W3 - MALE REPRODUCTIVE PROCESSES

Question 1

Describe sexual reproduction

Answer 1

• Spermatogenesis
  
  • Allows genetic mixing in order to increase offspring fitness
• Testis, epididymis and accessory glands
  
  • Different reproductive anatomy permits production and delivery of genes (via gametes)

Question 2

What are the reprodutive cells? Desribe their functions in the reproductive process

Answer 2

• Mixing of genes occurs via gametes
  
  • Specialized meiotic cells
  • Spermatozoa (sperm) and oocytes (eggs)
• Different structure reflects different functions in reproductive process
  
  • Spermatozoa
    
    • Compact, streamlined, highly motile (delivery vessel for DNA)
  • Oocytes
    
    • Large, nutrient/protein-rich cells (facilitate embryo development)

Question 3

What is the primary function of the testes? Describe the path of sperm in the testes

Answer 3

• Lie within the scrotum
• Approximately 4cm long and 2.5cm wide
• Primary function
  
  • Produce sperm
  • Produce steroids (Androgens such as testosterone)
• Each testis is divided into lobules contains 1-4 tightly coiled seminiferous tubules (site of spermatogenesis)
• Sperm pass from the seminiferous tubules through the rete testis and into the efferent ducts of the epididymis
• In the epididymis, sperm undergo further maturation (Acquire motility and fertilization ability) and storage until ejaculation

Question 4

What is the function of interstial (leydig) cells? Explain

Answer 4

• Interstitial (Leydig) cells
  
  • Produce androgens (testosterone) in response to LH from the anterior pituitary
  • Only testicular cells to express high-affinity LH receptors on their surface
  • Prolactin and inhibin also bind to the receptors and facilitate the stimulator action of LH on testosterone production
  • Found adjacent to the seminiferous tubule (In the interstitial space)

Question 5

What is the function of sustentacular (sertoli) cells? Explain

Answer 5

• Sustentacular (sertoli) cells
  
  • Amorphic shape - Generally wedged between cells
  • Support spermatogenesis and nourish it from a germ cell to spermatozoa
  • Sit on the basal membrane and also creates the blood-testis barrier
  • Tight junctions between them prevent blood from passing between
  • Devices the germ epithelium in the basal and abluminal compartments
  • Function is controlled by FSH
  • Produce inhibin, androgen-binding protein and anti-mullerian hormone
  • Cytoplasm invaginates and encloses
  • Basally located nucleus
  • Cytoplasm is attached to the basal membrane and extends to the lumen
  • Cytoplasm strands head towards the lumen
  • Provides nourishment as well as receiving cytoplasm from spermatocytes

Question 6

What is the function of myoid cells? Explain

Answer 6

• Myoid cells
  
  • Contract to move sperm and fluid through tubules
  • Surround the sertoli cells
  • Structural regulation of the forming testis cord, promotion of movement of mature sperm through the seminiferous tubules to export it (smooth muscle-like character)

Question 7

What is the function of spermatogonia, spermatocytes and spermatids? Explain

Answer 7

• Spermatogonia, spermatocyte and spermatid - Precursor sperm
  
  • Spermatogonia
    
    • Basement membrane
    • Not much DNA condensation
    • Some may be progressing to the abluminal compartment
    • Between tight junctions and basement membranes
  • Primary spermatocytes
    
    • Move through the tight junctions into the abluminal compartment
    • Undergo cell growth - Become larger
    • Nucleus undergoes condensation in preparation for separation
    • Generally, have a “spotted nuclei”
  • Round spermatids
    
    • Nucleus is condensed
  • Elongated spermatid
    
    • Round spermatid changes form
    • Releases much of the cytoplasm
    • Chromatin is much more condensed (stains more highly)
    • Grows flagellum

Question 8

Explain testicular blood flow and heat exchange

Answer 8

• Spermatogenesis requires a temperature approximately 4-6°C lower than body temperature
• Complex venous network (pampiniform plexus) wraps around the testicular artery in the spermatic cord
• Cools incoming arterial blood (37°C) before it enters the testis by heat exchange with outgoing venous blood (33°C)

Question 9

How is the blood testis barrier formed? What is its function?

Answer 9

• Formed by the sertoli cells and their tight junctions
  
  • Makes it fluid impermeable
• Forms a barrier between the basal and abluminal compartments of the seminiferous tubule
• Prevents the ‘naive’ immune system from mounting a response to ‘foreign spermatozoa’
• Controls the composition of luminal fluid

Question 10

What are the regions, structure and function of the epididymis?

