Mutable connective tissue

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The term mutable connective tissue or English mutable connective tissue , MCT for short , describes a special form of collagenous connective tissue that has so far only been detected in a single deuterostomy group , the Echinodermata (echinoderms).

The mutable connective tissue is a discontinuous collagen fiber network in which the individual fibers are organized into bundles via an elastic network of microfibrils. These bundles are connected to one another via a glycoprotein (stiparin).

The mutable connective tissue has special mechanical properties. It can change its passive mechanical properties (tensile strength, stiffness, viscosity) very quickly. A stiff and hard tissue can e.g. B. become soft and almost liquid within a very short time and vice versa. The special thing about this is that this process is largely without ATP-ADP change, i. H. So it is extremely economical in terms of energy.

These properties allow the echinoderms ( crinoids , starfish , brittle stars , sea urchins , sea cucumbers ) special services. For example, an Antedon mediterranea can sit as a solid structure in a current of water for a very long time and filter food particles because the tissue has been stiffened by the change in the internal fiber linkage. Such a rigid body posture that counteracts the flow of water can only be achieved through muscle work with enormous expenditure of energy.

Sea cucumbers (e.g. of the genus Holothuria) also impressively demonstrate the properties of mutable connective tissue: If you take a holothuria in your hand, its body wall first becomes firm, and shortly afterwards it becomes very soft, almost fluid, and literally closes with the fingers flow.

History of exploration

The first evidence of mutable connective tissue was found in holothurias (sea cucumbers). In fact, these also have the largest amount of MCT in their body wall. The MCT makes an important contribution to locomotion here, because the poorly developed circular muscles are hardly able to work as an effective antagonist against the longitudinal muscles. The peristaltic creeping mode of locomotion of the holothuria takes place via a stiffening and slackening of the body shell in sections by the mechanism described above. The Coelom filling acts here as a power transmitter and counterpart for the muscles.

Evolutionary history

The development of mutable connective tissue is an open question in evolutionary history. The similarities with the collagen tissues of vertebrates are striking. In contrast to these, the MCTs have a very high number of binding sites for proteoglucans, which influences the cohesion of the fibers with one another. Furthermore, the binding protein stiparin and the so-called Juxtaligamental cells, which play a role in the internment of the MCT, should be mentioned as special features. It is conceivable that the mutable connective tissue of the echinoderms arose from the muscle tissue of their evolutionary precursors, early choruses and enteropneusten (acorn worms) by degenerating the muscle cells into juxtaligamental cells. This assumption is supported by the fact that the irritation of the juxtaligamental cells occurs through a calcium-dependent cell mechanism similar to the irritation of muscle cells, in which the calcium release removes the myosin blockade and the muscle contraction is triggered,

Even if detailed knowledge about the origin of the MCT is not yet available, it is undisputed that the mechanical peculiarities of this tissue type made the formation and evolution of the echinoderms possible in the first place.

construction

The mutable connective tissue consists of collagen fibers that correspond to type I of vertebrates and can be detected with mouse anti-collagen type I and rabbit anti-fibrillin-I antibodies. The collagen fibers are linked by microfibrils (in particular proteoglucans) to form bundles, which are finally aggregated by a matrix of stiparin (a pressure-absorbing glycoprotein). The passive mechanical properties of the tissue change depending on the state of aggregation. In the vicinity of the fiber bundles, so-called juxtal ligament cells are arranged, which have characteristic electron-dense granules in the electron microscope image. These cells form extensions or extensions that extend into the fiber network. The juxtaligamental cells are also in synaptic contact with axons of the neurons of the echinoderms, which are often only distributed individually in the tissue ,

The change in mechanical properties is triggered by a change in interfibrillary cohesion, i.e. H. The connection of the collagen fiber bundles by the stiparin is built up or broken by the release of certain proteins, which are called "stiffener" and "plasticicer". The more fibrillar connections there are, the stiffer the tissue, the fewer fibrillar connections, the softer the tissue. The release of "stiffener" and "plasticicer" are related to calcium-dependent cell mechanisms. Some authors also suspect that a few contractile cells are involved in the mechanism.

histology

The histological detection of mutable connective tissues is carried out using special stains, which allow the proportion of proteoglucans and glycosaminoglucans to be detected. The colorations Movat Pentachrom, Safranin-O-light green and Alcian blue colorations (with a pH value of 1) have proven to be suitable for the detection of acidic mucopolysaccharides and PAS reactions.

Mechanical properties

The mutable connective tissue has special mechanical properties that are unique to it. First and foremost, this is the ability to switch from stiff and hard to soft and almost fluid without wasting energy (i.e. without ATP-ADP change). It is not about a contraction, but only a change in the passive mechanical properties (tensile strength, stiffness, viscosity). In addition, the MCT has a particular resistance to pulling. For example, while the strongest known muscle, the byssus retractor of the mussel ( Mytilus edulis ), can withstand a maximum isometric force of 1.4 MPa and only ruptures at around 10 MPa, the body wall of an Echinaster spinulosus is tear resistant. at a good 40 MPa (40 MPa corresponds to a load capacity of around 4 kg per square millimeter of fiber diameter).

The ability to regenerate known for echinoderms (especially sea lilies, brittle stars and starfish), or the slight loss of arms, is also a consequence of the properties of the MCT. On the one hand, histologically determined predetermined breaking points can be detected in the body segments; on the other hand, the body parts are actively separated, i.e. H. by softening the MCT by releasing the stiparin connections.

Web links

Individual evidence

  1. ↑ IC Wilkie: Variable Tensility in echinoderm Collage Nous Tissues: A Review. In: Marine and Freshwater Behavior & Physiology. 11, 1984, pp. 1-34.
  2. ^ A b M. Gudo: The echinoderms in the evolutionary field of the deuterostomians. Habilitation thesis . University of Göttingen, 2010.
  3. ^ RB Hill: Role of Ca2 + in excitation-contraction coupling in echinoderm muscle: comparison with role in other tissues. In: The Journal of Experimental Biology. 204, 2001, pp. 897-908.
  4. ↑ a b I. C. Wilkie: Is muscle involved in the mechanical adaptability of echinoderm mutable collagenous tissue? In: Journal of Experimental Biology. 205 (2), 2002, pp. 159-165.
  5. ^ IC Wilkie, M. Candia Carnevali, JA Trotter: Mutable collagenous tissue: Recent progress and an evolutionary perspective. In: T. Heinzeller, J. Nebelsiek (Ed.): Echinoderms. Balkema, London et al. 2004, ISBN 0-415-36481-7 , p. 644.
  6. ↑ MR Elphick, R. Melarange: Neural control of muscle relaxation in echinoderms. In: The Journal of Experimental Biology. 204 (5), 2001, pp. 875-885.
  7. ^ P. O'Neill: Structure and mechanics of starfish body wall. In: Journal of Experimental Biology. 147, 1989, pp. 53-89.