CdxA is a homeobox gene of the caudal type that was previously shown t
o be expressed in the endoderm-derived gut epithelium during early emb
ryogenesis. Expression of the CDXA protein was studied during intestin
e morphogenesis from stage 11 (13 somites) to adulthood in the chicken
. The CDXA protein can be detected during all stages of gut closure, f
rom stage 11 to 5 days of incubation, and is mainly localized to the i
ntestinal portals, the region where the splanchnopleure is undergoing
closure. In this region, which represents the transition between the o
pen and closed gut, the CDXA protein is restricted to the endoderm-der
ived epithelium. At about day 5 of incubation, the process of formatio
n of the previllous ridges begins, which marks the beginning of the mo
rphogenesis of the villi. From this stage to day 11 expression of CDXA
is localized to the epithelial lining of the intestine. In parallel,
a gradual increase in CDXA protein expression begins in the mesenchyme
that is close in proximity to the CDXA-positive endoderm. Maximal CDX
A levels in the mesenchyme are observed at day 9 of incubation. During
days 10 and 11 CDXA levels in the mesenchyme remain constant, and by
day 12 CDXA becomes undetectable in these cells and the epithelium aga
in becomes the main site of expression. From day 12 of incubation unti
l adulthood the CDXA protein is present in the intestinal epithelium.
Until day 18 of incubation expression can be detected along the whole
length of the villus with a stronger signal at the tip. With hatching
the distribution along the villi changes so that the main site of CDXA
protein expression is at the base of the villi and in the crypts. The
transient expression of CDXA in the mesenchyme between days 5 and 11
may be related to the interactions taking place between the mesenchyme
and the epithelium that ultimately result in the axial specification
of the alimentary canal and the differentiation of its various epithel
ia. The main CDXA spatial distribution during morphogenesis suggests a
tight linkage to the formation and differentiation of the intestinal
epithelium itself. CDXA appears to play a role in the morphogenetic ev
ents leading to closure of the alimentary canal. During previllous rid
ge formation the CDXA protein is transiently expressed in the mesenchy
mal cells thought to provide instructive interactions for the regional
ization and differentiation of the gut epithelium. Finally, CDXA is ex
pressed, from hatching until adulthood, in the crypts and the base of
the villi, in cells on their way to differentiate and replace those ag
ed by digestive activity.