STUDIES ON VISCERAL REGENERATION IN SEA-STARS. II. REGENERATION OF PYLORIC CAECA IN ASTERIIDAE, WITH NOTES ON THE SOURCE OF CELLS IN REGENERATING ORGANS

Abstract

1. The regenerative replacement of excised pyloric caeca has been studied in three species of sea-stars belonging to the Family Asteriidae. Regenerating specimens have been observed, and the histological events of regeneration followed by serial sections, to the point at which essentially normal organs are again in place and functioning. The process of regeneration is practically identical in all, but it occurs at a much slower rate in Leptasterias and Pisaster than in Asterias. Rate differences are attributable to differences in environmental temperature. 2. Caecal regeneration involves a sequence of changes resembling those previously observed in Henricia. Mesenteric tunnels guide the advance of simple tubular outgrowths from the pyloric duct; bifurcation of the caecum occurs at the point where the tunnel forks, giving rise to paired tubes growing distally parallel with each other. Differentiation of epithelial elements, with functional secretory cells, proceeds close behind the advancing tip of each tube. More proximally, lateral expansions produce pockets in which gland cells become localized, while less highly-specialized current-producing cells line the roof and floor of the median duct. The gradient of differentiation extends from the young, undifferentiated growing tip to the oldest region at the base of the regenerate. The pyloric caecum growing inside a regenerating ray following induced autotomy forms through the same sequence of events found in replacement of an excised caecum. Comparison with descriptions of post-larval formation of the pyloric caeca in the developing sea-star reveals that regeneration follows the same course as original formation. 3. Isolated segments of partially extirpated organs are resorbed without producing regenerative growth; their presence in the mesenteric tunnel does not affect normal outgrowth from the central stump. Pisaster and Asterias are capable of regenerating all 5 pairs of pyloric caeca at once, at rates somewhat slower than those involved in single-pair replacement. The return of function in these caecumless animals has been charted by observation of feeding behavior and digestive capability. 4. The suggestion that growth of the replacement caecum occurs through mobilization and incorporation of amoebocytes, based on earlier failure to identify mitotic activity in Henricia, is now shown to be invalid. Autoradiographs made from sections of two-week regenerates in Asterias after injection of thymidine-H³ reveal large numbers of cells in some phase of mitotic activity. Direct observation of mitotic figures in these same preparations confirms the fact that growth of the regenerating caecum involves mitotic proliferation. Cell division is most common among apparently undifferentiated cells with large, vesicular nuclei; it occurs where such cells occur, in both lining epithelium and covering peritoneum, at all levels of the advancing regenerate. In proximal regions, dividing cells are most numerous where the caecal walls are outfolding to form glandular pockets.