Ultrastructure of sea urchin tube feet

Abstract

An analysis of the ultrastructure of the tube feet of three species of sea urchins (Strongylocentrotus franciscanus, Arbacia lixula and Echinus esculentus) revealed that the smooth muscle, although known to be cholinoceptive, receives no motor innervation. The muscle fibers are attached to a double layer of circular and longitudinal connective tissue which surrounds the muscle layer and contains numerous bundles of collagen fibers. On its outside, the connective tissue cylinder is invested by a basal lamina of the outer epithelium to which numerous nerve terminals are attached. These are part of a nerve plexus which surrounds the connective tissue cylinder. The plexus itself is an extension of a longitudinal nerve that extends the whole length of the tube foot. It is composed of axons, but nerve cell bodies and synapses are conspicuously lacking, suggesting that the axons and terminals derive from cells of the radial nerve. Processes of the epithelial cells penetrate the nerve plexus and attach to the basal lamina. There is no evidence that the epithelial cells function as sensory cells. On the basis of supporting evidence it is suggested that the transmitter released by the nerve terminals diffuses to the muscle cells over a distance of several microns and in doing so affects the mechanical properties of the connective tissue.

The peristomial membrane of regular sea‐urchins: Functional morphology of the epidermis and coelomic lining inParacentrotus lividus(Lamarck)

“…9a) formed by two main populations of cells: apical monociliated peritoneocytes and subapical myocytes. This histological organization of the coelomic epithelium resembles very closely that described in the podial lining of other echinoids (Florey & Cahill, 1977;Rieger & Lombardi, 1987;Flammang & Jangoux, 1993). The peritoneocytes (Fig.…”

“…In particular, the reciprocal arrangement and adaptable shape of the epidermal support cells and the peritoneocytes are likely to be structural features which facilitate deformation. These features are found elsewhere in echinoderms (including echinoids) in other types of epithelia subjected to extreme distension (see for instance Rosati, 1968;Coleman, 1969;Florey & Cahill, 1977;Filimonova, 1979;Feral & Massin, 1982). For example a comparable structural organization was described by Weber & Grosmann (1977) in connection with the spinebase epidermis of another regular sea-urchin, i.e., an area of epidermis subjected to considerable deformation during movement of the spine itself.…”

“…The wide spaces between these slender epithelial cells are filled in the apical region by many other cell types (phagocytes, chromatophores, mucous cells and nerve cells), and in the basal region by the conspicuous network of the basiepithelial nerve plexus. The basic structural organization of these components resembles that of outer epithelia in other echinoids (Coleman, 1969;Florey & Cahill, 1977;Weber & Grosmann, 1977).…”

“…According to Cobb (1969) the ectoneural nerve tissue of sea-urchins usually contains both acetylcholine and catecholamines. Other authors (Coleman, 1969;Florey & Cahill, 1977;Wood & Cavey, 1981) have left unresolved the question of the innervation of the myocytes in the coelomic myoepithelium …”

Sea urchin tube feet are photosensory organs that express a rhabdomeric-like opsin and PAX6

“…Note the presence of the pigment cells, probably containing opsin, in and around the ossicles (figure 2g). Nerves are also present along the entire length of the tube foot under the epithelium, forming a nerve plexus (figure 2h) that terminates in a ganglion at the distal portion of the tube foot as previously described for sea urchins [30][31][32]. Using primers specific for opsins, a 1.563 Kbp cDNA was amplified from the green sea urchin tube foot.…”