Studies on the fetal development of the gubernaculum in cetacea

Adult cetacean males, like non-mammalian vertebrates and other testicond mammals, have intra-abdominal testes. There is no evidence of a processus vaginalis in them. Testicondia in cetaceans is considered secondary as they are judged, evolutionarily, the descendants of terrestrial mammals (ungulates) with testis descent. A possible argument in support of the latter contention would be that cetacean fetuses develop gubernacula which are the primordia of the processus vaginalis and other structures associated with testis descent in other placental mammals. The present study intended to analyse cetacean fetuses in this respect.

Serial sections of 25 fetuses (total body length between 39.5 and 160 mm) of 4 cetacean species (Delphinus delphis, Phocoena phocoena, Eschrichtius robustus, Physeter catodon) were examined with special attention to the presence or absence of structures homologous to the gubernaculum of other placental mammals (rats and humans).

Gubernacular primordia were observed in fetuses from about the time of onset of sexual differentiation. Their shape and anatomical relationship with the surrounding structures were similar as those in mammals with testis descent. The gubernaculum in males developed into a large mass of dense connective tissue in the ventral-caudal abdominal region at the site of the insertion of the mesonephric inguinal ligament and associated to the tip of the internal abdominal oblique muscle. No (or only very little) development of a processus vaginalis was noticed.

The results demonstrate initial emergence of mammalian-like gubernacular primordia in cetacean fetuses without their further development to elaborate structures required for testis descent. The findings support the view that cetaceans are secondarily testicond. It is suggested that (1) absence of the pelvic girdle together with (2) the development of structures in and beyond the caudal abdominal region, particularly the caudal hypaxial musculature, precludes the outgrowth, into caudal direction, of hollow organs (such as the processus vaginalis) from the abdominal cavity.