Pre-gut endoderm of chick embryos is regionalized by 1.5 days of development.
In this study, we set out to test the ability of endoderm from 1.5-day-old chick embryos (just before digestive tube formation) to develop region-specific characteristics when cultured heterotopically. Various parts of the 1.5-day endoderm were cultured in combination with the flank somatic mesoderm of 3- to 3.5-day chick embryos, and these cultures were analyzed for the expression of several transcription factors and the differentiation of the endoderm. By 1.5 days of normal development, the transcription factors, which are expressed in specific digestive organs, cSox2, CdxA, and cHoxb9/a13 were already expressed in the endodermal cells of the presumptive areas of their later expression domains. When 1.5-day pre-gut endoderm was cultured for 14-15 days, it showed specific differentiation into appropriate organ structures. In general, the more anterior part of the pre-gut endoderm formed the more rostral digestive organ structures while the posterior part became the caudal gut. The differentiation of these regions of endoderm matches their normal fate as recently elucidated (Matsushita [1996a] Rouxs Arch. Dev. Biol. 205:225-231; Matsushita [1999] Dev. Growth Differ. 41:313-319). Expression of cSox2, CdxA, and cHoxb9/a13 in endoderm cultured for 4-5 days is also consistent with their normal fate. Thus, each part of the pre-gut endoderm appears to be already regionally committed to some extent, in accordance with its fate by 1.5 days of development.