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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]
Gastruloids lack brain as well as extraembryonic tissues but characterisation of the cellular complexity of gastruloids at the level of single cell and spatial transcriptomics, reveals that they contain representatives of the three germ layers including neural crest, Primordial Germ cells and placodal primordia. [12] [13]
The tissue from the donor embryo was therefore referred to as the inducer because it induced the change. [9] While the organizer is the dorsal lip of the blastopore, this is not one set of cells, but rather is a constantly changing group of cells that migrate over the dorsal lip of the blastopore by forming apically constricted bottle cells.
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.
The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). [1] Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm. [2] The endoderm consists at first of flattened cells, which subsequently become columnar. It forms the epithelial lining of multiple systems.
One of the early gastrulation movements in sea urchins is the invagination of a region of cells at the vegetal side of the embryo (vegetal plate) to become the archenteron, or future gut tube. There are multiple stages of archenteron invagination: a first stage where the initial folding in of tissue occurs, a second stage where the archenteron ...
Germ cells are able to migrate to their final locations to rearrange themselves and some organs are made of two germ layers; one for the outside, the other for the inside. [20] The endoderm cells become the internal linings of organisms, such as the stomach, colon, small intestine, liver, and pancreas of the digestive system and the lungs.
Mesoderm cells condense to form a rod which will send out signals to redirect the ectoderm cells above. This fold along the neural tube sets up the vertebrate central nervous system. The endoderm is the inner most germ layer of the embryo which gives rise to gastrointestinal and respiratory organs by forming epithelial linings and organs such ...