Cuvierian tubules of the holothuroid Holothuria forskali (Echinodermata): a morphofunctional study |
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Authors: | D VandenSpiegel M Jangoux |
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Institution: | (1) Laboratoire de Biologie marine (CP 160), Université Libre de Bruxelles, 50 avenue F.D. Roosevelt, B-1050 Bruxelles, Belgium |
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Abstract: | Quiescent Cuvierian tubules of Holothuria forskali (Delle Chiaje, 1823) have a pseudostratified mesothelium made of ciliated cells that cover elongated cells containing lipoproteie
granules. Together these cells form conspicuous transversal folds that penetrate the underlaying connective tissue. Circular
and longitudinal muscle cells occur under the tips of the mesothelial folds. These cells are presumably myoepithelial cells;
they divide the connective tissue into a thin outer layer (where collagen fibers are directed perpendicularly to the tubule
long axis) and a thick inner layer (where collagen fibers are arranged in helices parallel to the tubular long axis). Nerve
processes are associated with the longitudinal muscles; they also occur between the spirally arranged collagen fibers. A spiral
nerve runs along the tubules; it innervates the circular muscles. The tubule lumen is narrow with a highly convoluted limiting
epithelium. Epithelial cells are not ciliated; they have basal processes that contain enlarged spherules including mucosubstances
and proteins. While having the same diameter as quiescent tubules, elongated tubules have a much thiner wall surrounding an
enlarged cylindrical lumen. The mesothelium has no adluminal cells and consists of unfolded granular cells. In contact with
a solid surface, granules become a sticky secretion. Longitudinal muscle fibers disintegrate and circular fibers form conspicuous
rings along the tubules. Collagen helices still occur, but are considerably elongated and flattened. Inner epithelial cells
dissociate from each other and typically lack basal processes and spherules, the latter being secreted into the lumen of elongating
tubules. Discharge of Cuvierian tubules results from tubule expulsion and elongation. Tubule expulsion is triggered by the
opening of the anus of a contracted holothuroid and results from the rupture of the cloaca with subsequent expulsion of coelomic
fluid. Tubule elongation results both from the mechanical stimulation of the tubule sphincter by hydrostatic pressure and
the entry of water into the tubule. Mechanical stimulation of the sphincter results in a rapid contraction of the circular
muscles that causes the disintegration of adluminal cells, the unfolding of granular cells and the stretching of inner epithelial
cells. Entry of water immediately follows sphincter stimulation and complete tubule elongation. Water pressure causes the
irreversible elongation of collagen helices, which results both in a considerable thinning of the tubule wall and the formation
of enlarged water-filled lumen. It is suggested that slippage of collagen fibers in elongating tubules is nervously mediated. |
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