in the original experiment the dorsal lip of the blastopore

This fibronectin rich layer appears on the undersurface of the upper layer shortly before the formation of the primitive streak and disappears in the region of the streak. Hyaluronic acid may be able to keep the cells separated by its ability to expand in water.įurther, the cellular movement of these cells is correlated with the presence of a fibronectin meshwork in the extra cellular basal lamina of the epiblast cells.

Due to this coating the cells remain separated from each other. They facilitate individual cell migration by coating the surfaces of incoming cells. Extracellular polysaccharides like hyaluronic acid may play an important role in this migration. Like the amphibian gastrulation, the cells of chick embryo passing through the blastopore lip constrict their apical ends to become bottle cells. (1) The primitive pit represents the dorsal opening of the blastopore (neurenteric canal). The only difference is that the avian blastopore is elongated whereas amphibian blastopore is circular. The primitive streak with its Hensen’s node is analogous to the blastopore and its dorsal lips of amphibian gastrula. Thus the fully formed chick gastrula consists of these germ layers-ectoderm, chorda-mesoderm and the endoderm. 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. The enclosure of the yolk by the ectoderm takes greater part of 4 days to complete and involves the continuous production of new cellular material at the expense of the yolk and the migration of the- presumptive ectodermal cells along the undersides of the vitelline envelope. While the presumptive mesodermal and endodermal cells are moving inward, the ectodermal precursors surround the yolk by epiboly. 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. This sheet gives rise to mesodermal portions of the embryo and extra embryonic membranes. The second migrating stream spreads throughout the blastocoel as a loose sheet, roughly mid-way between the hypoblast and the epiblast. These deep-moving cells give rise to all the endodermal organs of the embryo as well as to most of the extra embryonic membranes. One stream moves deeper and joins the hypoblast along its mid-line, displacing the hypoblast cells to the sides. As they enter the blastocoel, these cells separate into two streams. Meanwhile, cells continue migrating inward through the primitive streak. These early ingressing cells have all moved anteriorly, pushing up the anterior midline region of the epiblast to form the head process. These cells remain between the endoderm and the epiblast to form the head mesoderm and the chorda mesoderm (notochordal) cells. The next cells entering the blastocoel through Hensen’s node also move anteriorly, but they do not move as far ventrally as the presumptive endodermal cells. Inside the blastocoel, these cells migrate anteriorly and eventually displace the hypoblast cells in the anterior portion of the embryo. The first cells to migrate through the primitive streak are those destined to become the foregut. At the same time, the secondary hypoblastic cells continue to migrate anteriorly from the posterior margin of the blastoderm. Moreover, no true archenteron is formed in avian gastrula.Īs the cells enter the primitive streak, the streak elongates toward the future head region. The cells entering the inside of the avian embryo form a lossely connected mesenchyme. The cells which migrate through the Henson’s node pass down into the blastocoel and migrate anteriorly form head mesoderm and notochord, and those cells which pass through the lateral portion of the primitive streak forms the majority of endodermal and mesodermal tissues. As soon as the primitive streak is formed, blastoderm cells begin to migrate over the lips of the primitive streak and into the blastocoel (Fig. There is a funnel shaped depression in the centre of the node through which cells can pass into the blastocoel.

In the original experiment the dorsal lip of the blastopore