Lecture 2 - The male tract and spermatogenesis

Question 1

Sperm production: what is the optimal temperature, what helps keep this temperature stable, and where is sperm stored and matured?

Answer 1

34-35°C

• Scrotum allows for release of heat thanks to thin skin with no subcutaneous fat, keeping tesis cool
• Cremaster muscle keeps testis warm by contracting and elevating them closer to the body when cold (opposite happens when too hot)

Epididymis

Question 2

Vas deferens

Answer 2

Long slender tube where sperm travels down

Question 3

Where does sperm empty into?

Answer 3

Ejaculatory duct, which runs through the centre of the prostate gland

Question 4

Semen: what is its main component, what glands add volume to semen, and how does nerve activation result in fluid expulsion?

Answer 4

• Seminal fluid 46-80%
• Epididymis & Vas deferens (sperm-rich fraction): 5% of the ejaculate volume
• Prostatic secretion: 13-33% of the ejaculate volume
• Bulbo-urethral gland secretion: 5% of the ejaculate

Sympathetic nerve stimulation of epididymis, vas, prostate and seminal vesicles during sexual arousal results in smooth muscle contraction and expulsion of fluids which compose semen

Question 5

Seminal vesicles: what are they and what do they do?

Answer 5

The seminal glands

• Secrete a viscous material rich in prostaglandins, fructose, and other nutrients, providing an energy/nutrient source for sperm following ejaculation
• Coats sperm with inhibitors of capacitation e.g. CRISP1 (especially in the epididymis)
• Helps to create a coagulate after release into vagina, seminal plasma is slightly alkaline
• Subsequent fluidization of the ejaculate (proteases from prostate) further aids motility of sperm

Question 6

Bulbo-urethral gland: what does it do?

Answer 6

Secretes a clear viscous liquid that adds volume to the semen

Question 7

Urethra: what does it do?

Answer 7

Both:
Transports urine

Males:
Transports semen down the length of the penis.

Question 8

Can males urinate and ejaculate at the same time?

Answer 8

No, a sphincter in the prostate contracts during ejaculation and seals off exit from the bladder

Question 9

Spermatogenesis: what is it, where does it take place, and what cells are required for it?

Answer 9

The production or development of mature spermatozoa

Seminiferous tubule

Sertoli cells provide mechanical and nutritional support for spermatogenesis.
Interstitial cells (Leydig cells) important for the synthesis of androgens (especially testosterone) which is important for spermatogenesis.

Question 10

Testosterone: what do sertoli cells do to it and what is its function in the testis?

Answer 10

Converted to dihydrotestosterone (DHT) in Sertoli cells through the action of 5 alpha-reductase

• Maintenance of blood-testes barrier
• Spermatid to sertoli cell adhesion
• Spermiation

Question 11

DHT: what is it, how is it produced, and what does it do?

Answer 11

Dihydrotestosterone (DHT)

In Sertoli cells through the action of 5 alpha-reductase

• Maintenance of blood-testes barrier
• Spermatid to sertoli cell adhesion
• Spermiation

Question 14

Blood-testes barrier: what is it, what does it do, what is it formed by, and when does it develop?

Answer 14

The barrier between basal and adluminal compartment

Prevents the leakage of sperm into the blood, prevents an immune response to the spermatozoal antigens

Combination of adherens, gap and tight junctions.

Develops once puberty starts and prior to the onset of spermatogenesis

Question 15

Spermatogenesis: where does it occur, what are the stages and what drives the process?

Answer 15

Spermatogenesis occurs within the seminiferous tubule

Divided into 2 major parts, early and late

• Differentiation of spermatogonia into spermatocytes driven by FSH from the pituitary gland
• Testosterone is critical for spermatogenesis, stimulates the formation of spermatogonia and is important for the second meiotic division (formation of spermatids)

Question 16

Three major elements of spermatogenesis

Answer 16

• Mitotic proliferation to increase number of cells (1 cell - 64 spermatids)
• Meiotic division which generates genetic diversity and halves the chromosome number
• Cytodifferentiation which packages the chromosomes for effective delivery to the oocyte (very compact chromatin- heterochromatin)

Then spermiogenesis which involves differentiation of the sperm into their final form

Question 17

Meiosis

Answer 17

Leccy?

