Epidermis-Keratinocytes

The stratum lucidum is a thin, clear layer only found in specific areas of exceptionally thick skin, and it lacks hair (e g., plantar and palmar surfaces). This layer appears as a translucent, homogeneous line between the stratum granulosum and stratum comeum and consists of several layers of fully keratinized, closely compacted, dense cells devoid of nuclei and cytoplasmic organelles. The cytoplasm contains eleidin, a protein that is similar to keratin but has a different staining affinity and protein-bound phospholipids. 

The stratum comeum cell layers may vary in thickness from one body site to another. An interregional analysis was conducted in nine species to assess the variability in thickness and blood flow between body sites as well as the effect histologic techniques have on these metrics (Table 1.1). Such regional differences 

These keratinized cells are surrounded by a plasma membrane and a thick submembranous layer that contains a protein, involucrin. This protein is synthesized in the stratum spinosum and cross-linked in the stratum granulosum by an enzyme that makes it highly stable. Involucrin provides structural support to the cell but does not regulate permeability. 

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Site-of-contact tissues might be preferable for such compounds (Burlinson 1989 Furihata et al. 1984 Furihata and Matsushima 1987 Mori et al. 1999 Sawyer et al. 1988), if sufficiently validated. The main technical limitation of this assay with respect to other tissues is the need to isolate the cells after in vivo treatment and to get them to incorporate tritiated thymidine in vitro. Because UDS measurement does not require cell division, it can potentially be applied to many different tissues, provided that the cells can be isolated and maintained in primary culture for the few hours required for tritiated thymidine incorporation. The literature contains reports of UDS-based studies of stomach, colon, kidney, pancreas, tracheal epithelium, nasal epithelium, epidermis, keratinocytes, and spermatocytes (Burlinson 1989 Furihata et al. 1984 Furihata and Matsushima 1987 Sawyer et al. 1988 Loury et al. 1987 Mori et al. 1999 Latt et al. 1981 Helleday 2003). 

In the skin, ENaC is expressed in keratinocytes of the epidermis and in hair follicles. It could play a role in terminal differentiation by modulating keratinocyte calcium signaling. The skin expresses MR, GR, and 11 3HSD2, but the role of aldosterone and glucocorticoids on ENaC activity and keratinocyte differentiation is not yet understood. 

Of all the epidermal cells studied in vitro with respect to antioxidant status, perhaps keratinocytes are the most important. These cells differentiate as they move upwards through the epidermis and there is evidence that, in... 

IL-13 on keratinocytes, two Th-2 cytokines which function via the same receptor in this cell type [26]. IL-13 or IL-4-stimulated keratinocytes attract CCR4+CD4-I- Th-2 cells via CCL22 [27]. Moreover, IL-13 induces the expression of MMP-9 in keratinocytes [28] which may play a crucial role in atopic skin inflammation by facilitating the migration of leukocytes into the epidermis. 

It has been shown that IFN-y induces Fas on keratinocytes which renders them susceptible to apoptosis induction by infiltrating FasL+ T cells. This has been interpreted as an important event in eczema, mainly in atopic dermatitis. There is further evidence that cleavage of E-cadherin and sustained desmosomal cadherin contacts between keratinocytes that are undergoing apoptosis result in spon-gioform morphology in the epidermis as a hallmark of eczematous lesions. Suppression of keratinocyte activation and apoptosis thus remains a potential target for the treatment of atopic dermatitis [2]. 

Klunker S, Trautmann A, Akdis M, Verhagen J, Schmid-Grendelmeier P, Blaser K, et al A second step of chemotaxis after transendothelial migration keratinocytes undergoing apoptosis release IP-10, Mig and iTac for T cell chemotaxis towards epidermis in atopic dermatitis. J Immunol 2003 171 1078-1084. 

EPISKIN is a three-dimensional human skin model composed of a human collagen (Types El and I) matrix, representing the dermis, covered with a film of Type IV human collagen and stratified differentiated epidermis derived from human keratinocytes. 

The EpiDerm (EPI-200) skin model is mechanistically and functionally related to EPISKIN. The assay consists of normal human epidermal keratinocytes, which have been cultured in a chemically defined medium to produce a stratified, highly diEerentiated, organotypic tissue model of the human epidermis. 

