Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 97

The central role of the mesoglea (“or mesenchyme”) in the biology of cnidarians is best expressed by the words of Chapman (1966): “the coelenterate has long been regarded as two layers of epithelium stuck to that something which is the mesoglea.” Because it is largely acellular, the mesoglea has never been accorded the status of a germ layer, and its character, its importance, and even its name, differ among the cnidarian taxa. It can be regarded as the substance between the inner and outer epithelium. It assumes its greatest importance in the medusae where it acts variously as a source of buoyancy, an anchor for muscle fibers, a primitive skeleton, and even a source of nutrition.

The mesoglea consists of two or three components: fibers, a gel matrix, and where present, cells. Consensus on the chemical composition of the fibers is that they are composed of a collagen‐like protein. Evidence supporting the collagen‐like nature of the fibers comes from many sources including appearance in the electron microscope, thermal contractility, X‐ray diffraction patterns, amino‐acid composition, and histochemical staining. Overall, the evidence is quite convincing, and agreement is good among multiple authors (e.g. Arai 1997; Chapman 1966, 1974). The composition of the gel matrix is less well described, but the work of Gross et al. (1958) found mucopolysaccharides (glycosaminoglycans) associated with the collagen fibers, strongly suggesting a mucoprotein (proteoglycan)‐based gel. Similar gels are found in the synovial fluid lubricating the joints in vertebrates and are also found along with collagen in the cornea of the vertebrate eye. Proteoglycans and glycosaminoglycans are often highly transparent and quite resilient (Lehninger 1975).

Collagen fibers of varying diameters cross the mesogleal layer of the medusan bell, conferring a memory and resiliency to its shape that is important to the locomotory process described above. Helical elastic fibers of uncertain composition that aid in maintenance of bell shape and elastic recoil have been described in a few species, e.g. Chrysaora quinquecirrha (Arai 1997). To put the relative stiffness of jellyfish mesoglea in perspective, in a discussion of the comparative stiffness of a variety of biological materials, Vogel (1988) observed that the mesoglea of a sea anemone was about 500 times more deformable than rubber. Rubber was about 1000 times more deformable than collagen (as animal tendon). Clearly, the ratio of collagen fibers to gel matrix is important in determining the overall resilience of the medusan bell.