REDOX INDICATOR PATTERNS IN RELATION TO ECHINODERM EXOGASTRULATION. II. REDUCTION PATTERNS

Abstract

1. The paper consists primarily of a new investigation of intracellular reduction patterns of certain redox indicators in relation to exogastrulation, with the echinoids, Strongylocentrotus purpuratus, and Dendraster excentricus, and the asteroid, Patiria miniata, as material. Its purpose is: first, to record results of recent studies of these patterns, made with more adequate conditions for reduction than in earlier work; and second, to attempt somewhat further physiological analysis of the patterns and of their relation to oxidation patterns, than was undertaken in the earlier study.2. Conditions which make the reduction patterns visible involve, not only the differentially inhibiting action of the exogastrulating agent, but also differentially inhibiting effects of oxygen decrease externally, and in some cases, of intracellular concentrations of oxidized dye or indophenol, and perhaps also the physiological age of the exogastrula. Usually the significance of these different factors for individual exogastrulae is not certainly distinguishable, but the differentially inhibiting effect of the exogastrulating agent is probably in general the most important.3. In the less inhibited and less extreme forms of echinoid exogastrulae, in which ectoderm attains or approaches fully developed pluteus differentiation, the evaginated entoderm almost always reduces progressively from the tip toward the ectoderm, though occasional alterations of this pattern appear. In Patiria exogastrulae, dissociation of entoderm cells from the most susceptible and most inhibited entodermal tip and adjoining regions into the blastocoel occurs very frequently. Entodermal reduction progressing from the tip has been observed less frequently in Patiria than in the echinoids and thus far only when little or no dissociation from the entodermal tip occurs. In the echinoids entodermal dissociation may also increase the cells in the blastocoel far beyond the usual number of mesenchyme cells. In general, it appears evident that the larger the number of dissociated cells in the blastocoel, the less frequently does reduction progress from the entodermal tip.4. When entodermal reduction does not progress from the tip, it begins at or near the ect-entodermal junction, or in Patiria in the entoderm near this junction, and progresses toward the entodermal tip and often acropetally for a short distance in the adjoining ectoderm. Under natural conditions, these regions are the least rapidly reducing regions of the entoderm after its activation preceding gastrulation, and of the adjoining ectoderm. In these exogastrulae they have become the regions of most rapid reduction, probably not by change in their own conditions, but by more extreme inhibition of other parts and obliteration or perhaps reversal in direction of their polar gradients.5. In completely radial exogastrulae with rounded ectoderm lacking differentiation a slight polar reduction gradient may still be visible in the apical region, usually in cases of some degree of differential recovery after return to water, perhaps sometimes with development of differential tolerance to the exogastrulating agent, or a polar ectodermal reduction gradient may be completely absent. The ventrodorsal ectodermal gradient is completely obliterated by less extreme inhibition than the polar gradient.6. Even if oxygen content in the blastocoel differs little or not at all from that in the external water, as the oxidase gradient of normal development seems to indicate, it may become much lower in the blastocoel than outside, in consequence of oxygen uptake of cells of the cell-wall and of dissociated cells in the blastocoel with sufficient decrease of external oxygen tension. Under these conditions reduction must occur most rapidly in dissociated cells in the blastocoel, which are evidently not dead in most cases, and in the cell-wall reduction will progress from the blastocoelar surface outward in exogastrulae, as well as in normal development, and not only in entoderm, but also in ectoderm unless this has become so thin that a cell-wall gradient is not distinguishable. As might be expected, the cell-wall reduction gradient does not undergo reversal in exogastrulation, as does the oxidase gradient. The reduction gradient in the cell-wall is not directly related to exogastrulation.7. In intracellular indicator reduction the indicator becomes a hydrogen acceptor and an intracellular substrate undergoes oxidation catalyzed by one or more dehydrogenases. Both intracellular oxidation of reduced redox indicators, catalyzed by oxidase, and intracellular reduction, catalyzed by dehydrogenase, are directly visible evidences of certain characteristics of oxidative metabolism, though different enzyme systems and undoubtedly different intracellular substrates are concerned in the two oxidative reaction systems.