3.7 Gastrulation of the Avian
which cells can pass into the blastocoel. Hensen's
node is the functional equivalent of the dorsal lip
of the amphibian blastopore (i.e., the organizer) and the fish embryonic
shield.
As soon as the primitive streak has formed, epiblast cells begin to migrate through it and into the blastocoel
The primitive streak has a continually changing cell population. Cells migrating through Hensen's node pass down into the blastocoel and migrate anteriorly, forming foregut, head mesoderm, and notochord; cells passing through the lateral portions of the primitive streak give rise to the majority of endodermal and mesodermal tissues.
Unlike the Xenopus mesoderm, which migrates as sheets of cells into the blastocoel, cells entering the inside of the avian embryo ingress as individuals afterundergoing an epithelial-to-mesenchymal transformation. At Hensen's node and throughout the primitive streak, the breakdown of the basal lamina and the release of these cells into the embryo is thought to be accomplished by scatter factor, a 190-kDa protein secreted by the cells as they enter the streak. Scatter factor can convert epithelial sheets into mesenchymal cells in several ways, and it is probably involved both in downregulating E-cadherin expression and in preventing E- cadherin from functioning
Migration through the primitive streak: formation of endoderm and mesoderm.
The first cells to migrate through Hensen's node are those destined to become the pharyngial endoderm of the foregut. Once inside the blastocoel, these endodermal cells migrate anteriorly and eventually displace the hypoblast cells, causing the hypoblast cells to be confined to a region in the anterior portion of the area pellucida.
This region, the germinal crescent, does not form any embryonic structures, but it does contain the precursors of the germ cells, which later migrate through the blood vessels to the gonads ..
The next cells entering the blastocoel through Hensen's node also move anteriorly, but they do not move as far ventrally as the presumptive foregut endodermal cells. Rather, they remain between the endoderm and the epiblast to form the head mesenchyme and the prechordal plate mesoderm. These early-ingressing cells all move anteriorly, pushing up the anterior midline region of the epiblast to form the head process (Figure 11.. Thus, the head of the avian embryo forms anterior (rostral) to Hensen's node. The next cells migrating through Hensen's node become chordamesoderm (notochord) cells. These cells extend up to the presumptive midbrain, where they meet the prechordal plate. The hindbrain and trunk form from the chordamesoderm at the level of Hensen's node and caudal to it.
Meanwhile, cells continue migrating inwardly through the lateral portion of the primitive streak. As they enter the blastocoel, these cells separate into two layers. The deep layer joins the hypoblast along its midline and displaces the hypoblast cells to the sides. These deep-moving cells give rise to all the endodermal organs of the embryo as well as to most of the extraembryonic membranes (the hypoblast forms the rest). The second migrating layer spreads between this endoderm and the epiblast, forming a loose layer of cells. These middle layer cells generate the mesodermal portions of the embryo and extraembryonic membranes. By 22 hours of incubation, most of the presumptive endodermal cells are in the interior of the embryo, although presumptive mesodermal cells continue to migrate inward for a longer time.
Regression of the primitive streak.
Now a new phase of gastrulation begins. While mesodermal ingression continues, the primitive streak starts to regress, moving Hensen's node from near the center of the area pellucida to a more posterior position. It leaves in its wake the dorsal axis of the embryo and the notochord. As the node moves posteriorly, the notochord is laid down, starting at the level of the future midbrain. While the anterior portion of the notochord is formed by the ingression of cells through Hensen's node, the posterior notochord (after somite 17 in the chick) forms from the condensation of mesodermal tissue that has ingressed through the primitive streak (i.e., not through Hensen's node). This portion of the notochord extends posteriorly to form the tail of the embryo Finally, Hensen's node regresses to its most posterior position, forming the anal region. At this time, all the presumptive endodermal and mesodermal cells have entered the embryo, and the epiblast is composed entirely of presumptive ectodermal cells.
As a consequence of the sequence by which the head mesoderm and notochord are established, avian (and mammalian) embryos exhibit a distinct anterior-to-posterior gradient of developmental maturity. While cells of the posterior portions of the embryo are undergoing gastrulation, cells at the anterior end are already starting to form organs . For the next several days, the anterior end of the embryo is more advanced in its development (having had a "head start," if you will) than the posterior end.
Epiboly of the ectoderm
While the presumptive mesodermal and endodermal cells are moving inward, the ectodermal precursors proliferate. Moreover, the ectodermal cells migrate to surround the yolk by epiboly. The enclosure of the yolk by the ectoderm (again reminiscent of the epiboly of amphibian ectoderm) is a Herculean task that takes the greater part of 4 days to complete. It involves the continuous production of new cellular material and the migration of the presumptive ectodermal cells along the underside of the vitelline envelope. Interestingly, only the cells of the outer edge of the area opaca attach firmly to the vitelline envelope. These cells are inherently different from the other blastoderm cells, as they can extend enormous (500 µm) cytoplasmic processes onto the vitelline envelope. These elongated filopodia are believed to be the locomotor apparatus of these marginal cells, by which they pull the other ectodermal cells around the yolk. The filopodia appear to bind to fibronectin, a laminar protein that is a component of the chick vitelline envelope. If the contact between the marginal cells and the fibronectin is experimentally broken (by adding a soluble polypeptide similar to fibronectin), the filopodia retract, and epidermal migration ceases.
Thus, as avian gastrulation draws to a close, the ectoderm has surrounded the yolk, the endoderm has replaced the hypoblast, and the mesoderm has positioned itself between these two regions. We have identified many of the processes involved in avian gastrulation, but we remain ignorant as to the mechanisms by which many of these processes are carried out.