Stratum corneum reduction

The human skin model assay involves measuring the effects of corrosives on viable cells in a reconstituted human skin equivalent. To be accepted as a valid human skin model, several criteria must be met. The artificial skin must comprise a functional stratum corneum with an underlying layer of viable cells. Furthermore, the barrier function of the stratum corneum, as well as the viability of the epidermis, must be verified with appropriate experimental setups. The chemicals to be tested are applied up to 4 h as a liquid or a wet powder onto the skin model. Afterwards, careful washing has to be performed, followed by investigation of the cell viability [e.g., with a (MTT)] reduction assay). 

Psoriasis is a chronic skin disorder, in which an abnormally fast transition of basal cells into corneocytes results in a thickening of the stratum corneum. Transmission electron microscopy studies show an aberrant stratum corneum lipid ultrastructure in psoriatic skin [66], which is expected to be related to abnormalities in the lipid profile. Particularly, a significant reduction in ceramide 1 and a predominance of sphingosine ceramides at the expense of phytosphingosine ceramides are reported in psoriatic stratum corneum [67,68],... 

Chemical PEs have recently been studied for increasing transdermal delivery of ASOs or other polar macromolecules [35]. Chemically induced transdermal penetration results from a transient reduction in the barrier properties of the stratum corneum. The reduction may be attributed to a variety of factors such as the opening of intercellular junctions due to hydration [36], solubilization of the stratum corneum [37, 38], or increased lipid bilayer fluidization [39, 40]. Combining various surfactants and co-solvents can be used to achieve skin penetration, purportedly resulting in therapeutically relevant concentrations of ASO in the viable epidermis and dermis [41]. In summary, it appears feasible to deliver ASO to the skin using a number of different delivery techniques and formulations. 

Dithranol in combination with urea is widely used in psoriasis to improve the clinical efficacy, to minimize the dithranol concentration, to achieve the desired effect, to shorten the contact, to get a better hydration of the stratum corneum, and to decrease the proliferation rate of the keratinocytes. Gabard and Bieli showed an increased keratolytical effect of salicylic acid by adding 10% urea.54 Hagemann and Proksch55 showed in 10 patients with psoriasis under a 2-week treatment with a 10% urea ointment increased stratum corneum hydration, a small decrease in TEWL, a reduction in epidermal thickness (-29%), and a decreased epidermal proliferation (-51%). The altered expression of involucrin and cytokeratins as marker for epidermal proliferation was partially reversed.55... 

Skin Source. A surgically-removed human skin source is used for all in vitro diffusion cell tests. Skin is obtained from 37 + 13 year-old healthy females undergoing reduction mammaplasty operations. The epidermis-stratum corneum layer is separated from full-thickness skin using published techniques (2). Briefly, the full-thickness skin is immersed in distilled water heated to 60 + 1°C for 45 seconds. The epidermal-stratum corneum layers are then peeled from the dermis. The epidermis-stratum corneum layers are then excised with a cork borer into circular disks 1.4 cm in diameter. 

Figure 27. Upper curve effect of chloroform-methanol and ether on 196°C longitudinal shrinkage in newborn rat stratum corneum. Lower curve, chloroform-methanol reduction of 50°C longitudinal shrinkage (He atm) (11).

Before a compound can induce an adverse skin reaction upon dermal exposure, it has to penetrate into and permeate across the stratum corneum. Many of the strategies for the reduction of adverse reactions are based on reducing or modulating this process. 

The rate-limiting barrier to skin absorption is generally considered to be the outermost layer, the nonviable stratum corneum. Consequently, the skin is frequently thought of as a passive, inert barrier and percutaneous absorption of chemicals was thought to be dominated by laws of mass action and physical diffusion. This reduction of percutaneous absorption... 

The increase in permeability is also due to a reduction in cor-neocyte cohesion related to the unusual hyperhydration of the stratum corneum. 

The effect of age on percutaneous absorption has been examined in vivo in man with variable results. It was postulated (Roskos et al. 1989) that reduced hydration levels and lipid content of older skin may be responsible for a demonstrated reduction in skin permeability where the permeants were hydrophilic in nature (no reduction was seen for model hydrophobic compounds) (Table 14.2). The reduced absorption of benzoic acid demonstrated in the elderly (Rougier 1991) was in line with this suggestion, but not the reduction in absorption of testosterone (lipophilic) (Roskos et al. 1986), or lack of change in the absorption of methyl nicotinate (more hydrophilic) with age (Guy et al. 1983). There are a number of potential physiological changes which may be responsible for age-related alterations, including an increase in the size of individual stratum corneum corneocytes, increased dehydration of the outer layers of the stratum corneum with age, decreased epidermal turnover and decreased microvascular clearance (reviewed in Roskos and Maibach 1992). The issue of age-related variability, however, is far from resolved. 

Fig. 2. Cow s nose epidermis, formaldehyde fixation. Reduction in thioglycolic acid. Prussian blue reaction in stratum corneum.

The moisture content can be measured in vitro by means of gravimetry or electron microscopy, or by magnetic resonance techniques in vivo. The resolution of the latter technique is, however, currently not sufficiently high to enable isolated measurements on the stratum corneum. Compared with these techniques, assessment of stratum corneum hydration by means of electrical measurements (susceptance) represents an important reduction in instrumental cost and complexity. 

All compounds that we have studied for effects on keratinization have been initially evaluated on a series of over twenty patients with ichthyosis of several types. Many of the compounds have been evaluated for therapeutic effects on larger series of patients with dry skin several have been used therapeutically for comedonous acne and a few have been used in therapy of a diverse group of disorders including palmar and plantar hyperkeratosis, Darier s disease, lichen simplex chronicus and others. Reduction in stratum corneum accumulation has been readily demonstrable in all disorders in response to two-four times daily topical application of the compounds formulated as 5-20% oil-in-water creams, as solutions and as gels. 

The sequence of histologic and clinical changes in lamellar ichthyosis suggests that the effect of these compounds on the skin are mediated by means of their influence at the level of the underlying epidermis insofar as after several days of topical treatment a normal skin surface abruptly appears clinically, consequent to sheet-like desquamation of the entire thickened stratum corneum. This is correlated histologically with the emergence of a normally thin stratum corneum and reduction in thickness of the epidermis. No clinical... 

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