Gastrulation establishes the three primary tissues of an embryo: the ectoderm, mesoderm, and endoderm. This developmental process relies on a series of intricate cellular movements, which in humans transforms a flat, “bilaminar disc” composed of two cell sheets into a three-tiered structure. In the resulting embryo, the endoderm serves as the bottom layer, and stacked directly above it is the intermediate mesoderm, and then the uppermost ectoderm. Respectively, these tissue strata will form components of the gastrointestinal, musculoskeletal and nervous systems, among other derivatives.
Depending on the species, gastrulation is achieved in different ways. For example, early mouse embryos are uniquely shaped and appear as “funnels” rather than flat discs. Gastrulation thus produces a conical embryo, arranged with an inner ectoderm layer, outer endoderm, and the mesoderm sandwiched in between (similar to the layers of a sundae cone). Due to this distinct morphological feature of mice, some researchers study other models, like rabbit or chicken—both of which develop as flat structures—to gain insights into human development.
One of the main morphological features of avian and mammalian gastrulation is the primitive streak, a groove that appears down the vertical center of the embryo, and through which cells migrate to establish the mesoderm and endoderm. At the tip of the streak lies another important structure, termed the node, which appears as a conical indentation. Cells that migrate through the node not only contribute to the muscles and connective tissues of the head but also form a transient mesodermal structure called the notochord (future spinal cord) which plays a key role in directing the development of certain neurons. In addition, the node also “organizes” development in the embryo, due to the signals it produces. For example, chordin and noggin proteins emanating from the node help to direct nearby ectoderm to form neural tissue. In fact, if a mouse node is removed and transplanted into another mouse embryo, it can partially generate a second neural axis, complete with neural folds.
Since gastrulation relies on intricate cell movements to generate the three tissue layers, researchers have also tracked such migration by injecting cells of model organisms with dye and then culturing embryos. Paired with time-lapse microscopy, these techniques have revealed that in the chicken, epiblast cells are swept into the primitive streak by sweeping circular movements, and similar patterns of migration have been demonstrated in the rabbit. These techniques have also been extended to not only look at how cells shift during gastrulation, but also to track the tissue types that labeled cells will go on to form, generating detailed “fate maps” of early embryos.
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