Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts

Cutaneous dyskeratosis, a disorder of free-ranging desert tortoises, is characterized by loss of integrity of the horny layer of the carapace, plastron and thickened foreleg scutes. The disorder is thought to be associated with a defect in keratinization or cornification. This has prompted us to examine the composition of shell keratin of desert tortoises with and without cutaneous dyskeratosis to determine if biochemical or ultrastructural differences can be identified.

Keratin comprises a diverse group of molecules that have been characterized by x-ray diffraction techniques, biochemical studies of the composition and structure of keratin intermediate filament and matrix proteins, and ultrastructural analysis. Patterns identified by x-ray diffraction analysis include the a-keratin pattern of mammals (also found in soft epidermal stratum corneum of reptiles and birds), and the ß keratin pattern of avian feathers and reptilian shells, beaks, claws, and scales. The unique protein structure of a-type keratin is a modified a helix (a-helical coiled coil) while that of ß-type keratin is a ß-sheet. Recently, it has been demonstrated that the framework of ß-keratin filaments is composed of twisted ß-sheets, each containing four segments of polypeptide chains. Biochemically, the only similarity between reptilian hard ß-keratin structures (shell, claws, etc.) and mammalian hard a-keratin structures (hooves, quills, etc.) is the presence of a large proportion of zinc-soluble (high sulfur) matrix material. In general, the keratinization process involves intermediate filament (IF) and matrix formation while cornification involves cell surface modification. Pre-keratin polypeptides are synthesized intracellularly and incorporated into intermediate filaments (ß-keratin IF are 4.0 nm while a-keratin IF are 7.0 to 8.0 nm). The IF are organized into macrofibrils by matrix proteins (IF associated proteins). There is complete dissolution of nucleus and cytoplasmic organelles in fully keratinized cells. Cornification of ß-keratin structures continues as the boundaries of keratinized epithelial cells coalesce. Numerous desmosomes anchor the undulant base of the horny layer of tortoise shell to the underlying layer of keratinized epithelial cells. Due to extensive disulfide bonding, the final keratin product is insoluble except in certain protein solvents at extreme pH or in the presence of dissociating and reducing agents, such as urea and dithiothreitol respectively.