Keratinous epidermal layer

Figure 20. Photomicrograph of the keratin epidermal layers (annuli) of turtle shell fragments from an individual collected at the Ash Basin site, a Se map, and Se Ka XANES spectra of the epidermal layers and inorganic and organo-Se compounds (Hunter and Bertsch, unpublished).

Because papillomaviruses are specialized for replication in external ep-ithelia, infection is usually confined to the epithelium exposed to the external environment and does not result in systemic dissemination of the virus. While viral replication occurs in differentiated epithelial cells in the upper epidermal layer, viral particles are also often located in the deep epidermal basal layer. The epidermis becomes thickened and hyperkera-totic, and keratinocytes (keratin-containing cells) in the epidermal granular layer become vacuolated as a result of viral infection. The mechanisms by which virions penetrate the stratum corneum and infect viable keratinocytes is poorly understood, as there is a lack of practical in vitro culture systems available for these viruses to serve as study aids. 

The skin is an excellent barrier to microbial and parasitic infections. The most superficial layer of the skin is composed of flattened squamous cells, which are highly keratinized. Beneath this is the epidermal layer composed of cells tightly interconnected by desmosomes and other intercellular structures. These, in turn, are attached to the basement membrane composed of covalently bound or interwoven macromolecules. Between the basement membrane and a target blood vessel is an extracellular matrix rich in type I collagen, elastin and proteoglycan. Elastin and type I collagen are both interwoven fibrillar molecules, whereas the carbohydrate-rich proteoglycan behaves like a hydrated gel. For details of these macromolecular interactions, the reader is referred to reviews on the structure of skin. 

Colorimetric, spectrophotometric and histochemical studies have shown that the homy layer is the epidermal barrier to nickel absorption [270]. The carboxyl groups of the keratin have been found to be important in the binding of nickel [271, 272],... 

In the outermost layer, the stratum corneum, the eleidin has been converted into keratin, which represents the ultimate fate of the epidermal cell. Keratin, continuously sloughed off or worn away, is replaced by the cells beneath it. The time required for a basal cell to migrate from the stramm germinativum to the outer part of the stratum corneum is estimated at 26-28 days. 

Appendageal structures commonly found within the skin are the hairs, hair follicles, associated sebaceous glands, apocrine and eccrine sweat glands, and arrector pili muscles. Hairs are formed by epidermal invaginations. These keratinized structures traverse the dermis and may extend into the hypodermis. The free part of the hair above the surface of the skin is the hair shaft, and the part deep within the dermis is the hair root, which forms an expanded knob-like structure called the hair bulb. This is composed of a matrix of epithelial cells in different stages of differentiation. Hair is composed of three concentric epithelial cell layers the outermost thin cuticle, a densely packed keratinized cortex, and a central medulla of cuboidal cells. The hair follicle consists of four major components (1) internal root sheath (internal root sheath cuticle, granular layer, pale epithelial layer) (2) external root sheath (several layers similar to the epidermis) (3) dermal papilla (connective tissue) and (4) hair matrix (comparable to the stratum basale of the epidermis). 

Intercellular Volume and Penetration. The surface coats of epidermal cells occupy intercellular space and these gelatinous layers probably act as watery diffusion channels for nutrients (35). Surface coats are quite different from keratinized intercellular cement (32, 43) kera-tinization modifies desmosomes and intercellular materials to resist diffusion and premature separation in SC (32, 43, 44). 

Epidermis The epidermis is the outer layer and has five layers. Epidermal cells migrate from the stratum basal layer to the stratum comeum layer where they die. Their cytoplasm converts to keratin, which forms keratinocytes, a hard rough texture of the skin. Cells then stratum dysjunction (shed) and new cells take their place. 

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