Answer 10

• Regions
  
  • Caput (Head)
  • Corpus (Body)
  • Cauda (Tail)
• Structure
  
  • Highly coiled duct approximately 6m
  • Pseudo-stratified epithelia with stereocilia
• Function
  
  • Sperm maturation (acquisition of motility and fertilizing ability)
  • Sperm storage in cauda (tail) possible for several months
  • Sperm are ejaculated from the caudal epididymis not the testes

Question 11

What is gametogenesis?

Answer 11

• Formation of gametes (sex cells)
  
  • Spermatogenesis → Testes
  • Oogenesis → Ovaries
• Involves halving of the chromosomes via meiosis
  
  • Avoids polyploidy → Detrimental
• Diploid adult (2n = 46) divide 2 = Haploid gamete (n = 23)
• Haploid sperm (n = 23) + Haploid egg (n = 23) = Diploid zygote (2n = 46)

Question 12

What is spermatogenesis?

Answer 12

• The formation of sperm occurs in the seminiferous tubules of the testes and takes approximately 64 to 72 days
• Process involves both mitotic and meiotic cell divisions

Question 13

Explain the process of mitotic proliferation

Answer 13

• Mitosis is the equal division of a cell to produce two identical daughter cells with the same number of chromosomes
• The stem cell – spermatogonia – forms the basal layer of the germinal epithelium
  
  • This divides into type A daughter cells (remain as a further stem cell), and type B (go on to form spermatozoa)
• The type B daughter cells move to the abluminal compartment and become primary spermatocytes
  
  • They pass through the tight junctions between the sertoli cells and grow (DNA condenses)
• While nuclear division is complete, cytoplasmic division is incomplete – all cells derived from one spermatogonium are linked by a thin cytoplasmic bridge
  
  • This persists throughout meiotic divisions (cell only breaks off in the last states)

Question 14

Explain the process of meiotic division

Answer 14

• Generates genetic diversity and halves the chromosome number
  
  • Freshly created primary spermatocyte enters meiosis
• Process
  
  • The cell goes into the S Phase - doubling the DNA and leaving the basal compartment, going into the abluminal compartment
  • Prophase I - chromosomes pair up and recombination occurs (crossing of genetic material)
  • This can take up to 24 days and is especially sensitive to damage
  • Metaphase I - chromosomes line up at the equator on the meiotic spindle and the microtubules attach
  • Anaphase I - microtubules shorten, pulling homologous chromosomes to opposite poles
  • Telophase I and cytokinesis - chromosomes arrive at their poles
    
    • Each cell has half the number of chromosomes, but the chromosome consists of a pair of chromatids
    • The microtubules disappear and a new nuclear membrane is formed
• Meiosis II - secondary spermatocyte undergoes prophase II – the disappearance of nucleoli and envelope, shortening and thickening of chromatids
  
  • The centrosomes move the polar regions and arrange spindle fibres for the 2nd division
• Metaphase II - the centromeres attach to the spindle and the chromosomes line up
• Anaphase II - the sister chromatids separate and move towards the spindle poles
• Telophase II - the spindle disassembles, decondenses and the chromosomes lengthen
  
  • The nuclear envelope reforms, cell cleavage and cytokinesis occur
    
    • These products are called early spermatids (round spermatids)
    • Secondary spermatocytes and spermatids are difficult to distinguish from one another

Question 15

What is cytodifferentiation?

Answer 15

• The packaging of the chromosomes for effective delivery to the oocyte
  
  • Cytoplasmic remodelling - spermiogenesis
    
    • The spermatids change shape from being round, to being elongated
  • Histones are replaced by protamine’s
  • The tail is generated for forward propulsion
  • The midpiece forms, containing mitochondria
  • Equatorial and postacrosomal cap regions form (important for sperm-oocyte fusion)
  • Acrosome cap forms
• Nucleus contains the compact packaged haploid chromosomes
• Residual body acts as a “bin” for the cytoplasm excess, which is then phagocytosed by sertoli cells
• Centrioles - reduce to a central form structure linking the mid-piece to the head
• Completed with the formation of a spermatozoon (Found in the lumen)
• Thin cytoplasmic syncytium bridges rupture, releasing cells
• Cells are released into the lumen via spermiation

Question 16

Explain the process of spermatid differentiation

Answer 16

• Golgi forms acrosomal vesicle
• Acrosomal vesicle spreads over nucleus and distal centriole forms early flagellum
• Nucleus and cytoplasm elongate
• Mitochondria form midpiece, cytoplasm is shed and principle piece (tail) matures

Question 17

Explain how waves of spermatogenesis are controlled and calculate how long it takes for the completion of spermatogenesis by one spermatogonia

Answer 17

• Spermatogenesis occurs in waves - controlled temporally and spatially within the seminiferous tubules by sertoli cells
  
  • Rounds occur in waves
• This allows for the constant production of sperm cells opposed to a mass dump of sperm and periods of no sperm
• Further, there are six stages that make up the cycle of seminiferous tubules in humans
  
  • The progression from a spermatid to spermiation takes 16 days
    
    • Each spermatogonia need to go through the cycle 4.5 times to become a mature spermatozoa in the lumen
  • Thus, it takes 72 days for the completion of spermatogenesis by one spermatogonia

Question 18

What factors account for a reduction in sperm count?

