Maintaining the line of defense: Regeneration of cuvierian tubules in the sea cucumber Holothuria forskali (Echinodermata, Holothuroidea)

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 expulsi on, the lost tubules are readily regenerated. When only a few tubules have been expelled, there is often a latent period before the regeneration start s. In contrast, when many tubules have been expelled, the regenerative proc ess 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 c omplete regeneration of a given tubule takes about 5 weeks and may be divid ed into three successive phases: an initial repair phase including the over all 48-h post-autotomy period, a true regenerative phase taking about 4 wee ks to complete, and a growth phase of about one more week. Initial regenera tion events occur by epimorphosis, cell proliferation being essential to th e regenerative process, whereas late events occur mainly by morphallaxis, w ith migration of the newly differentiated cells. The mesothelium is the tis sue layer in which cell proliferation is the most precocious and the most i mportant, involving both peritoneocytes and undifferentiated cells (which s eem to be dedifferentiated peritoneocytes). As regeneration proceeds, the p ercentage of undifferentiated cells regularly decreases in parallel with th e differentiation of granular (adhesive-secreting) cells and myocytes. The myocytes then separate off from the mesothelium and migrate within the conn ective tissue layer. Three types of pseudopodial cells follow one another i n the tubule connective tissue during regeneration. Type 1 cells have all t he characteristics of echinoderm phagocytes and may have a fibroclastic fun ction, cleaning the connective tissue compartment before new collagen synth esis starts. Type 2 cells are rather undifferentiated and divide actively. The presence of type 3 cells is closely associated with the appearance of c ollagen fibers, and it is suggested that they have a fibroblastic function. In the inner epithelium, cells also divide actively, but only those in whi ch spherules have not yet differentiated in the basal intraconnective proce sses. 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 effici ent defensive mechanism: their large number, sparing use, and particular re generation dynamics make them an almost inexhaustible line of defense maint ained at limited energy cost.