STUDIES ON VISCERAL REGENERATION IN SEA-STARS. I. REGENERATION OF PYLORIC CAECA IN HENRICIA LEVIUSCULA (STIMPSON)

¹ Department of Zoology, Cornell University, Ithaca, N. Y.

1. Regeneration of pyloric caeca has been studied grossly and histologically in a series of specimens sacrificed one, two, three, four, six, and eight weeks following operative removal of the caeca by way of a median longitudinal incision through the aboral body wall of one ray.

2. After healing of the incision, which is fairly complete at the close of the second week, the cut stumps of the caeca are found to have healed over. The original web-like suspensory mesenteries have become continuous mesothelial sheets and have fused in pairs at their free edges to form mesenteric tunnels leading outward into the ray. The parietal peritoneum has become conspicuously hypertrophic, its normally flattened cells now tall and filled with deeply-staining cytoplasmic spherules.

3. During subsequent weeks one or both of the mesentenic tunnels may be invaded by an outgrowth from the central stump of the caeca, the outgrowing tongue of cells pushing through the mesenchymal thickening at the summit of the tunnel enclosed between the mesothelial layers. During early weeks one outgrowth usually lags behind the other; in later stages the two are more nearly equal in length and development. The tip of the outgrowth consists of a rod of large, undifferentiated cells sharply marked off from the surrounding mesenchyme. A short distance behind the tip a lumen develops simply as a cleft in the packed cells of the cylindrical regenerate. Nearer the base, the cells are seen to have undergone rearrangement to form a more regular lining, composed of taller cells showing the beginnings of a brush border and flagella. At all stages the outgrowth presents a regular gradient of differentiation between tip and base, the cells forming the tip being undifferentiated and young while those nearer the base show a gradual increase in degree of differentiation. At later stages this culminates, basally at least, in the formation of an essentially normal epithelium, provided with mucous gland cells and zymogen cells. Subepithelial components, such as the characteristic muscular and connective-tissue layers, are slower to form but are in evidence at the base of the regenerate by the eighth week. The normal marked distinction between oral and aboral parts of the caecum begin to be suggested also by the eighth week; it is apparent that Tiedemann's pouch will form not by outgrowth along a broad front from its remnant stump but by specializations in the floor of the tubular caecal regenerate.

4. The process of regeneration in the pyloric caecum differs in at least one important respect from that involved in replacement of the parietal parts of a regenerating ray. In the regeneration of the entire ray, the highly differentiated distal tip with its sensory specializations forms first and precedes the more proximal, less differentiated areas as the ray elongates. By contrast, in the caecal outgrowth there is clearly no precociously differentiated tip region; the tip is obviously the youngest, least-differentiated portion. The sequence of events in regeneration of the pyloric caecum resembles to a degree that which has been described for replacement of the digestive tract following evisceration in Holothuria.

5. A particularly puzzling aspect of the regenerative process involves the fact that no zone of mitotic activity has been found in the caecal outgrowth, although such a zone might be expected to lie near the advancing tip of the regenerate. This raises the question as to the source of cells in the newly forming caecum, and it is suggested that the cells may come not from proliferation but from recruitment and incorporation of amoebocytes gathering in the mesenchymal layer through which the regenerate grows. A more general question is raised concerning the role of cell proliferation in other cases of regeneration in echinoderms; only in the somewhat atypical case of gut-replacement from mesenchyme in the sea-cucumber Stichopus is there explicit description of mitotic activity as a feature of regeneration.