Keratinized tissue

Emollients are often added to cream formulations to modify either the characteristics of the pharmaceutical vehicle or the condition of the skin itself to promote penetration of the active ingredient to act either locally or systemically. The stratum corneum, being keratinized tissue, behaves as a semipermeable artificial membrane, and drug molecules can penetrate by passive diffusion. The rate of drug movement depends on the drug concentration in the vehicle, its aqueous solubility, and the oil/ water partition coefficient between the stratum corneum and the product s vehicle. Commonly used emollients include glycerin, mineral oil, petrolatum, isopropyl pal-mitate, and isopropyl myristate. 

The structure, organization, and ratio of matrix and fibrous proteins contribute to the physiochemical properties of keratinous tissues. For example, a primary difference between hair and nails is the arrangement of fibrous proteins and the concentration of matrix proteins present in each tissue. In cells destined to form the cortex of hair, fibrous proteins are oriented to form filaments which cluster to form fibrils. In the keratogenous zone, fibrils undergo lateral fusion to ultimately produce the cortex. The medulla also contains keratin which has been characterized as a collection of irregular fibrous proteins. Fibrous proteins form a trabecular framework comprising 95% of the medulla, and medullary proteins are less resistant to chemical degradation than proteins in the cortex. The cell membrane complex. 

Mercer, E. H., The electron microscopy of keratinized tissues, in The Biology of Hair Growth, Montagna, W. and Ellis, R. A., Eds., Academic Press, New York, 1958, 91. 

Baden, H. R, Goldsmith, L. A., and Fleming, B., A comparative study of the physicochemical properties of human keratinized tissues, Bioch. Biophys. Acta, 322, 269,1973. 

Owing to the economic importance of wool most investigators have used this material as a convenient source of a-keratin. When parallel studies have been made on hairs from other animals and on nails, claws, hoofs, and quills it has been found that conclusions reached by studying wool proteins apply, with only minor qualifications, to other keratinized tissues. Feathers are only of slight economic value and correspondingly less attention has been devoted to their chemistry, despite the fact that feather proteins are more readily solubilized and purified and that feather rachis yields X-ray diffraction patterns of excellent quality. 

Keratinized tissues contain several types of cells and these cells in turn contain many protein constituents. It is possible therefore for keratins to differ in amino acid composition either because the constituent proteins differ or because of differences in the relative amounts of identical proteins. It is rarely possible therefore to relate differences in over-all amino acid composition to differences in character or content of a particular protein constituent. 

There is one other bone which could be useful to geochemists interested in assessing environmental exposures. Antlers, but not horns, are shed annually. Both these organs have a bony core but horn is covered with dead keratinous tissues while antlers are covered initially by velvet, an epidermal tissue with separate blood and neural supply. Once the velvet has been rubbed off, the bony super-stmcrnre that has formed and mineralized very rapidly provides the animal with a remarkable headdress. After the breeding season osteoclasts resorb at the base where the antler is attached to the frontal bone of the skull and antlers may be relatively easily obtained for analyses. Male deer, reindeer, and caribou regrow their membranous bone excrescences each year with an increase in the complexity and size of these unique structures... 

Salicylic acid (1) is not employed internally as an analgesic due to its local irritating effect on the gastrointestinal tract. It is employed externally on the skin, where it exerts a slight antiseptic action and a marked keratolytic action. The latter property makes salicylic acid a beneficial agent in the local treatment of warts, corns, fungous infections, and certain forms of eczematoid dermatitis. Tissues cells swell, softer, and ultimately desquamate. Salicylic acid is applied as a 2 to 20% concentration in collodion, lotions, or ointments, and as a 10 to 40% concentration in plasters. Salicylic acid plaster is used for the destructive effect of salicylic acid on hardened, keratinized tissue. The so-called corn plaster are typical. 

The electrical admittance of keratinized tissue is typically logarithmically dependent on water content or ambient RH. An example from human hair is shown in Figure 4.23, in which the 1-Hz conductance of 50 fibers in parallel is plotted against ambient RH (Martinsen et al., 1997b). 

Nail is also keratinized tissue, but it is harder than SC. This is partly due to the hard a-keratin in nail as opposed to the more soft p-keratin in SC (Baden, 1970 Forslind, 1970). 

In keratinized tissue such as the outer layers of the human skin and in hair, nad, and horn, the change and ultimate death of the cells are genetically programed to occur during a period of approximately 30 days. The dead keratinized cells need no intra- or extracellular fluids, and they may be in a dry state wifli low admittance values. 

Dermatophyte A fungus that invades keratinized tissue of the skin and nails. 

There are intrinsic problems with hair Pb in terms of understanding Pb toxicokinetics in hair or other keratinizing tissue. Hair Pb loadings appear to vary with hair color, highly variable deposition rates, etc. (Barbosa et al., 2005 Mushak, 1992 U.S. ATSDR, 2007 U.S. EPA, 2006). Of particular concern, Pb-hair is poorly correlated with PbB, especially at low Pb levels currently encountered by nonoccupational human populations (Campbell and Toribara, 2001). The latter is to be expected, given what each biomarker is reflecting in time scales of exposures. 

Structural abnormalities are demonstrable in some dominant types, in keratohyalin (defective in autosomal dominant ichthyosis vulgaris), and tono-filaments (impaired or disturbed in their arrangement in hystrix-like ichthyoses clump formation in bullous ichthyosiform erythroderma or epidermolytic hyperkeratosis shell formation associated with high numbers of binucleate cells in ichthyosis hystrix type Curth-Macklin impairment in ichthyosis hystrix gravior type Rheydt). Keratohyalin and tonofilaments are both structural proteins of keratinizing tissues. 

---