File:Chicken endoderm origin.jpg

Schematic drawing of definitive endoderm origin.

(A) Precursors of the primitive streak have been mapped to Koller's sickle (KS) at prestreak stages (green dots), and (B) are within the primitive streak at stage 2 (red, PS).

(C) The rostral third of the extending streak (pale red) gives rise to the definitive endoderm, which ingresses displacing the hypoblast laterally and eventually forming the whole lower layer of the embryo (arrows to D).

(D) Gene expression is regionally restricted within the lower layer. The rostral germ cell crescent and displaced hypoblast (pale green hexagons) and junctional mesendoderm (pale green bars) are Gata4/5/6 positive. Rostral definitive endoderm is Sox17 positive (yellow), with more caudal definitive endoderm expressing only Gata6 transcripts by stage 4 (green in area pellucida). The area of overlap at the boundary between Sox17 and Gata6 is marked by asterisks (blue).

Original File name: 1471-213X-7-91-2.jpg

Specification of germ layer identity in the chick gastrula. Chapman SC, Matsumoto K, Cai Q, Schoenwolf GC. BMC Dev Biol. 2007 Jul 30;7:91. PMID: 17663788

"Our results demonstrate that germ layer identity is fixed at the time populations of endoderm and mesoderm cells ingress through the primitive streak, whereas their gene expression patterns remain labile. In addition, our results show that inductive and repressive signals are present, and that these signals regulate gene expression of both ingressed endoderm and mesoderm cells. Thus, gastrula cells display elements of both pre-patterning and plasticity, with endoderm the first germ layer becoming committed to its fate during early gastrulation stages."

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

BMC Dev Biol. 2007; 7: 91. Published online 2007 July 30. doi: 10.1186/1471-213X-7-91. Copyright © 2007 Chapman et al; licensee BioMed Central Ltd.