When irritated, individuals of the sea cucumber Holothuria forskali expel a few Cuvierian tubules which lengthen, instantly become sticky, and rapidly immobilize most organisms with which they come into contact. After expulsion, the lost tubules are readily regenerated. When only a few tubules have been expelled, there is often a latent period before the regeneration starts. In contrast, when many tubules have been expelled, the regenerative process starts immediately but proceeds in successive waves of 10 to 30 tubules that begin to regenerate at 10-day intervals. However, in all cases, the complete regeneration of a given tubule takes about 5 weeks and may be divided into three successive phases: an initial repair phase including the overall 48-h post-autotomy period, a true regenerative phase taking about 4 weeks to complete, and a growth phase of about one more week. Initial regeneration events occur by epimorphosis, cell proliferation being essential to the regenerative process, whereas late events occur mainly by morphallaxis, with migration of the newly differentiated cells. The mesothelium is the tissue layer in which cell proliferation is the most precocious and the most important, involving both peritoneocytes and undifferentiated cells (which seem to be dedifferentiated peritoneocytes). As regeneration proceeds, the percentage of undifferentiated cells regularly decreases in parallel with the differentiation of granular (adhesive-secreting) cells and myocytes. The myocytes then separate off from the mesothelium and migrate within the connective tissue layer. Three types of pseudopodial cells follow one another in the tubule connective tissue during regeneration. Type 1 cells have all the characteristics of echinoderm phagocytes and may have a fibroclastic function, cleaning the connective tissue compartment before new collagen synthesis starts. Type 2 cells are rather undifferentiated and divide actively. The presence of type 3 cells is closely associated with the appearance of collagen fibers, and it is suggested that they have a fibroblastic function. In the inner epithelium, cells also divide actively, but only those in which spherules have not yet differentiated in the basal intraconnective processes. It appears, therefore, that in the three tissue layers of the tubules, regeneration proceeds by cell dedifferentiation, then proliferation, and finally by differentiation. Cuvierian tubules thus constitute a very efficient defensive mechanism: their large number, sparing use, and particular regeneration dynamics make them an almost inexhaustible line of defense maintained at limited energy cost. |
