What is the process of testis descent in embryology?

Testicular Descent Embryology

Testicular descent occurs in two distinct phases: the transabdominal phase (8-15 weeks gestation) controlled by gubernacular enlargement and cranial ligament regression, followed by the inguinoscrotal phase (25-35 weeks gestation) requiring gubernacular migration from groin to scrotum under androgen influence. 1

Transabdominal Phase (First Trimester)

The initial phase of testicular descent involves critical anatomical changes that release the testis from its original position:

• The transabdominal phase occurs between 8-15 weeks of gestation, during which the caudal genito-inguinal ligament (gubernaculum) enlarges while the cranial suspensory ligament regresses 1
• Insulin-like 3 (INSL3) from fetal Leydig cells is the primary stimulator of gubernacular growth, augmented by Müllerian inhibiting substance/anti-Müllerian hormone 1, 2
• Testosterone causes regression of the cranial ligament, allowing the testis to be released from its urogenital ridge location 1, 2
• In females, the craniosuspensory ligament is maintained, which explains why the ovary retains its dorsal retroperitoneal position rather than descending 3

Inguinoscrotal Phase (Third Trimester)

The second phase represents the actual migration of the testis through the inguinal canal into the scrotum:

• The inguinoscrotal phase occurs between 25-35 weeks of gestation and is androgen-dependent 1
• At approximately 22-25 weeks gestational age, the testes are positioned at the internal ring 4
• Normal testicular descent into the scrotum occurs between 25-30 weeks of gestational age 4
• The gubernaculum must migrate from the groin to the scrotum, guided by the genitofemoral nerve releasing calcitonin gene-related peptide under androgen influence 1, 2

Timing and Speed of Descent

Research examining human fetal anatomy reveals the rapid nature of inguinoscrotal migration:

• The transition through the inguinal canal is a very fast process occurring primarily between 20-26 weeks postconception, observed in only 13% of fetuses at any given time 5
• All fetuses beyond 30 weeks postconception have testes located in the scrotum 5
• The first cases of inguinal testis position appear at 17 weeks postconception, but inguinoscrotal migration rarely occurs before 20 weeks 5

Mechanical Factors in Descent

Beyond hormonal control, mechanical forces play essential roles:

• Intraabdominal pressure transmitted through the patent processus vaginalis applies traction to the gubernaculum, introducing the testis into the inguinal canal 6
• Androgens alter the viscoelastic properties of the gubernaculum, reducing its turgidity and allowing intraabdominal pressure to push the testis into the scrotum 6
• Gubernacular outgrowth results in masculinization of the inguinal canal, creating the anatomical pathway for testicular migration 6

Clinical Implications of Failed Descent

Understanding embryology explains the prevalence patterns of cryptorchidism:

• 1-3% of full-term male infants have undescended testes at birth, while 15-30% of premature infants are affected 4
• Nearly 100% of extremely low birth weight infants (<900g) have undescended testes, reflecting incomplete gestational development 4
• Spontaneous descent may occur in the first six months of life but is unlikely after six months of corrected age, as the hormonal and mechanical factors driving descent diminish postnatally 4

Hormonal Control Summary

The endocrine regulation involves multiple factors working in concert:

• INSL3 controls the transabdominal phase, with definitive evidence from Insl3-null mice showing bilateral cryptorchidism 2
• Androgens mediate the inguinoscrotal phase through effects on the genitofemoral nerve and gubernacular regression 1, 2
• Anti-Müllerian hormone and HOX gene family members may play supplementary roles in the descent process 2