Testicles

The testicles, also known as the testes or the male gonads, are a pair of egg-shaped glands suspended within the scrotum. The testicles have multiple layers: an outer tunica vaginalis, an intermediate tunica albuginea, and an innermost tunica vasculosa. The testicles are composed of testicular lobules (contain interstitial tissue) and seminiferous tubules (produce spermatozoa). Blood supply to the testicles is primarily provided by the testicular artery. Venous drainage is through testicular veins.

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Embryology

Male development

  • Male development is driven by the Y chromosome:
    • The Y chromosome contains the testis-determining gene SRY (sex-determining region on the Y chromosome).
    • Development of male genitalia occurs in the presence of the SRY gene.
  • Gonads appear as a pair of longitudinal ridges (gonadal ridges).
  • Gonadal ridge:
    • Formed by a proliferation of the epithelium and a condensation of mesenchyme
    • Epithelial cells penetrate the mesenchyme and form primitive sex cords (at the stage of indifferent gonads).
    • Germ cells appear after the 6th week of gestation.
  • Primordial germ cells originate in the epiblast and migrate:
    • By the 3rd week of gestation, the primordial germ cells reside in the yolk sac close to the allantois.
    • By the 4th week of gestation, the primordial germ cells reside along the dorsal mesentery of the hindgut.
    • By the 7th week of gestation, the primordial germ cells invade the gonadal ridges.

Testicular development

  • Under the influence of the SRY gene
  • Mesonephric (wolffian) duct:
    • Present in males → development of testes 
    • Degenerates without the presence of the SRY gene
    • Sertoli cells: 
      • Secrete müllerian inhibitory factor
      • Blocks formation of female internal genitalia
    • Leydig cells: 
      • Secrete testosterone
      • Lead development of the wolffian duct to male internal genitalia
    • Testosterone:
      • Converted to dihydrotestosterone (DHT) by 5-alpha reductase
      • Important for prostate development 
    • At puberty:
      • Testis cords acquire a lumen, become seminiferous tubules, and join the rete testis tubules.
      • The wolffian duct becomes the vas deferens.

Gross Anatomy

Testes (the male gonads) are responsible for the production of sperm and testosterone:

  • The testes are enclosed by 3 layers:
    • Tunica vaginalis:
      • Outermost layer 
      • Continuation of the peritoneal processus vaginalis
    • Tunica albuginea:
      • Intermediate layer
      • Where vessels, nerves, and ducts enter the testicular capsule to begin formation of the testicular lobules
    • Tunica vasculosa:
      • Innermost layer 
      • Contains blood vessels
      • Borders the inner surface of the tunica albuginea
      • Encloses lobules composed of seminiferous tubules and interstitial tissue
  • Spermatic cord:
    • Testes are suspended by the spermatic cord within the scrotum. 
    • Allows for maintenance of a lower temperature
  • Scrotum:
    • A pouch composed of highly glandular, hair-bearing skin and multiple layers of fascia/muscle:
    • Contents:
      • Testes
      • Epididymis
      • Vas deferens (ductus deferens)
      • Cremaster muscle: lifts the scrotum to maintain an ideal temperature
      • Dartos muscle: corrugates skin to moderate temperature
      • Spermatic, cremasteric, and infundibuliform fascia (attached to the parietal layer of the tunica vaginalis) 
    • Raphe (scrotal midline):
      • Extends from the urethral meatus → ventral penile shaft → anus
      • Represents the “seam” where the genital tubercles fused in development
Anatomy of the testis

Various layers and structures comprising the testes:
Note the suspension from the spermatic cord, which permits temperature regulation.

Image: “Anatomy of the testis” by Phil Schatz. License: CC BY 4.0
The scrotum and testes

External scrotal view:
The central raphe on the far left represents the “seam” where the genital tubercles fused in development. In the central image, note the cremaster and dartos muscles, which are important for temperature regulation. On the right, note a deep layer demonstrating the external testes, epididymis, and neurovasculature.

Image: “The Scrotum and Testes” by Phil Schatz. License: CC BY 4.0

Microscopic Anatomy

Blood vessels, lymphatics, and the genital ducts enter the mediastinum of the testis and give rise to numerous lobules composed of seminiferous tubules and interstitial tissue:

Seminiferous tubules

  • Site of spermatogenesis 
  • Contains spermatozoa of various stages of development:
    • Spermatogonia = germ cell for all spermatozoa
    • Primary spermatocytes → meiosis I → 2 secondary spermatocytes → meiosis II → 4 haploid spermatids
  • Accounts for the majority of testicular volume
  • Lumen lined by Sertoli cells:
    • Vital supporting cells in spermatogenesis
    • Secrete fluid to facilitate sperm movement
    • Regulate hormones in the testicles via secretion of inhibin B and androgen-binding protein in response to gonadotropins