Answer 18

• Sperm is made in very high quantities
• Up to 1/3 sperm degenerate due to apoptosis
  
  • Sperm also degenerate during prolonged sperm storage in the epididymis
• Many are lost during the transport of sperm in the female tract
  
  • Of the 300 million inseminated, less than 1,000 reach the site of fertilisation
    
    • Vaginal loss, cervix, UTJ barriers, immune attack, lose energy
• This is maintained by proliferation and renewal of spermatogonia
  
  • Constant replication allows for constant production
• Low sperm count is called oligospermia
• No sperm count is called azoospermia

Question 19

Which hormones control spermatogenesis? Explain their function

Answer 19

• Spermatogenesis is controlled by a variety of hormones
  
  • Gonadotropin releasing hormone (GnRH) - Protein
    
    • Stimulates the secretion of LH and FSH
    • Released from the hypothalamus
  • Luteinising hormone (LH) - Protein
    
    • Stimulates the secretion of testosterone
    • High affinity receptors only on leydig cells
    • Acts synergistically with prolactin and inhibin to facilitate the stimulation of testosterone production
    • Released from the anterior pituitary
  • Follicle stimulating hormone (FSH) - Protein
    
    • Supports sertoli cell function
    • Receptors on basolateral surface of the sertoli cells
    • Stimulates production of ABP and inhibin for negative feedback
    • Released from the anterior pituitary
  • Androgen binding protein (ABP) - Protein
    
    • Sequesters testosterone to the site of spermatogenesis (allows it to travel through hydrophilic environments)
    • Released from sertoli cells
  • Testosterone (T) - Steroid
    
    • Essential for the initiation of spermatogenesis
    • Stimulates primary and secondary characteristics
    • Produced by leydig cells
      
      • Diffuses into the tubule lumen and binds to ABP
      • Some enter the sertoli cells and bind to androgen receptors
    • Target cell – sertoli cells
      
      • Maintains the blood-testis barrier
      • Permits sertoli-spermatic adhesion
      • Essential for mature sperm release
    • Negatively feeds back on the hypothalamus and pituitary
• Inhibin - Protein
  
  • Regulates the rate of spermatogenesis (about 20 million/mL)
  • FSH specific

Question 20

Explain the endocrine control of spermatogenesis

Answer 20

• Processes
  
  • GnRH - LH and FSH
  • LH –> Leydig cells –> Testosterone –> Negative feedback on hypothalamus and pituitary (mostly for LH), initiates spermatogenesis in sertoli cells, secondary characteristics, maintains blood-testis barrier, mature sperm released
  • FSH –> Sertoli cells –> Inhibin –> Reduces sensitivity of anterior pituitary to FSH cells to GnRH and ABP –> Sequesters testosterone to site of spermatogenesis
    
    • Inhibin specifically inhibits FSH (not GnRH)

Question 21

What are the accessory glands in the male reproductive system? Explain their functions

Answer 21

• Seminal vesicles (2)
  
  • Contracts during ejaculation
  • Secrete yellow viscous alkaline fluid containing fructose, ascorbic acid, coagulating enzymes and prostaglandins (enhance sperm motility/fertilization)
  • Approximately 70% of semen volume
• Prostate (1)
  
  • Contracts during ejaculation causing secretions to enter the urethra
  • Secretes milky acidic fluid containing citrate, several enzymes and prostate specific antigen (activates sperm)
  • Up to 30% of semen volume
• Bulbourethral glands (2)
  
  • Secretes a thick clear alkaline mucus (neutralises traces of acidic urine in urethra prior to ejaculation)
• The accessory glands produce the bulk of the seminal plasma

Question 22

What is semen and the volume of a typical ejaculate? Explain its function and components

Answer 22

• Mixture of sperm, testicular fluid and accessory gland secretions
• Function
  
  • Transport medium for sperm
  • Provides nutrients and chemicals that protect and activate sperm
  • Facilitate sperm movement
• Components
  
  • Prostaglandins
    
    • Stimulate contraction in female tract (Facilitates sperm uptake)
  • Relaxin
    
    • Enhances sperm motility
  • Clotting factors
    
    • Promote semen coagulation
• Typical ejaculate is 2-5mL of semen containing 15–150 x 10⁶sperm/mL