Pharyngeal regeneration in the land planarian Bipalium kewense

Abstract

Bipalium kewense Moseley, which propagates only asexually by fission in Japan, will regenerate a head and pharynx in both spontaneously fissioned and experimentally cut pieces. The regeneration is completed within two weeks of the time the cut or break is produced. Histological preparations of regenerating pieces at 5–6 d after transection or fission showed accumulations of numerous basophilic cells in the parenchyma between the ventral nerve cord and the disintegrating intestinal wall. At 7–8 d, a slit formed among these cells, and by 8–9 d, they had organized themselves into an anlage of the pharynx and the slit had extended. The newly formed pharynx eventually became fully functional. The basophilic cells appeared to be derived from different sources, some from so-called neoblasts originating in the parenchyma and some from the intestinal wall. The neoblasts seemed to migrate along the ventral nerve cord into the region where the new pharynx was to be formed. At the same time, the intestinal wall in this vicinity disintegrated considerably, and cells of the intestine were extruded into the parenchyma where they appeared to dedifferentiate. As undifferentiated cells, these were likely to take part in the formation of the new pharynx in concert with the neoblasts.