Testicular Development and Descent: The Journey to the Scrotum Before Birth

The testicle does not form inside the scrotum; it forms on the back wall of the abdominal cavity, right next to the kidneys. Around the sixth and seventh weeks of pregnancy, an undifferentiated gonadal structure appears, not yet developed as male or female; signals from the Y chromosome direct this structure to develop as a testicle.

The descent of the testicle is not a single movement but two phases, each governed by different forces. The first is the intra-abdominal (transabdominal) phase, occurring roughly between weeks 10 and 15 of pregnancy: as the gubernaculum thickens and shortens, the upper ligament loosens, and the testicle moves relatively lower within the growing abdomen, approaching the internal opening of the inguinal canal. This phase is largely directed by the hormone INSL3.

• Hormones: INSL3 (intra-abdominal phase) and testosterone (inguinoscrotal phase)
• Gubernaculum: the guiding ligament that directs the testicle to the scrotum
• Genitofemoral nerve: signals that determine the direction of the gubernaculum
• Intra-abdominal pressure: the force that pushes the testicle through the inguinal canal
• Processus vaginalis: the extension of the abdominal lining that accompanies descent and should close afterward

Whichever phase of descent is interrupted, the testicle remains where that phase left it. Interruption of the intra-abdominal phase is uncommon; the testicle stays near the kidney or within the abdomen, and this pattern is more often seen bilaterally or together with other hormonal problems. Interruption of the inguinoscrotal phase is far more common; the testicle stops in the inguinal canal or at the canal's exit, just above the scrotum.

In the first three to six months after birth, male infants experience a temporary hormonal surge known as "mini-puberty": testosterone levels rise, the testicles enlarge somewhat, and some testicles that had not completed their descent move into the scrotum during this period. After six months, this surge fades; this is why six months is the cutoff point beyond which spontaneous descent can no longer be expected.

• During this same period, important maturation takes place in testicular tissue: primitive germ cells transform into precursor cells that will later produce sperm. The cool environment of the scrotum is required for this transformation; in a testicle that remains in a warmer environment, this step is disrupted, and the number of precursor cells declines over the years. This is the biological reason guidelines set the surgical window at 6 to 12 months.

In most cases, undescended testicle does not have a single cause; genetic predisposition, hormonal signals, and the pregnancy environment all play a role together. The strongest associated factors are preterm birth, low birth weight, and being small for gestational age. A family history of undescended testicle increases the risk somewhat.

• Pregnancies with impaired placental function, maternal smoking during pregnancy, and exposure to certain environmental chemicals have been associated with undescended testicle in various studies, but it is not possible to say that any of these factors alone is the cause. When undescended testicle occurs together with hypospadias or bilateral non-palpable testicles, hormonal and, when needed, genetic evaluation comes into consideration. In our practice, this evaluation is carried out by pediatric urology together with endocrinology.