Emergency Ultrasound > Introduction > Embryology

Embryology of the Testis

You can advance to the next section if only interested in the imaging findings and would like to skip the overview of embryology. Understanding the embryologic origin of the testes and especially their descent can help remember the layers involved and to better comprehend the pathophysiology of certain conditions. For an excellent detailed review of the embryology, please refer to Module 21 of the website www.embryology.ch, the following link is an overview of descent of the testes with great images and an animation:

http://www.embryology.ch/anglais/ugenital/diffmorpho04.html (embryology.ch)

Between the 3rd month of pregnancy and its end the testes become transferred from the lumbar area (ventro-medial to the mesonephros) into the future scrotum. This transfer is due to a combination of growth processes and hormonal influences. The gubernaculum testis also plays a decisive role in this phenomenon.

The gubernaculum testis arises in the course of the 7th week from the lower gubernaculum, after the mesonephros has atrophied. Cranially it has its origin at the testis and inserts in the region of the genital swelling (future scrotum). At the same time, at the inguinal canal along the lower gubernaculum, an evagination of the peritoneum arises, the vaginal process, on which the testes will slide through the inguinal canal.

In that the vaginal process lengthens downwardly, it takes the muscle fibers of the oblique internal muscle and the transverse muscle with it. The muscle fascia of the transverse muscle is the innermost layer and in the scrotal region, it forms the internal spermatic fascia of the spermatic cord and the scrotum.

The muscle layer of the musculus cremaster is formed from fibers of the oblique internal and transverse muscles. Externally, the external spermatic fascia is formed from the superficial aponeurosis of the oblique external abdominal muscle.

The region, where the testes pass through the abdominal wall, is called the inguinal canal. Between the 7th and the 12th week the gubernaculum shortens and pulls the testes, the deferent duct and its vessels downwards.

Between the 3rd and 7th month the testes stay in the area of the inguinal canal so they can enter into it. They reach the scrotum at roughly the time of birth under the influence of the androgen hormone.

While in the first year of life the upper part of the vaginal process becomes obliterated, there remains only the peritoneo-vaginal ligament. The lower portion persists as the tunica vaginalis testis, which consists of a parietal and a visceral layer.

Descent of Testes, (from biology.ucf.edu)

Briefly, a few important points regarding the gonads themselves are provided (from embryology.ch):

  • In the testicular cords primordial germ cells (future spermatozoa) are to be found. The somatic cells differentiate themselves into Sertoli's supporting cells, responsible for nourishing the spermatozoa and secreting the antimüllerian hormone (AMH), which promotes the atrophy of the paramesonephric duct (Müller).
  • The rete testis forms the continuation of the centrally-lying testicular cords or the straight seminiferous tubules.
  • The efferent tubules connect the rete testis with the mesonephric duct (Wolff), the future epididymis, which continues with the deferent duct.
  • The interstitial mesenchymal cells of the testes develop into Leydig's interstitial cells. They are responsible for the production of testosterone that, among other things, assures that the mesonephric duct (Wolff) does not atrophy.
  • The stroma, made of connective tissue, subdivides the testes into lobules and forms the tunica albuginea.

8 week, 16-20 week, (from embryology.ch)

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