<i>Sox17</i> Expression Patterns during Gastrulation and Early Neurulation in the Rabbit Suggest Two Sources of Endoderm Formation

Most gastrointestinal tract and associated gland epithelia originate from the endoderm germ layer discovered by Pander in 1817. The recent surge in stem cell concepts revived interest in the findings of 30 years ago that the endoderm layer itself originates from the epiblast (which since Pander's time had been held to be the forerunner of the ectoderm and mesoderm germ layers only). However, the question as to which parts of the mammalian gastrulation-stage embryonic disc generate endoderm cells is still unresolved. Therefore, the expression of the gene coding for the transcription factor Sox17, a key transcription factor involved in endoderm formation in mouse, chick, frog, and zebrafish, was analyzed in the rabbit, a model organism for mammalian gastrulation morphology, using whole-mount in situ hybridization and high-resolution histological analysis of embryos at gastrulation and early neurulation stages. Sox17 mRNA in the mesoderm and lower layer (hypoblast) compartments within and adjacent to Hensen's node and the anterior segment of the primitive streak confirmed the validity of this approach, as this region had previously been shown to form endoderm in mouse and chick. However, Sox17 expression in central and posterior epiblast at pregastrulation stages together with a transient expression at the posterior extremity of the primitive streak suggest that endoderm (possibly hindgut) may be formed close to the emerging cloacal membrane, as well.