Microfilaments in the external surface layer of the early amphibian embryo.

A comparison was made by transmission electron microscopy of the microfilaments in the surface layers of the early embryos of Triturus alpestris and Xenopus laevis at stages of development up to neurulation. Actin-like filaments which bound heavy meromyosin (HMM) were found in cell extracts of all stages, but were comparatively rare in the newly fertilized egg. Ten nm microfilaments were present throughout development in Xenopus, and from the mid-neurula stage in Triturus. Both kinds of microfilament were located in the circumferences of superficial ectoderm cells at the level of the apical junctions, the 10 nm microfilaments in association with desmosomes which began to develop before gastrulation in Xenopus. The accumulations of microfilaments in the apical constrictions, which form in ectoderm cells of Triturus early gastrulae when dissociated in a calcium-free medium, suggest that they are contractile elements. In the absence of such accumulations in the cell apices, the reverse curling exhibited by Xenopus ectodermal explants is attributed rather to a separation of the cells' lateral borders. Cytochalasin B (5 mug/ml) caused ectodermal explants from the early gastrulae of both species to disaggregate. With the rupture of the apical junctions there was a disorganization of the associated microfilamentous layer.