Transdifferentiation in holothurian gut regeneration.

It has recently been shown that the whole spectrum of cell types constituting a multicellular organism can be generated from stem cells. Our study provides an example of an alternative mechanism of tissue repair. Injection of distilled water into the coelomic cavity of the holothurian Eupentacta fraudatrix results in the loss of the whole digestive tract, except the cloaca. The new gut reforms from two separate rudiments. One rudiment appears at the anterior end of the body and extends posteriorly. The second rudiment grows anteriorly from the cloaca. In the anterior rudiment, the luminal epithelium (normally derived from endoderm) develops de novo through direct transdifferentiation of the coelomic epithelial cells (mesodermal in origin). In the posterior rudiment, the luminal epithelium originates from the lining epithelium of the cloaca. After 27 days, the two rudiments come into contact and fuse to form a continuous digestive tube lined with a fully differentiated luminal epithelium. Thus in this species, the luminal epithelia of the anterior and posterior gut rudiments develop from two different cell sources-i.e., from the mesodermally derived mesothelium and the endodermally derived epithelium of the cloacal lining, respectively. Our data suggest that differentiated cells of echinoderms are capable of transdifferentiation into other cell types.