Skin As Mechanical Support From Crossed Fibers To A Dermal

The functional and evolutionary morphology of the dermal layer of the skin has been particularly neglected by comparative vertebrate biologists. It is widely recognized that the dermis, whether ossified or not, provides structural integrity to the skin (Harkness, 1968). However, biomechanical study of vertebrate support and movement has focused largely on the bony internal skeleton, whereas the mechanics of the external dermis have been largely ignored. In the few cases in which dermal mechanics have been investigated in detail, this largest of organs, not surprisingly, is revealed to play an important role in musculoskeletal function including locomotion (Wainwright et al., 1978) and ventilation (Brainerd et al., 1989).

Most commonly, the dermis of vertebrates takes the form of a fabric with two sets of crossed fibers. This fabric is wrapped helically around the animal, "at a bias" to use the dressmaker's term (see Fig. 2). This helically wound, crossed fiber dermis is likely to be primitive for vertebrates because it occurs early in the ontogeny of every group that has been examined and is retained throughout life in all fish groups (Fig. 3). Developmental repatterning to a more matted dermis is known to occur only in terrestrial vertebrates-birds and mammals among amniotes as well as frogs and salamanders among lissamphibians. Lepidosaurs and turtles, underneath their epidermal scales, appear to retain crossed fibers in the dermis as adults. However, aside from salamanders, dermal fiber patterning in most vertebrate groups is poorly sampled (see Table 3) and the extent of variability within each group is unknown.

The primitive vertebrate dermis can be mechanically modeled as a pressurized cylinder formed from crossed fibers that are helically wound around the animal. This type of structure has several mechanical properties that are important during laterally undulating locomotion (Wainwright, 1988). It resists torsion and bending (common forces during laterally undulating locomotion) and thus helps maintain the structural integrity of the cylinder. In addition, a helically wound dermis serves as an "exotendon" that transmits forces posteriorly to the region of the tail, which is the main propulsive site during laterally undulating swimming (Wainwright et al., 1978). Thus, a helically wound crossed fiber dermis functions to enhance the efficiency of axial lateral undulation, especially during swimming when traveling waves pass axial forces posteriorly to the tail.

Lateral undulation is the primitive mode of axial movement for vertebrates as evidenced by its predominance in all fish groups. Early land vertebrates, including the ancestors of amniotes, were also lateral undulators as evidenced by the morphology of their vertebrae (Edwards, 1977) and the retention of lateral undulation in modern lissamphibians (except frogs) and lepidosaurs.

In amniotes, as well as in lissamphibians, decreasing reliance on traveling waves of lateral undulation has led to the evolution of a thick mat of more randomly oriented dermal fibers (Frolich and Schmid, 1991). Lateral undulation during terrestrial locomotion is often done using standing waves as opposed to the traveling waves most frequently used during swimming (Frolich and Biewener, 1992) and the advantages offered by a helically wound dermis no longer apply. Terrestrial salamanders uniformly exhibit a developmental shift to a more matted dermis, although the few lepidosaurs surveyed retain the helically wound pattern (Table 3). However, axial movement during locomotion in lizards is poorly studied (and they may still use the primitive traveling wave pattern), whereas snakes are well known to regularly use traveling waves of lateral undulation (Jayne, 1986, 1988). Further study of axial locomotion and the role of the dermis is needed for all vertebrates and virtually nothing has been done in this area on amniotes.

Simply because a structure such as the helically wound crossed fibers of the dermis, is no longer useful is not good reason, evolutionarily, to modify it. Some advantage for evolving an ontogenetic shift to a matted dermis in amniotes (and lissamphibians) is likely to exist. However, the paucity of good data on whole skin mechanical studies from amniotes makes it difficult to do more than speculate on what functional advantage a thick matted dermis provides. Loss of bony dermal scales for speedier locomotion renders

Table 3. Dermal Collagen Fiber Patterns in Chordates.

Species Fiber Pattern Reference

Table 3. Dermal Collagen Fiber Patterns in Chordates.

Aganathans

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  • Amira
    How does the skin Mechanical support?
    5 months ago

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