Structure and cytoskeletal organization of migratory mesoderm cells from the Xenopus gastrula

Prospective mesoderm cells from the Xenopus gastrula exhibit interesting motile behavior, e.g., a transition from a nonmigratory state to an active translocation that can be induced experimentally, and directional substrate-guided locomotion. We examine the cytoskeletal organization of these embryonic cells. We show that the large, globular cells are enclosed in a triple shell consisting of an actomyosin cortex, a peripheral cytokeratin layer, and a peculiar microtubule basket that surrounds the cell body and constrains the distribution of large inclusions such as yolk platelets. Consistent with the migratory phenotype of these cells, no stress fibers or focal contacts are present. Mesoderm cells possess typical lamellipodia with fine protruding filopodia. The leading edge of the lamellar part exposes binding sites for the fodrin SH3 domain. Lamellipodia are connected to the cell body through actin filament bundles of the upper cell cortex. Myosin II is present in the cell body and extends to varying degrees into lamellipodia. We present indirect evidence that myosin II is located in the upper part of lamellipodia and propose a model that involves myosin II in a dynamic linkage between lamellipodium and the cortex of the cell body.