the embryo initiates gastrulation on the same side as sperm entry. One can even produce two gastrulation initiation sites by combining the natural sperm-oriented rotation with an artificially induced rotation of the egg.
3.5.1 The convergent extension of the dorsal mesoderm
Involution begins dorsally, led by the pharyngeal endomesoderm* and the prechordal plate. These tissues will migrate most anteriorly beneath the surface ectoderm.
The next tissues to enter the dorsal blastopore lip contain notochord and somite precursors. Meanwhile, as the lip of the blastopore expands to have dorsolateral, lateral, and ventral sides, the prosepective heart mesoderm, kidney mesoderm, and ventral mesoderm enter into the embryo.
Figures F depict the behavior of the IMZ cells at successive stages of Xenopus gastrulation. The IMZ is originally several layers thick. Shortly before their involution through the blastopore lip, the several layers of deep IMZ cells intercalate radially to form one thin, broad layer. This intercalation further extends the IMZ vegetally. At the same time, the superficial cells spread out by dividing and flattening. When the deep cells reach the blastopore lip, they involute into the embryo and initiate a second type of intercalation. This intercalation causes a convergent extension along the mediolateral axis that integrates several mesodermal streams to form a long, narrow band.
This is reminiscent of traffic on a highway when several lanes must merge to form a single lane. The anterior part of this band migrates toward the animal cap. Thus, the mesodermal stream continues to migrate toward the animal pole, and the overlying layer of superficial cells (including the bottle cells) is passively pulled toward the animal pole, thereby forming the endodermal roof of the archenteron. The radial and mediolateral intercalations of the deep layer of cells appear to be responsible for the continued movement of mesoderm into the embryo.
The adhesive changes driving convergent extension appear to be directed by two cell adhesion molecules, paraxial protocadherin and axial protocadherin. The former is initially found throughout the dorsal mesoderm, but then is turned off in the precursors of the notochord. At that time, axial protocadherin becomes expressed in the notochordal tissue
An experimental dominant negative form of
paraxial protocadherin (which is secreted instead of being bound to the cell membrane) prevents convergent extension. 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
3.5.2 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 (IMZS) and the prospective mesodermal cells, including those of the notochord, in its deep region (IMZD). 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. This may in part be a consequence of the different protocadherins in the axial and paraxial mesoderms. 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 embry.
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 IMZS cells that form the lining of the archenteron roof and from the subblastoporal vegetal cells that become the archenteron floor.
3.5.3 Epiboly of the ectoderm While involution is occurring at the blastopore lips,the ectodermal precursors are expanding over theentire embryo. 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.
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During early gastrulation, three rounds of cell division increase the number of the deep cell layers in the animal hemisphere. At the same time, complete integration of the numerous deep cells into one layer occurs. The most superficial layer expands by cell division and flattening. The spreading of cells in the dorsal and ventral marginal zones appears to proceed by the same mechanism, although changes in cell shape appear to play a greater role than in the animal region. The result of these expansions is the epiboly of the superficial and deep cells of the animal cap and NIMZ over the surface of the embryo. Most of the marginal zone cells, as previously mentioned, involute to join the mesodermal cell stream within the embryo. As the ectoderm epibolizes over the entire embryo, it eventually internalizes all the endoderm within it. At this point, the ectoderm covers the embryo, the endoderm is located within the embryo, and the mesoderm is positioned between them.
*The pharyngeal endoderm and head mesoderm cannot be separated experimentally at this stage, so they are therefore sometimes referred to collectively as the pharyngeal endomesoderm. The notochord is the basic unit of the dorsal mesoderm, but it is thought that the dorsal portion of the somites may also have similar properties.
Dominant negative proteins are mutated forms of the wild-type protein that interfere with the normal functioning of the wild-type protein. Thus, a dominant negative protein will have an effect similar to a loss-of-function mutation in the gene encoding the particular protein.