The Questions of Developmental Biology

An experimental dominant negative form of paraxial
protocadherin (which is secreted instead of being bound to the cell
membrane) prevents convergent extension (Kim et al. 1998). Moreover,
the expression domain of paraxial protocadherin separates the trunk
mesodermal cells, which undergo convergent extension, from the head
mesodermal cells, which do not.
Migration of the involuting mesoderm
As mesodermal movement progresses, convergent extension continues to narrow and
lengthen the involuting marginal zone. The IMZ contains the prospective endodermal roof of the
archenteron in its superficial layer (IMZ S ) and the prospective mesodermal cells, including those
of the notochord, in its deep region (IMZ D ). During the middle third of gastrulation, the
expanding sheet of mesoderm converges toward the midline of the embryo. This process is driven
by the continued mediolateral intercalation of cells along the anterior-posterior axis, thereby
further narrowing the band. Toward the end of gastrulation, the centrally located notochord
separates from the somitic mesoderm on either side of it, and the notochord cells elongate
separately (Wilson and Keller 1991). This may in part be a consequence of the different
protocadherins in the axial and paraxial mesoderms (Kim et al. 1998). This convergent extension
of the mesoderm appears to be autonomous, because the movements of these cells occur even if
this region of the embryo is experimentally isolated from the rest of the embryo (Keller 1986).
During gastrulation, the animal cap and noninvoluting marginal zone (NIMZ) cells
expand by epiboly to cover the entire embryo. The dorsal portion of the NIMZ extends more
rapidly toward the blastopore than the ventral portion, thus causing the blastopore lips to move
toward the ventral side. While those mesodermal cells entering through the dorsal lip of the
blastopore give rise to the dorsal axial mesoderm (notochord and somites), the remainder of the
body mesoderm (which forms the heart, kidneys, blood, bones, and parts of several other organs)
enters through the ventral and lateral blastopore lips to create the mesodermal mantle. The
endoderm is derived from the IMZ S cells that form the lining of the archenteron roof and from the
subblastoporal vegetal cells that become the archenteron floor (Keller 1986).
Epiboly of the ectoderm
While involution is
occurring at the blastopore lips,
the ectodermal precursors are
expanding over the entire
embryo. Keller (1980) and
Keller and Schoenwolf (1977)
have used scanning electron
microscopy to observe the
changes in both the superficial cells and the deep cells of the animal and marginal regions.
The major mechanism of epiboly in Xenopus gastrulation appears to be an increase in cell number
(through division) coupled with a concurrent integration of several deep layers into one (Figure
10.15).