In the context of skin sensitization bioavailability can be seen as the capacity of the compound to reach the viable epidermis, where it interacts with keratinocytes and Langerhans cells. This capacity is dependent on its molecular weight and solubility in polar and apolar solvents [115]. Importantly, potency prediction solely on the basis of cell culture models (steps 3 and 4) does not account for skin penetration rate and may thus wrongly predict potency in vivo. Possible in vitro approaches to detect allergic capacity of chemicals/pharmaceuticals are presented in Table 18.5. 

The skin consists of two main compartments, the epidermis, a stratified squamous epithelium, and the underlying dermis, a richly vascularized tissue embedded in a connective tissue matrix (Fig. 41.1). The epidermis consists of multiple layers of keratinocytes, which differentiate into the outermost layer, the stratum corneum. This layer contains the hydrophilic structural... 

Unlike isotretinoin, acitretin (Soriatane) is not primarily sebosuppressive. Rather, it promotes normalization of dysregulated keratinocyte proliferative activity in the epidermis and is also antiinflammatory. Oral absorption is optimal when acitretin is taken with a fatty meal peak levels are reached approximately 3 hours after ingestion, while steady-state plasma levels are achieved after approximately 3 weeks of daily dosing. The mean terminal elimination half-life of the parent compound is 49 hours. However, when consumed with ethanol, acitretin may be transesterifled to form etretinate, a retinoid that is stored in adipose tissue, resulting in a much longer half-life (3-4 months or longer). 

The most superficial layer of skin is the stratum comeum (SC), which consists of terminally differentiated keratinocytes (comeocytes) that originate from actively proliferating keratinocytes in lower epidermis (basale, spinosum, and granulosum cells), and contain a lamellar lipid layer secreted from lamellar bodies (Fig. 7a). Flydration of the SC is an important determinant of skin appearance and physical properties, and depends on a number of factors including the external humidity, and its structure, lipid/protein composition, barrier properties, and concentration of water-retaining osmolytes (natural moisturizing factors, NMFs) including free amino acids, ions, and other small solutes. 

Machesney M, Tidman N, Waseem A, Kirby L, Leigh 1 (1998) Activated keratinocytes in the epidermis of hypertrophic scars. Am J Pathol 152 1133-1141... 

Mammalian skin must be tough, water-resistant, self-renewing, and rapidly healing. The outer layers of cells or epidermis consist principally of keratinocytes, epithelial cells specialized for formation of keratin (Fig. 7-31). In the inner layer of the epidermis the basal stem cells divide, providing a constant outward flow of cells which become progressively flattened, dehydrated, and filled with keratin fibrils.3 The outer layers contain only dead cells which are finally sloughed or abraded from the surface. Human epidermis is completely renewed in about 28 days ... 

Diagram of a dendritic melanocyte surrounded by satellite keratinocytes. The Golgi area (G), where the melanosomes are synthesized, is shown around the nucleus. The other branched cell, higher in the epidermis, is a Langerhans cell with its tennis racquet-shaped granules. Courtesy of Dr. W. Quevedo, Jr. From Montagna et al.m... 

Finally, a similar approach was undertaken to evaluate the origin of factor 3 in Fig. 15.3c whose score image reveals several small-localized pockets in the epidermis. The loading from factor 3 (Fig. 15.3b) depicts one broad feature at 1090cm 1 not present in the other factor loadings. Based on both the position of this particular band and the spatial distribution of the factor score, the band probably arises from a phosphodiester mode of DNA [30, 31] present in the nuclei of keratinocytes or other cells. An additional feature at 780 cm 1 in both spectra and factors (not shown) arises from cytosine in DNA [32] and is consistent with this interpretation. 

In their initial studies, Tfayli et al. [33] acquired spectra from an Episkin model. This model is comprised of human adult keratinocytes which produce stratified epidermis following a 13 h culture period. Raman spectra from this model were compared with normal human skin. Significant differences were noted, particularly in spectral features arising from the 850/830 tyr Fermi doublet (which is sensitive to the H-bonding state of the -OH group [34]) and in the protein amide III region. Usable spectra were acquired to a depth of 15-20 pm. 

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