Abstract
Coelomocyte filopodia are composed primarily of actin and a 57000 D protein, which is antigenically related to the actin cross-linking protein found in needles produced in vitro from extracts of sea urchin oocytes. In this report we present electron micrographs of the filopodia which show longitudinal striations corresponding to bundled, F-actin filaments and transverse bands corresponding to a cross-linking protein. The periodicities of the spacings are 90 and 130 Å respectively. Interestingly, the filaments in the filopodia are less well ordered than those in needles, probably as a result of the speed with which the filopodia are assembled in the transforming coelomocytes. These results indicate that the well ordered needles are a good in vitro model for important in vivo structures.