germ cells

Question 47

30 year old man with Sertoli cell only syndrome.
What is the likely effect on the HPG axis hormones.

Answer 47

Normal sexual development
Will not have feedback from testoseteone so reduced FHS and LH
Negative feedback still in place but will be reduced.