The Gubernaculum and the Evolution of Testicular Descent

Abstract

Adding an evolutionary perspective to anatomy teaching can enrich student learning. One way of introducing evolutionary concepts into a course is by "sneaking it in" by presenting interesting and sometimes entertaining "stories" that add anatomical detail, encourage critical thinking, and illustrate underlying evolutionary history and concepts. The gubernaculum tells one such story; a story that involves the movement of the testes through the fetal abdominal cavity and finally into the scrotum. While it is well understood that the proximate importance of this migration is to guarantee an appropriate thermal environment for sperm production, our understanding of the role of the gubernaculum in anchoring, pulling, and clearing the way for the testes as they descend is only now coming to light. Even less clear is our understanding of the evolutionary forces that ultimately led to such a seemingly vulnerable positioning of the testes. Multiple hypotheses have been proposed, and whether one or more can ultimately explain the seemingly complicated process and curious anatomy, the essential evolutionary test has obviously been met-it works.The information contained in this article will enhance student comprehension of the gubernaculum and their appreciation for the evolutionary history of testicular descent and related concepts associated with the pedagogy of courses in Human Anatomy and Human Anatomy and Physiology.

Reappraising the exteriorization of the mammalian testes through evolutionary physiology

“…Although most mammals have descended scrotal testes, others do not. Some have 'descended ascrotal' testes in which the testes emerges from the abdominal wall, but do not move into a scrotum, instead, staying just under the skin, a condition seen in whales, pinnipeds (seals and walruses), sloths, moles, and rhinoceros [24]. Other animals demonstrate 'testicondy' in which the testes do not move out of the abdominal cavity and stay close to the kidneys.…”

“…This group includes the Afrotheria clade of elephants, manatees, and the duck-billed platypus and other monotremes. In theory, all should have low core body temperatures for spermatogenesis, but only monotremes do [24].…”

“…There is no specific aspect of anatomy that provides a greater differentiation between males and females than the reproductive system. After a brief indeterminate period in humans lasting six weeks, the testes and ovaries originate from the same embryological tissues [24]. Beginning in week seven, the onset of the production of testosterone instigates the process of differentiation [7].…”

“…The ovary and testes both originate adjacent to the kidney, but the testes takes the long journey in most mammals. In humans, that transit involves at least two steps, a transabdominal phase and an inguinoscrotal passage [5,24]. Importantly, movement of the testes is relative.…”

“…Its position shifts in a highly choreographed set of coordinate movements between the testes and other body tissues. Although it has been proposed that the abdominal cavity shifts and the testes is held in a generally rigid position by the gubernaculum, there is direct evidence that transabdominal testicular movement occurs as a combined push and pull interchange [24,104]. Crucially, the gubernaculum, which is more developed in the male fetus compared to the female provides a uniform trans-species pathway of testicular descent [105].…”