Horny layer

Hom-klee, m. bird s-foot trefoil (Lotus corni-culatus). -kobalt, n. asbolite. -Ibffel, m. horn spoon, -mohn, m. horn poppy (Glau-cium). -quecksilber, n. horn quicksilver (native mercurous chloride), -schicht, /. horny layer, specif, epidermis, -silber, n. horn silver (cerargyrite). -spatel, m. horn spatula, -stein, m. homstone (variety of quartz). 

The outermost layer of the skin appearing in the exploded epidermal sketch of Fig. 1(b) represents the stratum corneum (the horny layer). The principal barrier element of the skin, it is an essentially meta-bolically inactive tissue comprised of acutely flattened, stacked, hexagonal cell building blocks formed from once-living cells. These cellular building blocks are... 

Gender, too, affects the appearance of human skin. Nevertheless, there is little evidence that the skin of males and females differs greatly in permeability. However, there are established differences in the barrier properties of skin across the races of humans. While the horny layers of Caucasians and Blacks are of equal thickness, the latter has more cell layers and is measurably denser [30]. As a consequence, black skin tends to be severalfold less permeable [30,31],... 

Salicylic acid is one of the most commonly used keratolytics. It causes a disruption in corneocyte-to-corneocyte cohesion in the abnormal horny layer of psoriatic skin. This serves to remove scales, smooth the skin, and decrease hyperkeratosis. The keratolytic effect enhances penetration and... 

H.-J. Weigmann, J. Lademann, H. Meffert, H. Schaefer, and W. Sterry. Determination of the horny layer profile by tape stripping in combination with optical spectroscopy in the visible range as a prerequisite to quantify percutaneous absorption. Skin Pharmacol. Appl. Skin. Physiol. 12 34-45 (1999). 

The other major route of chemical hazards to plant workers is penetration through the skin, which has a surface area of about 1.8 m for a male adult, and is lined with a tough horny layer of densely packed cells without nuclei. So the skin is reasonably impervious to the penetration of chemical substances, and penetration depends on the lipid solubility of the compound, where the diffusion speed is inversely related to molecular weight. Skin penetration hazard becomes relatively more important than breathing for high boiling point liquids, which have lower vapor pressures. 

Papilloma. A benign epithelial neoplasm producing finger-like or verrucous projections from the epithelial surface. Parakeratosis. Persistence of the nuclei of the keratinocytes into the stratum corneum (horny layer) of the skin. Parakeratosis is normal in the epithelium of true mucous membrane of the mouth and vagina. Parkinson s disease. Neurological disorder characterized by hypokinesia, tremor, and muscular rigidity. 

Today the population is becoming increasingly exposed to ultrafine particles (< 20 nm, e.g., Aerosil, 2) in bodycare and household products. Ion microscopy studies revealed that such particles can, for example, penetrate the horny layer of the skin and can result in unexpected interactions. SIMS and Fourier transform laser microprobe mass spectrometry (FT LMMS) have been applied to study 2 stimulated interaction in thin layers of dermatological gels as a result of UV irradiation.175 For future studies of distribution of ultrafine particles LA-ICP-MS will be employed. 

Arsenious oxide in dilute solution is not absorbed by the unbroken skin, but absorption occurs from concentrated solution.6 The action is not directly on the cells of the horny layer of the epithelium, but primarily on the capillaries.7... 

Accelerants operate in complex, interacting ways to change the intercellular region of the horny layer by fluidization, alteration of polarity, phase separation, or lipid extraction. More... 

FIGURE 12.1 Penetration enhancer activity, (a) Action at intercellular lipids. Some of the ways by which penetration enhancers attack and modify the well-organized intercellular lipid domain of the stratum comeum. (b) Action at desmosomes and protein structures. Such dramatic disruption by enhancers (particularly potent solvents) as they split the stratum corneum into additional squames and individual cells would be clinically unacceptable, (c) Action within comeocytes. Swelling, further keratin denaturation and vacuolation within individual horny layer cells would not be so drastic but would usually be cosmetically challenging (see Menon and Lee [69] for further details). (Reproduced from Barry, B.W., Nat. Biotechnol. 22, 165, 2004. With permission.)... 