Interstitial tissue

  • Leydig cells: release testosterone in response to gonadotropins secreted by the pituitary gland
  • Immune cells (mast cells and macrophages)
  • Nerves and blood vessels

Epididymis

The epididymis is posterolateral to the testis and consists of 3 sections:

  • Head:
    • Testicular tubules enter the epididymis at the location
    • Contains a high concentration of principal cells with stereocilia for reabsorption of secretory fluid
  • Body:
    • Contains smooth muscle aiding in the transmission of spermatozoa to the vas deferens during ejaculation
    • Plays a role in reabsorption of secretory fluid
  • Tail:
    • Leads to the vas deferens (ductus deferens)
    • Contains prominent smooth muscle

Vas deferens (ductus deferens)

  • A tubular structure originating at the tail of the epididymis 
  • Transports sperm to the urethra

Spermatic cord

  • Functions to suspend the testis
  • Terminates in the mediastinum of the testis
  • Arises at the deep inguinal ring, traverses the inguinal canal, and exits at the external inguinal ring

Neurovasculature

Arterial supply

  • Testes:
    • Testicular artery: the major blood supplier, originating from the aorta
    • Artery to vas deferens: supplies the vas deferens and epididymis
  • Scrotum:
    • External pudendal branches of the femoral artery
    • Scrotal branches of internal pudendal artery
    • Cremasteric branch of inferior epigastric artery
Vasculature of the testis

The robust vasculature of the testis and surrounding structures:
Note the largest arterial supply is the testicular artery, arising from the aorta. Also, note the large testicular vein and the pampiniform plexus.

Image by Lecturio.

Venous drainage

  • Testes:
    • The right testicular vein drains into the inferior vena cava (below the renal vein).
    • Left testicular vein → left renal vein → inferior vena cava 
  • Scrotum: follows arterial supply with frequent arteriovenous (AV) anastomoses
Venous drainage of the testis

Venous drainage of the testes demonstrating the different routes of venous emptying between the left testicular vein and the right testicular vein:
Note the left testicular vein drains into the left renal vein, but the right testicular vein drains directly into the inferior vena cava.

Image by Lecturio.

Lymphatic drainage

  • Testes:
    • The right testis drains to the interaortocaval nodes and paracaval nodes.
    • The left testis drains to the left para aortic and interaortocaval nodes.
  • Scrotum:
    • The right side drains to right superficial inguinal nodes.
    • The left side drains to left superficial inguinal nodes.

Innervation

  • Testes: via renal and aortic plexuses arising from T10 and T11
  • Scrotum:
    • Ilioinguinal nerve (anterior scrotum)
    • Genital branch of the genitofemoral nerve (anterolateral scrotum)
    • 2 posterior scrotal branches of the perineal nerve (posterior scrotum)
    • Perineal branch of the posterior femoral cutaneous nerve (inferior scrotum)
Nerve supply to the testis

Nerve supply to the testes

Image by Lecturio.

Clinical Relevance

  • Hypogonadism: characterized by decreased sex steroid production in the gonads. In men, hypogonadism can be the result of primary testicular failure or secondary testicular failure (secondary to pituitary or hypothalamic disorders). Symptoms of hypogonadism include erectile dysfunction, decreased libido, and regression or absence of secondary sexual characteristics.
  • Hydrocele: scrotal masses can result from various causes, hydroceles are 1 of the most common. A hydrocele is the pathological collection of fluid in the tunica vaginalis around the testes. Varicoceles occur when there is dilatation of the pampiniform venous plexus, which is connected to the internal spermatic vein. Varicoceles are associated with impaired testicular hormonal function and infertility. Spermatoceles are paratesticular epididymal cysts with sperm-containing fluid.
  • Testicular torsion: the sudden rotation of 1 of the testes around the axis. Testicular torsion refers to the twisting of the spermatic cord structures either in or below the inguinal canal. Compromised blood flow to and from the testis results from the acute rotation. The impeded venous return and reduced arterial perfusion of the testis can lead to hemorrhagic infarction of the testicular parenchyma. Testicular torsion is a medical emergency and surgical intervention is usually necessary.

References

  1. Isidori A. M., Lenzi A. (2017). Scrotal and Testicular Anatomy. In: Ultrasound of the Testis for the Andrologist. Trends in Andrology and Sexual Medicine. Springer, Cham. https://doi.org/10.1007/978-3-319-51826-8_1
  2. Patel A. P. (2017). Anatomy and physiology of chronic scrotal pain. Translational andrology and urology, 6(Suppl 1), S51–S56. https://doi.org/10.21037/tau.2017.05.32
  3. Raghu M. R. (2020). Testicle and Epididymis Anesthesia: Overview, Indications, Contraindications. Emedicine. Com. https://emedicine.medscape.com/article/82983

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