| As part of the deuterostomes, echinoderms constitute excellent model systems to study regeneration. Few holothurian regeneration studies have been accomplished, and even less have focused at regeneration at the ultrastructural level. This is the first ultrastructural analysis on the holothurian intestine regeneration that has been reported, investigating the different cell populations of the tissue layers of the H. glaberrima regenerating intestine and mesentery, and suggesting their possible roles in the regeneration process. This study has found that during the first two weeks of regeneration, both the muscle cells and coelomic epithelial cells of the mesentery adjacent to the regenerating intestine undergo dedifferentiation, resulting in the formation of undifferentiated cells that have a possible important role in intestinal regeneration. At the same time, different cell components of the perivisceral nervous system undergo degeneration and phagocytosis. These include the visceral plexos nerve cells of the regenerating mesentery mesothelium, and the bipolar nerve cells and nerve fibers of the mesentery and intestinal connective tissue. These nerve cell components suffer degeneration and phagocytosis as part of the necessary nervous system remodeling that occurs during the H. glaberrima intestinal regeneration. Finally, different populations of cells that have the coelomic fluid and body wall as their possible source, have very important roles in the formation of the new intestine. These include spherule cells, phagocytic amoebocytes, fibroblasts, lymphocytes, and other coelomic fluid cells. While spherule cells have important roles in the formation of the new connective tissue ECM fibers, phagocytic amoebocytes are involved since the first week of regeneration in the removal of foreign materials, such as bacteria and parasites, and different degenerating cell components that include nerve fibers, collagen fibrils, and apoptotic cell debris. Amoebocytes are thus part of the echinoderm defense system, together with spherule cells and other cells of the coelomic fluid. Also, a possible transformation of lymphocytes or undifferentiated cells could be responsible for the formation of important cell populations, which could include peritoneocytes or intestinal epithelial cells. This study has provided essential information on the different cellular aspects of the H. glaberrima intestinal regeneration, an important phase in the study of any regeneration model system, that has to be investigated as completely as possible before accomplishing the subsequent genetic analysis. |
