Gastrulation is the defining morphogenetic event of embryology wherein the blastula is restructured through massive coordinate cell rearrangements into a triploblastic body plan comprising ectoderm, mesoderm, and endoderm.
1. Cellular Mechanics of Gastrulation Movements
- Invagination: Infolding of an epithelial sheet of cells into the blastocoel, similar to poking a soft rubber ball (e.g. sea urchin vegetal plate endoderm).
- Involution: Inturning of an expanding outer cell layer over the inner surface of an outer rim (e.g. amphibian mesoderm over the dorsal blastopore lip).
- Ingression: Migration of individual cells from the blastoderm surface into the blastocoel cavity via epithelial-to-mesenchymal transition / EMT (e.g. chick primitive streak, sea urchin primary mesenchyme).
- Delamination: Splitting of one cellular sheet into two roughly parallel sheets (e.g. mammalian hypoblast and epiblast delamination).
- Epiboly: Movement of epithelial sheets (usually ectodermal) spreading unitarily as a unit to enclose the deeper layers of the embryo.
2. The Spemann-Mangold Organizer (1924)
Hans Spemann and Hilde Mangold discovered that transplanting the dorsal blastopore lip of an amphibian gastrula into the ventral side of a host gastrula induced a complete conjoined secondary embryo containing a brain and neural tube.
Molecular Mechanism of Neural Induction: The default fate of ectoderm is neural. BMP-4 secreted ventrally actively induces ectoderm to become epidermis. The Organizer secretes BMP antagonists—Noggin, Chordin, and Follistatin—which bind BMP-4 in the extracellular space, permitting dorsal ectoderm to adopt its default neural plate fate.