Developing Inside a Layer of Germs-A Potential Role for Multiciliated Surface Cells in Vertebrate Embryos

This paper reviews current research on the microbial life that surrounds vertebrate embryos. Several clades are believed to develop inside sterile-or near-sterile-embryonic microhabitats, while others thrive within a veritable zoo of microbial life. The occurrence of embryo-associated microbes in some groups, but not others, is an under-appreciated transition (possibly transitions) in vertebrate evolution. A lack of comparable studies makes it currently impossible to correlate embryo-associated microbiomes with other aspects of vertebrate evolution. However, there are embryonic features that should instruct a more targeted survey. This paper concludes with a hypothesis for the role of multiciliated surface cells in amphibian and some fish embryos, which may contribute to managing embryo-associated microbial consortia. These cells are known to exist in some species that harbor in ovo microbes or have relatively porous egg capsules, although most have not been assayed for embryo-associated microbiota. Whether the currents generated within these extraembryonic microhabitats contribute to culturing consistent microbial communities remains to be seen.