Induction of Testicular Descent
The actual mechanism by which the descent of the testes occurs has been the subject of debate for many years and the object of much research. It is clear that there is considerable variation between species not least because of anatomical and physiological differences.
Endocrine factors certainly appear to have a significant role. The requirement for the descent of the mammalian testes to an extra-abdominal location is due to the need to lower the temperature by 2-3°C of the body temperature for spermatogenesis (see Section 16.5). However, the testes of the elephant and the hyrax are intra-abdominal. Inguinal passage occurs at about 120 days of pregnancy in the ox, 80 days in the sheep and 90 days in the pig. In the horse, however, the testis is drawn to the inguinal ring, but at 100-120 days the foetal gonad starts to enlarge rapidly and is unable to pass through the inguinal canal. The gonads reach maximum size at about 210-240 days of pregnancy. The testes then undergo shrinkage until inguinal passage is possible at about 300 days, although some distortion of the testes is still necessary. By the time the foal is born the testes are normally through the inguinal canal.
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