Question 18

Stages of sperm development

Answer 18

Spermatogenesis is the production of sperm

Spermiogenesis is the final stage of spermatogenesis, which sees the maturation of spermatids into spermatozoa

Spermiation is the release of mature spermatozoa into the seminiferous tubule

Question 19

The three divisions of a sperm cell

Answer 19

Head
Mid piece
Principal piece

Question 20

Development of sperm flagella

Answer 20

• Centrioles gather at the end opposite to the acrosomal vesicle
• Nine coarse fibres form along the axis of the tail, each aligned with a microtubule doublet of the flagellum
• Microtubules extend, forming a developing flagellum
• Mitochondria migrate to anterior of flagellum and condense around it as a series of rods forming a spiral
• Superfluous cytoplasm is shed
• DNA uses highly condensed heterochromatin (protamines replace histones)
• Developed flagellum formed

Question 21

Development of head of sperm

Answer 21

• Golgi apparatus give rise to lysosomal-like granules (glycoprotein rich) which coalesce to a single acrosomal vesicle
• The nucleus (with acrosomal cap attached) moves towards the cytoplasmic membrane and elongates
• Golgi apparatus detaches from acrosomal cap and moves posteriorly - The acrosome must adhere to the nuclear membrane in order to develop a normal shape

Question 22

Acrosome: what is its role in fertilisation?

Answer 22

It contains the enzyme hyaluronidase, which is released as sperm reach the oocyte to digest the cumulus cells

Question 23

How does the acrosome know to anchor to the nuclear membrane?

Answer 23

The DPY19L2 gene encodes a protein that anchors the nuclear membrane to the acrosome cap

Question 24

Sperm mid piece

Answer 24

leccy

1= mitochondrial sheath
2,6 = outer dense fibres (9)
3= plasma membrane
4= proximal centriole
7= striated column of connecting piece
8= capitulum
9- implantation fossa of nuclear envelope
5, 10 = microtubules of axenome

Question 25

Principal piece: what’s the difference between it and the midpiece?

Answer 25

Principle piece (tail) has different arrangement to the midpiece:
* Lack of mitochondria but microtubule structure akin to midpiece
* Dynein is an ATPase which utilises the ATP generated from the mitochondria in the midpiece - enabling the microtubules to ‘slide’ and allows movement of the flagellum

Question 26

Sperm motility: how does their motility work, when do sperms become motile, and what steps of maturation do they have?

Answer 26

The sperm tail (flagellum) contains microtubules and dynein (an ATPase) - hydrolysis of ATP generated in the adjacent mitochondria provides energy for motility

However sperm are not motile in the seminiferous tubules of the testis - they mature and become more motile in the epididymis as a result of the action of dihydrotestosterone (DHT)

Further maturation occurs in the female tract (capacitation)

Question 27

Time taken to produce sperm

Answer 27

~10⁸ per day

Sperm formation takes ~70d followed by ~14d to reach the ejaculatory ducts

Question 28

Ejaculation: how does sperm travel for ejaculation, what secretions are added to sperm, and why are they added?

Answer 28

Sperm empty into ejaculatory duct, which runs through the centre of the prostate gland.

• Prostatic secretions include proteases which fluidizes the ejaculate which supports motility and further maturation of the sperm - amines: spermine, spermidine, citric acid, etc
• Seminal vesicles secrete a viscous material rich in prostaglandins, fructose, and other nutrients. Provide energy source for sperm following ejaculation - alkaline which helps protect sperm in the vagina (pH of ~4.7)
• Bulbo-urethral gland secretes a clear viscous liquid that adds volume to the semen

Question 29

Female reproductive tract

Answer 29

• Ovary - where egg develops
• Fallopian tubes - where the egg travels follwoing ovulation
• Uterus - If an egg is fertilised, it will implant here and develop
• Cervix - secretes mucus aiding sperm movement, protects developing fetus from infection, opens for childbirth
• Vagina - canal for childbirth

Question 30

Cervical mucus: what is it composed of, what does it do, and what is its importance?

