REDOX INDICATOR PATTERNS IN RELATION TO ECHINODERM EXOGASTRULATION

Abstract

1. A modification of the Nadi or indophenol reaction, using very low concentrations of agents, and intracellular oxidation of dyes reduced by sodium hydrosulphite, demonstrates the existence of a cell-wall oxidation gradient, decreasing from the blastocoel outward and from the apical region basipetally, in normal blastulae and early gastrulae of Strongylocentrotus purpuratus, Dendraster excentricus and Patiria miniata. 2. In the evaginated entoderm of exogastrulae, derived either from the original prospective entoderm or from entodermized ectoderm, the cell-wall gradient is reversed in direction, decreasing from the external surface toward the blastocoel. 3. The cell-wall oxidation gradient of ectoderm which persists as ectoderm does not undergo reversal, but as the cell-wall decreases in thickness, it becomes difficult or impossible to determine whether it persists. 4. Reversal of the cell-wall oxidation gradient by an exogastrulating agent is regarded as resulting from two factors: first, more or less complete obliteration of the original gradient by differential inhibition; second, from establishment of the reversed gradient by the relation between diffusion of oxygen inward from the external entodermal surface and the oxygen uptake of the entodermal cells. 5. It is suggested that this reversal of the oxidation gradient pattern reverses the polarity of the entoderm, and the reversed polarity determines evagination instead of invagination. 6. In exentogastrulae entodermal evagination is followed by invagination. In some of the exentogastrulae appearing in Patiria material a "re-reversal" of the cell-wall gradient occurs in the terminal region of the invaginated part of the entoderm. It is suggested that in the exentogastrulae some trace of the original cell-wall gradient persists, although the visible gradient is reversed. After differential recovery, following return to water, some degree of reactivation of the original gradient in the cell-wall of the entodermal tip, the region of most rapid and most complete recovery, determines the beginning of invagination, and conditions in the blastocoel bring about further reactivation in some individuals. Exentogastrulae decrease in frequency or do not appear at all with increasing degrees of inhibition, presumably because the original cell-wall gradient has been completely obliterated. In entexogastrulae there is no "re-reversal" of the cell-wall gradient but merely persistence or recovery of the original gradient.