1.1 Scrotum

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  The scrotum is a muscular sac that lies behind the penis.

  
  
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  It is the location for sperm production.

  
  
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  The temperature in the scrotum is about 2–4 degrees less than the body core temperature.

  
  
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  The dartos muscle forms from the subcutaneous layer of the scrotum.

  
  
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  The dartos muscle divides the scrotum into two compartments in the middle, each housing one testis.

  
  
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  The cremaster muscle is the continuation of the internal oblique muscle and forms a muscular net around the entire scrotum.

1.2 Structure of the Testes

See Figure 14.1.

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  Each testis is 4–5 cm in length.

  
  
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  They are located in the scrotum.

  
  
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  The scrotum is lined with connective tissue:

  
  
  
  
  1. 
     

     ◦ the outer lining is the tunica vaginalis, and has a parietal and visceral layer

     
     
  2. 
     

     ◦ the tunica albuginea is a dense, tough, white, connective tissue

     
     
  3. 
     

     ◦ the tunica albuginea invaginates into the testes to form septa, which divide the testes into 300–400 lobules

     
     
  4. 
     

     ◦ within the lobules are the seminiferous tubules

     
     
  5. 
     

     ◦ tightly coiled seminiferous tubules form the bulk of the testes

  
  
  
  
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  Spermatozoa develop from the primitive germ cells (spermatogenesis) in the walls of the seminiferous tubules.

  
  
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  From the lumen of the seminiferous tubules, the sperm moves into the meshwork of tubules called the rete testis in the head of the epididymis.

  
  
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  From there, spermatozoa pass through the tail of the epididymis into the vas deferens.

  
  
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  Fifteen to twenty efferent ductules cross the tunica albuginea and leave the testes.

  
  
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  The ductules enter through the ejaculatory ducts into the urethra in the body of the prostate at the time of ejaculation.

  
  
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  Between the tubules in the testes are nests of cells containing lipid granules, the interstitial cells of Leydig, which secrete testosterone into the bloodstream.

  
  
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  The spermatic arteries that drain into the testes are tortuous, and blood in them runs parallel, but in the opposite direction to blood in the pampiniform plexus of spermatic veins. This anatomic arrangement may permit countercurrent exchange of heat and testosterone.

  
  
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  The testes move down the abdominal cavity and into the scrotal cavity through the inguinal canal.

  
  
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  Cryptorchidism is a failure of the testes to descend into the scrotal cavity.

Figure 14.1 Cross section of the testis. Used with permission from https://visualsonline.cancer.gov/details.cfm?imageid=1769

1.3 Embryological Origin of the Male Reproductive Tract

The embryological origin of the male reproductive tract is covered in detail in Section 2, Embryology.

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  Paramesonephric ducts give rise to the:

  
  
  
  
  1. 
     

     ◦ testes

     
     
  2. 
     

     ◦ epididymis

     
     
  3. 
     

     ◦ vas deferens

  
  
  
  
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  Mesonephric ducts give rise to the accessory glands:

  
  
  
  
  1. 
     

     ◦ prostate

     
     
  2. 
     

     ◦ bulbourethral or Cowper’s gland

     
     
  3. 
     

     ◦ seminal vesicle

  
  
  
  
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  Developing tubules within the testes secrete a polypeptide müllerian inhibitory factor, which causes the regression of the paramesonephric ducts 60 days after fertilisation.

1.4 Sertoli Cells

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  Elongated branching cells are supporting cells called sustentacular cells (typically found in the epithelium).

  
  
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  The walls of the seminiferous tubules are lined by primitive germ cells and Sertoli cells: large, complex glycogen-containing cells, which stretch from the basal lamina of the tubule to the lumen.

  
  
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  Tight junctions between adjacent Sertoli cells near the basal lamina form a blood–testis barrier, which prevents passage of molecules from interstitial tissue and the basal compartment to the lumen.

  
  
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  The barrier is penetrated by the maturing germ cells by progressive breakdown of the tight junctions above the germ cells, with concomitant formation of new tight junctions below them.

  
  
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  Sertoli cells secrete the following

  
  
  
  
  1. 
     

     ◦ androgen binding protein (testosterone binding protein)

     
     
  2. 
     

     ◦ Anti-müllerian hormone (AMH): secreted in early stages of fetal life

     
     
  3. 
     

     ◦ inhibin and activin: generally produced after puberty; this regulates follicle stimulating hormone (FSH) by positive and negative feedback

     
     
  4. 
     

     ◦ estradiol: aromatase from the Sertoli cell converts testosterone to 17β estradiol

     
     
  5. 
     

     ◦ transferrin: blood plasma protein helps to transport iron

  
  
  
  
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  FSH stimulates the androgen binding protein produced by Sertoli cells and formation of blood–testes barrier.

  
  
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  FSH also increases spermatozoa by preventing the apoptosis of type A spermatozoa and helps with maturation.

  
  
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  Inhibin acts as a negative feedback to reduce the release of FSH and GnRH.

  
  
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  Testosterone levels in the testes is 20–50 times higher than in the blood.

1.5 Functions of the Blood–Testis Barrier

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  The blood–testis barrier maintains the composition of the fluid in the lumen of the seminiferous tubules, which contains very little protein and glucose, but is rich in androgens, estrogens, K⁺, inositol, glutamic and aspartic acid.

  
  
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  Protects the germ cells from blood-borne noxious agents.

  
  
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  Prevents antigenic products of germ-cell division from entering the circulation and generating an autoimmune response.

  
  
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  Helps establish an osmotic gradient, which facilitates the movement of fluid into the tubular lumen.

1.6 Leydig Cells

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  Also called interstitial cells of Leydig, which lie adjacent to the seminiferous tubules.

  
  
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  Leydig cells have an eosinophilic cytoplasm.

  
  
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  Leydig cells produce testosterone in the presence of luteinising hormone (LH): LH binds to the Leydig cells to stimulate testosterone secretion and androgen production.

  
  
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  Contain lipofuscin pigment and inclusions called Reinke crystals (rod-like structures measuring 3–20 µm). These are seen in <50% of Leydig cells and have no function. Typically used to diagnose Leydig cell tumours histologically.

  
  
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  Leydig cell tumours are usually benign, but can be hormonally active and produce testosterone.

1.7 Germ Cells

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  Spermatogonia (least mature) line the basement membrane.

  
  
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  Spermatogonia are stem cells and can differentiate into different cell types (2 n).

  
  
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  Primary spermatogonia (2 n=46 chromosomes).

  
  
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  Secondary spermatogonia (n=23 chromosomes): first meiosis.

  
  
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  Spermatids (n=23 chromosomes): second meiosis.

  
  
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  Sperm.

1.8 Epididymis

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  The epididymis is nearly 6 m long, but lies in a tightly coiled state.

  
  
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  It takes roughly 12 days for the immotile sperm to traverse through the epididymis into the head, predominantly moved by the contraction of the smooth muscles lining the epidydimal tubules.

  
  
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  It consists of three parts: caput, corpus and cauda and is the main site of storage.

  
  
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  Mature sperm are stored in the tail (cauda) of the epididymis.

  
  
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  Individual ducts join to make the vas deferens.