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Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body (e.g. dorsal–ventral, anterior–posterior), and internalized one or more cell types including the prospective gut. [2]
The next cells passing through Hensen's node become the chordamesoderm. The chordamesoderm has two components: the head process and the notochord. The most anterior part, the head process, is formed by central mesoderm cells migrating anteriorly, behind the prechordal plate mesoderm and toward the rostral tip of the embryo.
The cells continue to be rearranged until the shallow dip formed by invagination transforms into a deeper, narrower pouch formed by the gastrula's endoderm. This pouch narrows and lengthens to become the archenteron, a process driven by convergent extension. The open end of the archenteron is called the blastopore.
Before gastrulation, the cells of the trophoblast become differentiated into two layers: The outer layer forms a syncytium (i.e., a layer of protoplasm studded with nuclei, but showing no evidence of subdivision into cells), termed the syncytiotrophoblast, while the inner layer, the cytotrophoblast, consists of well-defined cells.
This matches with the "flaps-folding-over" model of gut formation, but an alternative view is that the original blastopore migrated forwards to one end of the ancestral organism before deepening to become a blind gut. [1] This is consistent with living Xenacoelomorpha, which are the sister taxon to protostomes and deuterostomes.
Epiboly in zebrafish is the first coordinated cell movement, beginning at the dome stage late in the blastula period and continuing throughout gastrulation. [3] At this point the zebrafish embryo contains three portions: an epithelial monolayer known as the enveloping layer (EVL), a yolk syncytial layer (YSL) which is a membrane-enclosed group of nuclei that lie on top of the yolk cell, and ...
[1] [2] [3] To quote from the original paper: "Altogether, these observations further emphasize the similarity between the processes that we have uncovered here and the events in the embryo. The movements are related to those of cells in gastrulating embryos and for this reason we term these aggregates ‘gastruloids’".
The morula then develops by cavitation to become the blastocyst, or in many other animals the blastula. Cell differentiation then further commits the morula's cells into two types: trophectoderm cells that surround the lumen and the inner mass of cells (the embryoblast). The inner cell mass is at the origin of embryonic stem cells. [15]