Origin of calcified tissue in regenerating spines of the sea urchin, Strongylocentrotus purpuratus (Stimpson): A quantitative radioautographic study with tritiated thymidine

Cellular proliferation in the calcified dermis during regeneration of fractured spines of the sea urchin, Strongylocentrotus purpuratus (Stimpson), was studied quantitatively in radioautograms of tissues labeled with tritiated thymidine in vivo or in vitro to establish the origin of regenerated calcified tissue. Non‐fractured, whole spines were included for comparison. In short term experiments, incorporation of tritiated thymidine occurred throughout the length of regenerating and non‐fractured spines, with a peak in the number and proportion of labeled cells in the shaft occurring near the milled ring, declining to low levels in the the base and tip. A localized apical growth zone was absent in these spines. In pulse‐labeling experiments, there was a marked distal shift in the distribution of labeled cells during regeneration of fractured spines and in whole spines presumably due to migration. Significantly, labeled cells appeared in increasing numbers throughout the regenerate during growth. Few migratory coelomocytes within the calcified dermis were observed to incorporate detectable quantities of tritiated thymidine. The results of this study are consistent with the hypothesis that regenerated connective tissue of the calcified dermis originates by distal migration of cells, primarily from proximal regions of relatively high mitotic activity within the shaft near the level of the milled ring.