gastrulaatio

Gastrulaatio, or gastrulation, is a key stage in animal embryogenesis. During gastrulation, the blastula, a hollow ball of cells, reorganizes into a multilayered structure called the gastrula. This transition establishes the three primary germ layers—ectoderm, mesoderm, and endoderm—and sets up the major body axes. Gastrulation follows cleavage and precedes organogenesis, with timing that varies among species.

The process involves coordinated cell movements and shape changes, such as invagination (folding inward), involution (sheet migration), ingression (individual cells moving inward), delamination (splitting into layers), and epiboly (expansion of cell layers). These movements rearrange cells to form the inner layer (endoderm) and outer layer (ectoderm), with mesoderm arising between them to give rise to muscles, bones, and other tissues.

In many animals, an organizer region directs patterning: the primitive streak in birds and mammals or analogous regions in other vertebrates; in amphibians, bottle cells mark the onset of involution. Signaling networks, including Nodal, BMP, Wnt, and FGF pathways, regulate tissue specification and axis formation. Proper gastrulation is critical for gastrulating tissues to produce functional organs.

Gastrulation exhibits variation across taxa, but remains a conserved mechanism for transforming a simple blastula into a structured embryo with distinct germ layers. Disruptions can result in severe developmental defects.