Corneous

Abstract: Keratinization involves two quite separate processes: the synthesis of keratin fibrous protein, resistant to proteolytic enzymes, and the enzymic cytolysis of most of the non-keratin components of the cell. The balance of these two factors largely determines the type of horny cell formed. Soluble substances in horny cells are probably mainly products of cytolysis. Keratin is a polymerized fibrous protein and it varies both in molecular and chemical composition in different keratinized structures. Various substances, including calcium compounds, phospholipids, nucleic acids and probably some enzymes, although synthesized independently, sometimese become bound to the keratin, probably through side chains of the keratin molecule. Bound phospholipid is importantin waterproofing the horny layer. (2) In amphibia the keratin is less resistant to denaturation than in higher vertebrates. The cells also show little cytolysis. (3) Reptilian horny scales resemble the amphibian horny layer but are more strongly keratinized. The cells are solid in structure but lose their nuclear staining. Similar horny scales occur on the legs of birds and the tails of mammals. (4) Epidermal growth and keratinization are intermittent in lizards and snakes, the horny layer being formed and shed as a whole. This process differs from the continuous growth and keratinization of other reptiles and mammals. In these groups the keratin is mostly lost in small flakes. (5) A more flexible horny layer is formed in the hinge regions separating the rigid reptilian scales, and also in bird epidermis. It is composed of separated thin layers of solid horny cells and is quite different from the flexible horny layer of mammals. (6) Feathers and hairs are analogous structures, but keratinization of these structures shows many differences. Thus, in the growing feather, protein synthesis shown by the distribution of ribosomal ribonucleic acid, continues to occur until shortly before keratinization is completed. In the hair a zone of protein synthesis is followed by a discrete keratogenous zone where the final stages of keratinization occur. (7) A true granular layer of the epidermis probably occurs only in mammals and was first developed in hair follicles. It contains keratohyalin, which, however, is probably not a precursor of keratin. A characteristic type of flexible horny layer occurs in mammals and is always formed over a granular layer. It is composed of hollow cells with only the peripheral cytoplasm keratinized. The interior of these cells is largely broken down to soluble constituents. Some other types of horny cell are formed from a granular layer, as in the soles of the feet. (8) A retrogressive change in epidermal keratinization has occurred in some marine mammals. The flexible horny layer is replaced by a nucleated (parakeratotic) horny layer. This shows close similarities to reptilian scales. Parakeratosis in these animals is associated with the loss of hairs.

Abstract: In all amniotes specialized intermediate filament keratins (IF-keratins), in addition to keratin-associated and corneous proteins form the outermost cornified layer of the epidermis. Only in reptiles and birds (sauropsids) the epidermis of scales, claws, beaks, and feathers, largely comprises small proteins formerly indicated as "beta-keratins" but here identified as corneous beta-proteins (CBPs) to avoid confusion with true keratins. Genes coding for CBPs have evolved within the epidermal differentiation complex (EDC), a locus with no relationship with those of IF-keratins. CBP genes have the same exon-intron structure as EDC genes encoding other corneous proteins of sauropsids and mammals, but they are unique by encoding a peculiar internal amino acid sequence motif beta-sheet region that allows formation of CBP filaments in the epidermis and epidermal appendages of reptiles and birds. In contrast, skin appendages of mammals, like hairs, claws, horns and nails, contain keratin-associated proteins that, like IF-keratin genes, are encoded by genes in loci different from the EDC. Phylogenetic analysis shows that lepidosaurian (lizards and snakes) and nonlepidosaurian (crocodilians, birds, and turtles) CBPs form two separate clades that likely originated after the divergence of these groups of sauropsids in the Permian Period. Clade-specific CBPs evolved to make most of the corneous material of feathers in birds and of the shell in turtles. Based on the recent identification of the complete sets of CBPs in all major phylogenetic clades of sauropsids, this review provides a comprehensive overview of the molecular evolution of CBPs.

Abstract: The proliferation of the epidermis in soft skin, claws, and scutes of the carapace and plastron in the tortoise (Testudo hermanni) and the turtle (Chrysemys picta) were studied using autoradiographic and immunocytochemical methods. During the growing season, a basal keratinocyte in the epidermis of soft skin and claws takes 5–9 days to migrate into the corneous layer. In the tortoise, during fall/winter (resting season) a few alpha-keratin cells are produced in soft epidermis and hinge regions among scutes and occasional beta-keratin cells in the outer scute surface. When growth is resumed in spring (growing season), cell proliferation is intense, mainly around hinge regions and tips of marginal scutes. No scute shedding occurs and numerous beta-keratin cells are produced around the hinge regions, while alpha-keratin cells disappear. Beta-cells form a new thick corneous layer around the hinge regions, which constitute the growing rings of scutes. Beta-keratin cells produced in more central parts of scutes maintain a homogeneous thickness of the corneous layer along the whole scute surface. In the turtle, a more complicated process of scute growth occurs than in the tortoise. At the end of the growing season (late fall) the last keratinocytes formed beneath the old stratum corneum of the outer scale surface and hinge regions produce more alpha- than beta-keratin. These thin alpha-keratin cells form a scission layer below the old stratum corneum, which extends from the hinge regions toward the center of scutes and the tip of marginal scutes. In the resting season (fall/winter) most cells remain within the germinative layer of the carapace and plastron and a few alpha-cells move in 7–9 days into the corneous layer above hinge regions. In the following spring/summer (growing season) a new generation of beta-keratin cells is produced beneath the scission layer from the hinge region and more central part of the scutes. The epidermis of the inner surface of scutes and hinge regions contains most of the cells incorporating thymidine and histidine, while the remaining outer scute surface is less active. It takes 5–9 days for a newly produced beta-cell to migrate into the corneous layer. These cells form a new corneous layer that extends the whole scute surface underneath the maturing scission layer. The latter contains lipids and eventually flakes off, determining shedding of the above outer corneous layer in late spring or summer. J. Morphol. © 2005 Wiley-Liss, Inc.