In terms of the mechanisms of action, the pyrrolidones partition well into human horny layer. They may act by altering the solvent nature of the membrane and pyrrolidones have been used to generate reservoirs within skin membranes. Such a reservoir effect offers potential for sustained release of a permeant from the stratum corneum over extended time periods. However, as with many other potential enhancers, clinical use of pyrrolidones is problematic due to adverse reactions. An in vivo vasoconstrictor bioavailability study demonstrated that pyrrolidones caused erythema in some volunteers, although this effect was relatively short-lived. Also, a toxic hygroscopic contact reaction to NMP has been reported recently [21]. 

At physiological pH the long-chain ceramides of the horny layer barrier in the presence of cholesterol and fatty acids have been shown to have equal capacity to form lamellar lipid structures as have phospholipids.24,25 The chain length of the ceramides is to a great extent longer than 18 carbons, even up to 34 carbons in one of the chains, and this suggests close packing of the crystalline type at normal skin temperatures. 

Several classes of ceramides have been described in human skin.14 Today it is considered that the ceramides are essential for the barrier properties. It has been suggested that the lower amount of ceramides found in stratum corneum in atopic dermatitis26,27 explains the increased TEWL seen in dry atopic skin. In this context it is of special interest to note that part of the long-chain ceramides of the horny layer are covalently bound to the proteins forming the corneocyte envelope.25 This suggests that such lipids constitute anchors of the hydrophobic phase to the corneocytes and thereby add to the cohesion of the cells of the horny layer. 

Water homeostasis is a strict requirement for normal physiological function. The most important task of the human skin is thus to create a watertight enclosure of the body to prevent water loss. It is the intercellular lipid matrix of the outermost keratinized horny layer of the skin (possibly together with recently reported claudin-based tight-junctions Furuse et al., 2002) that represents the skin barrier proper as once this lipid matrix (composed foremostly of saturated long chain ceramides ( 50% wt/wt) and cholesterol (—30% wt/wt) (Wertz and Norlen, 2002)) has been removed, substances diffuse freely into or out of the body system (Blank, 1952 Breathnach et al., 1973 Elias and Friend, 1975). At the same time the intercellular lipid matrix ensures that the stratum corneum remains hydrated and thus the skin surface appears healthy and smooth. 

In the initial studies of trace elements of human skin cross sections, a Ca profile evolved which was increasing from the basal, germinative level of the epidermis toward the horny layer. The drastic concentration drop of Ca concentration down to threshold values at the border of the stratum germinativum and the stratum corneum was a particularly interesting feature.15-17 Only a few years later this finding could be correlated to the fact that a Ca concentration >0.1 mmol was essential if a fully cornified stratum corneum was to be obtained in cell culture. This relationship between Ca and terminal differentiation of the epidermal cells was verified in a PIXE study of epidermal cell cultures.18... 

In a recent study of clinically normal skin from patients with psoriasis, a high Fe content of the horny layer was demonstrated, whereas there was no detectable Fe in the horny layer of normal healthy control individuals.38 Obviously, this finding in psoriasis demonstrates that the entire differentiating epidermis of these patients is involved in the disorder, whether clinically expressed or not. 

The lack of filaggrin and keratohyaline in IV horny layer results in a deficiency of the natural moisturizing factor (NMF) — composed of urocanic acid and pyrrolidone carboxylic acid (PCA) — that is, breakdown products of amino acids in profilaggrin.12 In XRI, on the other hand, there is... 

Wellner, K., Fiedler, G., and Wohlrab, W., Investigation in urea content of the horny layer in atopic dermatitis, Z. Hautkr., 67, 648-650, 1992. 

Blair, C., The action of a urea-lactic acid ointment in ichthyosis. With particular reference to the thickness of the horny layer, Br. J. Dermatol., 94, 145, 1976. 

HP3 is of questionable physiological significance. As expected, in both in vitro and in vivo models, a strong occlusion was seen after application of white petrolatum. This strong occlusive effect as observed with the lipophilic pastes led to a diminution of the TEWL because of the concomitant increase in the barrier function of the stratum corneum and despite an accumulation of moisture in the horny layer. This justifies the use of such pastes for skin protection, but not for drying the skin. 

In vivo, hydrophilic pastes showed different interactions with the skin. Some pastes clearly hydrated the skin, others could indeed remove water from a preliminary hydrated horny layer. Elements contributing to these properties may be the presence of humectants such as glycerol, contributing to a long-lasting presence of water on the skin in the first case, or the acceleration of skin surface water loss, contributing to an accelerated removal of water from a hydrated horny layer in the second case. However, this represents, in our opinion, at most one of the elements contributing to the measured events and may simply be due to a different water content of the pastes. 

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