Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst, is reorganized into a two-layered or three-layered embryo known as the gastrula.

Lewis Wolpert, pioneering developmental biologist in the field, has been credited for noting that "It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life.

Model organisms whose gastrulation is understood in the greatest detail include the mollusc, sea urchin, frog, and chicken.

[citation needed] The major distinctions between deuterostomes and protostomes are found in embryonic development: Sea urchins have been important model organisms in developmental biology since the 19th century.

Gastrulation – internalization of the prospective endoderm and non-skeletogenic mesoderm – begins shortly thereafter with invagination and other cell rearrangements the vegetal pole, which contribute approximately 30% to the final archenteron length.

[17] Hilde Mangold, working in the lab of Hans Spemann, demonstrated that this special "organizer" of the embryo is necessary and sufficient to induce gastrulation.

[citation needed] Specification of endoderm depends on rearrangement of maternally deposited determinants, leading to nuclearization of Beta-catenin.

Other signals such as Wnt and BMP also play a role in respiratory fate of the Xenopus by activating cell lineage tracers.

[7] The primitive streak is formed at the beginning of gastrulation and is found at the junction between the extraembryonic tissue and the epiblast on the posterior side of the embryo and the site of ingression.

[7] During the early stages of development, the primitive streak is the structure that will establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation.

[26] The primitive streak extends through this midline and creates the antero-posterior body axis,[27] becoming the first symmetry-breaking event in the embryo, and marks the beginning of gastrulation.

[28] This process involves the ingression of mesoderm and endoderm progenitors and their migration to their ultimate position,[27][29] where they will differentiate into the three germ layers.

[26] The localization of the cell adhesion and signaling molecule beta-catenin is critical to the proper formation of the organizer region that is responsible for initiating gastrulation.

[30] Fibroblast growth factors (FGF), canonical Wnt pathway, bone morphogenetic protein (BMP), and retinoic acid (RA) are all important in the formation and development of the endoderm.

These are associated with number of clear advantages in using tissue-culture based protocols, some of which include reducing the cost of associated in vivo work (thereby reducing, replacing and refining the use of animals in experiments; the 3Rs), being able to accurately apply agonists/antagonists in spatially and temporally specific manner[35][36] which may be technically difficult to perform during Gastrulation.

To illustrate this, the guided differentiation of mouse ESCs has resulted in generating primitive streak–like cells that display many of the characteristics of epiblast cells that traverse through the primitive streak[31] (e.g. transient brachyury up regulation and the cellular changes associated with an epithelial to mesenchymal transition[31]), and human ESCs cultured on micro patterns, treated with BMP4, can generate spatial differentiation pattern similar to the arrangement of the germ layers in the human embryo.

[32][33] Finally, using 3D embryoid body- and organoid-based techniques, small aggregates of mouse ESCs (Embryonic Organoids, or Gastruloids) are able to show a number of processes of early mammalian embryo development such as symmetry-breaking, polarisation of gene expression, gastrulation-like movements, axial elongation and the generation of all three embryonic axes (anteroposterior, dorsoventral and left-right axes).

The countries that believe this have created a 14-day rule in which it is illegal to study or experiment on a human embryo after the 14-day period in vitro.