Gastrulation is a key event, setting the stage for the inductive interactions that drive the determination and differentiation of the myriad of cell types that make life so interesting.
Gastrulation reorganizes the blastula into a three-layered embryo with a recognizable body plan.
The primary mechanisms involve cell movements rather than cell divisions as in cleavage stage. In this lecture we consider two superficially different examples, sea urchin and frog, to arrive at common properties of gastrulation. Subsequent lectures will cover avian and mammalian gastrulation. We begin this lecture by reviewing some relevant aspects of cell organization and cell behaviors in morphogenesis. Epithelia and mesenchyme Of fundamental importance in development are two basic states of cell organization and behavior termed epithelial and mesenchymal .