Answer 30

Composed of glycoproteins, salts and water

• Assemble its glycoproteins to form elongated fibres arranged in channels which allow sperm to penetrate the mucus and pass upward through the cervix
• Removes factors from sperm e.g. anti-capacitation factors (cholesterol) and tocopherols
• Acts as a potential auxillary energy source
• Prior to fertilisation - aids movement sperm, harder for abnormal sperm
• After fertilisation - forms a plug that forms a plug and inhibits invading infection/sperm

Question 31

Phalange: what is it in regards to sperm and what does it do?

Answer 31

Increase the surface area between cervical mucus and seminal fluid, and trap pockets of semen in the mucus

This protects the sperm from the hostile environment of vaginal fluid]

Lowkey leccy this

Question 32

Cervical mucus: how do hormones affect its production?

Answer 32

• Oestrogen stimulates the cervix to secrete large amounts of mucus (non-viscous) priot to fertilisation - these changes caused by oestrogen faciliate sperm migration at a time that coincides with ovulation (mid-cycle)
• Progesterone stimulates the cervix to secrete a thick, viscous, sticky mucus that lacks glycoprotein channels

Question 33

Hormonal dependence of cervical mucus properties: the comparison

Answer 33

Pre-ovulatory:
* Estrogen dominates
* Mucin production
* Maximum hydration
* Supports sperm entry

Post-ovulatory:
* Progesterone dominates
* Minimal production of mucin
* Loss of hydration – firmer gel
* Prevents sperm entry

Question 34

Sperm that cannot fertilise: what is one type, why does this type fail to fertilise, and what is the genetic explanation of this situation?

Answer 34

Round-headed sperm

Fail to fertilise because they have not developed an acrosome cap

The DPY19L2 gene causes production of a protein that causes anchoring of the acrosome to the nucleus - this gene is mutated in many cases of globozoospermia; inheritance is autosomal recessive

Question 35

Globozoospermia: what is it and what gene is it often caused by?

Answer 35

Production of round-headed sperm

DPY19L2 gene

Question 36

ICSI: what does this stand for and what is it?

Answer 36

Intracytoplasmic sperm injection

This a common procedure in assisted reproduction when male subfertility is diagnosed
(eg if sperm swim poorly or fail to fertilise)

Question 37

ICSI negatives?

Answer 37

In mice, ICSI with DNA-damaged sperm results in:
* Decreased preimplantation embryo development rate
* Decreased offspring number
* higher postnatal weight gain
* Increased rate of tumours
* Increased anxiety in female offspring
* Premature ageing

Question 38

What is the name of the long slender tube where sperm travels down?

Answer 38

vas deferens

Question 39

Capacitation: what is it, when does it occur, and what happens?

Answer 39

Biochemical process that prepares sperm to fertilize an egg

As they travel through the female reproductive tract

• Loss of stabilizing factors: sperm lose factors they acquired while in seminal plasma
• Reorganization of membrane lipids and proteins - sperm reorganize their membrane lipids and proteins
• Generation of reactive oxygen species - sperm generate controlled amounts of reactive oxygen species.
• Development of hyperactivated motility - sperm develop hyperactivated motility.

Question 40

Capacitation: what are the changes made to sperms?

Answer 40

• Removal of seminal glycoproteins e.g. glycodelin S tocopherol, etc and cholesterol to acceptors present in female reproductive track results in modification of the cell membrane ↑ membrane fluidity and fusigenic properties
• Change from linear to hyperactivated motility
• Sperm detach from epithelium
• Cytoskeletal changes
• Exposure of sperm receptors, notably those for Zona pellucida proteins ZP2 & ZP3

Question 41

Capacitation: what drives it?

Answer 41

Capacitation is pH independent but leads to the increase of intracellular pH due to capacitation events - requires external HCO₃⁻:
* ↑ HCO₃⁻ leads to an increase of cAMP concentration and cAMP dependent tyrosine phosphorylation of sperm proteins
* Then increased intracellular pH and [HCO₃⁻] leads to activation of cyclic nucleotide gated channels in sperm flagella membrane: linked to changed flagella movement
* Sperm membrane becomes hyperpolarized: through changes in K⁺ entry
* Ca²⁺ influx by activation of pH dependent Ca²⁺ channels
* Sterol binding proteins initiate loss of cholesterol