14. Development of the male genital organs. Malformations

This topic assumes that you know the development of the indifferent gonad and other genital organs, described in the previous topic.

Learning objectives

  • Describe the formation of the rete testis, seminiferous tubules, and Sertoli cells
  • Which molecular factor is involved in the development of the testis?
  • Where do Leydig cells develop from, and which role do they play in the development of male genital organs?
  • What is the gubernaculum and which role does it play in the descent of the testis?
  • Why are the testes covered by layers corresponding to those of the abdominal wall?
  • By which week is testicular descent complete?
  • Which embryonic structure gives rise to the male genital ducts?
  • What prevents the development of the Müllerian ducts in males?
  • Describe the formation of the prostate
  • Which hormone drives the development of the external male genital organs?
  • Describe the formation of the penis
  • Describe the formation of the scrotum
  • What is cryptorchidism?
  • What is hypospadias, and what causes it?
  • What is epispadias?
  • What is micropenis, and what causes it?

Development of the testes

Rete testis, seminiferous tubules, and Sertoli cells

The primitive sex cords continue to proliferate and penetrate deep into the medulla, forming the testis cords. This occurs due to the presence of testis-determining factor (TDF), encoded by the SRY gene.

The testis cords in the hilum of the develop gonad eventually form the rete testis, which is a network which connects the seminiferous tubules to the efferent ducts.

By the fourth month of development the testis cords are solid (without lumen) and composed of primitive germ cells and Sertoli cells, the latter of which are derived from the surface epithelium of the gonadal ridge.

The testis cords remain solid until puberty, at which point they are hollowed out, giving them a lumen. At this point the testis cords have become the seminiferous tubules, which carry the sperm cells to the rete testis.

Tunica albuginea

The tunica albuginea, a dense layer of fibrous connective tissue, forms and separates the testis cords from the surface epithelium.

Leydig cells

Leydig cells derive from the mesenchyme of the gonadal ridge. By the eighth week of development they begin to produce testosterone, which is essential for the further development of the other male genital organs.

Descent of the testes

The gonadal ridges, the precursors of the testes, develop in the abdominal region, but the final position of the testes is in the scrotum. The urogenital mesentery, introduced in the previous topic, forms the gubernaculum, a ligamentous structure which attaches to the in the inguinal region. The gubernaculum aids in “pulling” the testis into the scrotum. This migration is complete by week 33.

The testes enter the scrotum through the inguinal canal, “pushing” a layer of the abdominal wall in front of it. This explains why the testes are covered by layers derived from the abdominal wall. For example, the cremaster muscle arises from the internal abdominal oblique muscle.

Development of the male genital ducts

The male genital ducts derive from the mesonephros and the Wolffian (mesonephric) duct. This occurs under the influence of testosterone produced by the Leydig cells. Anti-Müllerian hormone, produced by the Sertoli cells, suppress the development of the Müllerian ducts.

Efferent ducts

The efferent ducts, which connect the rete testis to the epididymis, originate from some of the excretory tubules of the mesonephros.

Epididymis

The epididymis is formed from the Wolffian ducts, which don’t degenerate in males.

Seminal vesicle, ductus deferens, ejaculatory duct

The Wolffian ducts lead from the testis to the urogenital sinus. The seminal vesicles form as outbuddings of the duct, while the rest of the Wolffian duct forms the ductus deferens and the ejaculatory duct.

Prostate

The prostate develops from the urogenital sinus, but under the influence of dihydrotestosterone (DHT) rather than testosterone.

Development of external male genital organs

In the male the development of the internal genital organs is influenced by testosterone, while the development of the external genital organs is influenced by dihydrotestosterone (DHT).

Penis

In the male the urethral folds fuse, forming the shaft of the penis with the penile urethra inside. The genital tubercle enlarges and becomes the glans penis.

The external urethral meatus is formed from ectodermal cells from the tip of the glans, which grow inward.

Scrotum

The scrotum is formed from the fusion of the genital swellings.

Malformations

Cryptorchidism

Cryptorchidism is a condition where one or both testes fail to descend into the scrotum before birth. In most cases the testes complete their descent within four months of life. Infants with this condition should be monitored, and if the testes haven’t descended within four months after birth, surgery is needed.

Hypospadias

Hypospadias is a condition where the urethral folds fail to fuse completely, leaving abnormal openings on the urethra on the ventral aspect of the penis.

Epispadias

Epispadias is a condition where the genital tubercle is mispositioned, leaving abnormal openings on the urethra on the dorsal aspect of the penis. It’s less common than hypospadias. It often occurs with bladder exstrophy.

Micropenis

Micropenis is a condition when the penis is more than 2,5 standard deviations shorter than the mean. It usually occurs due to insufficient androgen stimulation, due to problems with the testes, hypothalamus, or pituitary.

Summary

  • Describe the formation of the rete testis, seminiferous tubules, and Sertoli cells
    • The primitive sex cords form the testis cords
    • The testis cords in the hilum of the gonad form the rete testis
    • The other testis cords contain primitive germ cells and Sertoli cells (derived from surface epithelium), and remain solid until they’re hollowed out at puberty, then forming the seminiferous tubules
  • Which molecular factor is involved in the development of the testis?
    • Testis-determining factor (TDF), encoded by the SRY gene
  • Where do Leydig cells develop from, and which role do they play in the development of male genital organs?
    • From mesenchyme of the gonadal ridge
    • They produce testosterone, which is essential for the development of the other male genital organs
  • What is the gubernaculum and which role does it play in the descent of the testis?
    • The gubernaculum is a remnant of the urogenital mesentery
    • It aids in the migration of the testes into the scrotum by “pulling” them inferiorly
  • Why are the testes covered by layers corresponding to those of the abdominal wall?
    • Because the testes push a layer of abdominal wall in front of them as they descend through the inguinal canal and into the scrotum
  • By which week is testicular descent complete?
    • Week 33
  • Which embryonic structure gives rise to the male genital ducts?
    • The Wolffian (mesonephric) duct
  • What prevents the development of the Müllerian ducts in males?
    • Anti-Müllerian hormone, produced by Sertoli cells
  • Describe the formation of the prostate
    • The prostate is derived from the urogenital sinus under the influence of DHT
  • Which hormone drives the development of the external male genital organs?
    • Dihydrotestosterone (DHT)
  • Describe the formation of the penis
    • The genital tubercle forms the glans, while the fusion of the two urethral folds form the shaft
    • The external urethral meatus is formed from ectodermal cells from the tip of the glans
  • Describe the formation of the scrotum
    • The scrotum is formed from the fusion of the genital swellings
  • What is cryptorchidism?
    • A condition where the testes are undescended into the scrotum at birth
    • Most cases resolve spontaneously within 4 months
  • What is hypospadias, and what causes it?
    • A condition where there are openings on the ventral aspect of the penis
    • It’s caused by improper fusion of the urethral folds
  • What is epispadias?
    • A condition where there are openings on the dorsal aspect of the penis
  • What is micropenis, and what causes it?
    • A condition where the penis is shorter than 2,5 standard deviations of the mean
    • It’s caused by insufficient androgen stimulation

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13. Development of the female genital organs. Malformations

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15. Formation of the fetal membranes and the placenta. Fetal circulation

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