Cell Biology

        
The usefulness of tube feet

          Echinoderms, including Stichopus chloronotus, use their tube feet not only to walk but also to hold onto surfaces. In order to hold onto the substratum, the tube feet utilise adhesive secretions, which can be quite strong, but can also easily detach when the animal wishes to (Thomas and Hermans 1985; Flammang 1996). The epidermis of the tube foot is used for both sensory and secretory purposes (Flammang et al. 2005). Non-ciliated secretory cells release the adhesive secretion, while ciliated secretory cells release a de-adhesive secretion (Flammang et al. 1994; Flammang 1996). The adhesive is present as a thin layer on the outermost part of the tube foot, and the tube foot is able to detach by secreting the de-adhesive and leaving this outer layer and the adhesive secretion behind (Flammang 1996; Flammang et al. 1998). The chemical interactions behind this adhesion have not yet been studied for S. chloronotus (Flammang et al. 2005), but in the asteroid Asterias rubens it seems that the adhesive ability may be due to ionic bonds through carbohydrates or proteins or to disulphide bonds (Waite 1987; Flammang et al. 2005).

        The reason the adhesive secretions of echinoderm tube feet are of so much interest to us is that they may have some technological applications. These include the development of water-resistant adhesives and new antifouling strategies (Flammang et al. 2005). We are also interested in their unique ability to adhere to a surface and then easily release when desired (Flammang et al. 2005). Water-resistant adhesives could be useful for surgical purposes, as well as dental purposes, and could be a useful and non-toxic way to prevent biofouling on ships, piers and other man-made structures (Flammang et al. 2005).


Unique properties of mutable collagenous tissue

          Mutable collagenous tissue (MCT) is found in all echinoderms, including Stichopus chloronotus (Wilkie 2005). It is extremely useful in these animals because it can undergo dramatic changes in terms of its mechanical properties (MCT). This was mentioned in the External Morphology section of this web page. The majority of the collagens are found in the extracellular matrix of the connective tissue, and they are constructed of proteins (Wilkie 2005). It is important for the sea cucumber that the MCT has certain properties including the ability to transmit, resist, and dissipate mechanical forces, as well as store and release elastic strain energy (Wilkie 2005). While MCT is found in nearly all animal life forms it is particularly unique in echinoderms, because it has the ability to change mechanical properties in a matter of seconds (Wilkie 2005). This involves neural control in conjunction with coordinated muscle activity (Wilkie 2005).

         MCT in echinoderms can change both reversibly, such as the changing stiffness of S. chloronotus dermis, and irreversibly (Wilkie 2002). In some echinoderms, such as crinoids, collagneous ligaments are capable of active contractility, although they lack the myocytes found in muscle (Wilkie 2005). The cells of mutable collagenous tissue can be myocytes, but this is not common (Wilkie 2005). It is more likely that the cells are either a cell with a hereogenous vacuole, fusiform shape and branching processes (Wilkie 1988; Wilkie et al. 1992) or a cell body containing large, electron-dense, membrane-bound granules (Wilkie 2005). The extracellular matrix of MCT is made mostly of fibres made up of collagen fibrils (Wilkie 2005). These fibrils can be arranged in a wide variety of ways (Wilkie 2005). Proteoglycans are also present in the extracellular matrix, and these can be both soluble and insoluble (Trotter et al. 1995). Other non-collagenous proteins that can be found in the MCT of echinoderms include stiparin, tensilin, and plasticiser, although their functions are not fully understood at present (Wilkie 2005). The exact way that these characteristics of MCT combine to allow mutability are not known, although several hypotheses have been proposed (see Wilkie 2005 for more detail).

         By studying MCT in echinoderms, some hope to make biotechnological advances. In the past, and even today, the body wall of holothurians has been used in herbal medicines. There is a scientific basis behind this as well, and it is now used for joint pain and HIV therapy (Jiaxin 2003), with more uses being studied (Fredalina et al. 1999; Villasin and Pomory 2000). Research is also looking into the ability of MCT to be used in mammals, to promote adhesion of tissue grafts, or even to create MCT-derived artificial tissue (Wilkie 2005). Before this can be accomplished, much research is still required into the properties and mechanisms of MCT.