Absence of furrowing activity following regional cortical tension reduction in sand dollar blastomere and fertilized egg fragment surfaces

The purpose of the present investigation was to test experimentally the pos sibility that division mechanism establishment at the equator of sand dolla r eggs may be a consequence of cortical tension gradients between the equat or and the poles. Cytochalasin has been shown to decrease tension at the se a urchin egg surface. The concave ends of cytochalasin D-containing agarose cylinders were held against regions of the surface of Echinarachnius parma blastomeres and enucleated fertilized egg fragments. The ability to interf ere with normal furrowing activity was used as a biological indicator of th e effectiveness of cytochalasin. When agarose containing 2 mu g/mL cytochal asin contacted the equatorial region of the blastomeres resulting from the first cleavage, or the equatorial surfaces of nucleated fertilized egg halv es, furrowing was blocked, stalled or delayed, indicating that the concentr ation of cytochalasin was effective. When the same concentration of cytocha lasin was applied to the poles, the cells and nucleated fertilized egg frag ments divided in the same way as the controls, indicating that the effectiv eness of the cytochalasin did not spread from the poles to the equator and that bisection did not interfere with the division of nucleated fertilized egg fragments. When the same concentration of cytochalasin was applied to d iametrically opposed surfaces of enucleated, spherical egg fragments, there was no evidence of furrowing activity between the areas that contacted the cytochalasin or in any other part of the surface, Because of the tension-r educing effect of cytochalasin, a tension gradient existed between the regi ons affected and unaffected by cytochalasin. The results strongly suggest t hat establishment of the division mechanism by simple gradients of tension at the surface is unlikely.