Monday, November 18, 2013
Stretching the lower jaw
Biomechanical properties of connective tissue have been relatively well studied and collagen is responsible for much of the skin's ability to resist mechanical failure. In mammalian skin, at low levels of strain (up to about 20%) there is little change in the tissue's mechanical properties because collagen undergoes reorientation prior to stretching and slippage. At increased loads collagen account for skin's viscoelastic properties. However, under prolonged periods of strain, collagen alone is incapable of full recovery from high levels of extension due to irreversible creep. Elastin in the deep dermis has been suggested as being responsible for skin's elastic behavior, but this has not been supported by in vivo or in vitro studies of mammalian skin. Given the known structure and function of elastin it is likely that elastin in the dermis of snakes is responsible for the refolding of interscale skin and subsequently for returning the skin to its resting condition after prolonged periods of stretch.
The skin and intermandibular soft tissues determine the lower jaw's extensibility and the upper jaws of snakes are limited in the degree to which they can move laterally. Therefore, suspensorial length and mobility, mandible length and lower jaw extensibility are the major determinants of gape size in snakes. Within the context of macrostomy - snakes being able to swallow exceptionally large prey - Cose and Cundall (2013) examine how snake skin between the two sides of the lower jaw behaves during swallowing and how its function is related to structure.
Video records of skin behavior in the lower jaw of watersnakes feeding on fish or anesthetized watersnakes being stretched on an Instron machine showed that most skin extension involves the interscale skin. The largest intermandibular separation recorded during feeding was 7.7x the resting distance, but intermandibular separation reached 10x without tissue failure during mechanical testing. Histological and anatomical analyses of lower jaws fixed in resting, moderately or highly stretched conditions showed that stretching had little effect on scale regions of the epidermis. However, stretching flattened folds of interscale regions at both gross and cellular levels and imposed changes in epidermal cell shape. Stretching of the dermis is primarily limited to realignment of collagen and stretching of elastin in the deep dermis. The configuration of dermal elastin suggests a model for passive recovery of epidermal folding following release of tension.
Close M, Cundall D. 2013. Snake lower jaw skin: Extension and recovery of a hyperextensible keratinized integument. J. Exp. Zool. 9999A:1–20.