Epiblast and primitive-streak origins of the endoderm in the gastrulating chick embryo
2003
Gastrulation is characterized by the extensive movements of cells. Fate
mapping is used to follow such cell movements as they occur over time, and
prospective fate maps have been constructed for several stages of the model
organisms used in modern studies in developmental biology. In chick embryos,
detailed fate maps have been constructed for both prospective mesodermal and
ectodermal cells. However, the origin and displacement of the prospective
endodermal cells during crucial periods in gastrulation remain unclear. This
study had three aims. First, we determined the primitive-streak origin of the
endoderm using supravital fluorescent markers, and followed the movement of
the prospective endodermal cells as they dispersed to generate the definitive
endodermal layer. We show that between stages 3a/b and 4, the intraembryonic
definitive endoderm receives contributions mainly from the rostral half of the
primitive streak, and that endodermal movements parallel those of ingressing
adjacent mesodermal subdivisions. Second, the question of the epiblast origin
of the endodermal layer was addressed by precisely labeling epiblast cells in
a region known to give rise to prospective somitic cells, and following their
movement as they underwent ingression through the primitive streak. We show
that the epiblast clearly contributes prospective endodermal cells to the
primitive streak, and subsequently to definitive endoderm of the area
pellucida. Finally, the relationship between the hypoblast and the definitive
endoderm was defined by following labeled rostral primitive-streak cells over
a short period of time as they contributed to the definitive endoderm, and
combining this with in situ hybridization with a riboprobe for
Crescent , a marker of the hypoblast. We show that as the definitive
endodermal layer is laid down, there is cell-cell intercalation at its
interface with the displaced hypoblast cells. These data were used to
construct detailed prospective fate maps of the endoderm in the chick embryo,
delineating the origins and migrations of endodermal cells in various
rostrocaudal levels of the primitive streak during key periods in early
development.
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