Developing embryos need to exchange respiratory gases to grow and survive. In egg laying species, gas exchange occurs directly with the surrounding environment, across moist membranes inside the egg. However, in live-bearing female amniotes, gas exchange occurs across the placenta, between the developing embryo and the bloodstream of the parent. The development of the placenta is facilitated by rapid remodelling of the uterine epithelium and surrounding tissues during pregnancy. This remodelling includes an increase in vascular growth to facilitate the increasing oxygen needs of the embryo.
The members of the family Syngnathidae have male pregnancy and are an ideal opportunity to examine the biology and evolution of pregnancy independent of the female reproductive tract. In the seahorse Hippocampus abdominalis, the female transfers yolk-rich oocytes to the brooding pouch of the male, where they are internally fertilised and kept within the sealed pouch during development. Seahorse young are released as independent and free-swimming fry. The lining of the father’s brood pouch shares the same function as an amniote placenta, in that it likely functions in waste removal, gas exchange, osmoregulation, and nutrient provision. H. abdominalis shares homologous genes for pregnancy with mammals, suggesting that the genetic pathways that regulate pregnancy could be shared. However, there is little known about the structural and cellular changes to the brood pouch of H. abdominalis during pregnancy, including the mechanisms by which embryonic gas exchange is achieved in the sealed pouch. Our study characterised the structural changes to the inner tissue layers of the brood pouch throughout pregnancy. We found an increase in the epithelial surface area and pouch vascularity during pregnancy, which likely facilitates respiratory gas exchange during male pregnancy in this species. These changes mirror the changes to the vascular bed of gestational tissue in viviparous amniotes.