Stratum corneum thickness

Tretinoin 0.05-0.1% which is the most critical component of this regimen as it results in decreased stratum corneum thickness, increases the kinetics of epidermal turnover, and decreases corneocyte adhesion... 

Fig. 12.3. Epidermal measurements, mitotic figures, and apoptotic keratinocytes in a chronic proliferative dermatitis mutant (Sharplncpdm/Sharplncpdm) mouse. Routine hematoxylin- and eosin-stained paraffin histologic sections can be used to determine proliferation rates based on mitotic index (number of mitotic figures, circled in the figure, in the stratum basale per 1000 cells) or the presence and numbers of apoptotic epidermal keratinocytes (dotted arrows) when present. Epidermal thickness can be measured at high dry magnification (40x) to include the malpigian, living cell, layer (M), the stratum corneum thickness (SC), or the full thickness of the epidermis (M+SC).

A problem that is rarely taken into account is related to the fact that the water concentration shows a conspicuous gradient within the stratum corneum thickness. These factors are likely to influence the physical state of lipids in bilayer formations, and therefore we expect lipid barrier structure to vary within the thickness of the stratum corneum.53... 

TABLE 35.1. Comparative Stratum Corneum Thickness, Epidermal Thickness, and the Number of Epidermal Cell Layers in the Ventral Abdomen (VAB) and at the Thoracolumbar Junction (TLJ) of Several Species (H E, Paraffin Sections)... 

Application site Epidermal and stratum corneum thickness, keratinization, blood flow, hair follicles/glands, skin condition (dermatoses, damage, hydration state, occlusion, pH)... 

Table 1.1 Epidermal Thickness, Stratum Corneum Thickness, and Number of Cell Layers from the Back and Abdomen of Nine Species...

Kalia, Y.N., Alberti, I., Naik, A., and Guy, R.H., 2001, Assessment of topical bioavailability in vivo, the importance of stratum corneum thickness. Skin Pharmacol. Appl. Skin Physiol, 14, 82-86. 

Fairley, J.A., Rasmussen, J.E., 1983. Comparison of stratum corneum thickness in children and adults. J. Am. Acad. Dermatol. 8, 652-654. 

Glycolic acid creams, cleansers, and exfoliating preparations are used to boost epidermal exfoliation, reduce stratum corneum thickness and augment the peel penetration, thus enhancing the uniformity of the light chemical peel. Pretreatment is also essential for combination medium-depth peels. 

All humans experience intrinsic aging. Typically, it is characterized by smooth, relatively atrophic, finely wrinkled or lax skin. Histologically, the stratum corneum is normal. However, the epidermis is atrophic and there is flattening of the dermo-epidermal junction. Dermal features include decreased thickness, loss of elastic fibers, and a decrease in the biosynthetic capacity of fibroblasts [i, 2] (Table 15.1). 

Stratum corneum normal thickness (basket weave pattern), epidermis thinned, atrophic, flattened rete ridges... 

Let us consider how the skin is structured to better understand how this tissue performs some of its vital functions. Consider the cross section of the skin sketched in Fig. 1. This illustration shows the readily distinguishable layers of the skin, from the outside of the skin inwards the 10 pm thin, fully differentiated, devitalized outer epidermal layer called the stratum corneum the 100 pm thin live, cellular epidermis and the 1000 pm thin (1 mm thin) dermis. Note that all the thicknesses specified here are representative only, for the actual thickness of each stratum varies severalfold from place to place on the body. Dispersed... 

The stratum corneum consists of separated, nonviable, cornified, almost nonpermeable corneocytes embedded into a continuous lipid bilayer made of various classes of lipids, for example, ceramides, cholesterol, cholesterol esters, free fatty acids, and triglycerides [6], Structurally, this epidermis layer is best described by the so-called brick-and-mortar model [7], The stratum corneum is crucial for the barrier function of the skin, controlling percutaneous absorption of dermally applied substances and regulating fluid homeostasis. The thickness of the stratum corneum is usually 10-25 /an, with exceptions at the soles of the feet and the palms, and swells several-fold when hydrated. All components of the stratum corneum originate from the basal layer of the epidermis, the stratum germinativum. 

The nails are composed of flattened, keratinized cells, fused into a dense and hard, yet slightly elastic plate. Their thickness varies from 0.5 to 1.0 mm. In contrast to the stratum corneum (10%), the total lipid content of the nails lies between 0.1% and 1%, and the keratin domain is harder, due to higher sulfur content (cystine). Moreover, the water content is only 7% to 12%, in comparison to 25% in the stratum corneum. The relative water gain may not exceed 25% at 100% relative humidity, in sharp contrast to 200-300% as found in the stratum corneum. 

The intercellular route is considered to be the predominantly used pathway in most cases, especially when steady-state conditions in the stratum corneum are reached. In case of intercellular absorption, substance transport occurs in the bilayer-structured, continuous, intercellular lipid domain within the stratum corneum. Although this pathway is very tortuous and therefore much longer in distance than the overall thickness of the stratum corneum, the intercellular route is considered to yield much faster absorption due to the high diffusion coefficient of most drugs within the lipid bilayer. Resulting from the bilayer structure, the intercellular pathway provides hydrophilic and lipophilic regions, allowing more hydrophilic substances to use the hydrophilic and more lipophilic substances to use the lipophilic route. In addition, it is possible to influence this pathway by certain excipients in the formulation. 

In the following section, methods of assessing the thickness of stratum corneum removed per tape are summarized. 

For untreated skin, repeated stripping may remove a constant amount of stratum corneum, independent of the number of strips already performed [125], The momentary depth may thus be calculated by dividing the mean thickness of the overall stratum corneum by the total number of tapes necessary to remove it completely and multiplying with the number of strips already performed. 

Variation in penetration at different anatomic sites Drug penetration is inversely related to thickness of the stratum corneum. Thus, permeability (and often toxicity) is greater in areas of thinner skin, such as the face or scrotum. 

FIGURE 2.2 A diagram of human skin. Epidermal thickness depends upon body site being thickest on the palms and soles (-1500 pm) and thinnest around the eyes (-10 pm). The stratum corneum is the only layer composed of anucleated, terminally differentiated kerati-nocyte cells called corneocytes. All other epidermal layers contain nucleated keratinocytes. The dermis is composed primarily of the structural proteins collagen and elastin. 

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