Epidermis composition

Fig. 13.2 Triple staining of a nipple duct and the adjacent nipple epidermis for K14 (red), ER (pink), and K8/18 8 (green), a Composite image. Nuclei are counterstained with DAPI (blue), b The same as a a, but without nuclear counterstaining. c Red component (for K14) of the composite image, d Pink component (for ER) of the composite image. Note the immature glandular epithe hum of the duct and the abmpt transformation to squamous cell epithelium of the nipple. ER (pink nuclei) is only expressed in glandularly differentiated cells...

Squier CA, Cox PS, Wertz PW, Downing DT (1986) The hpid composition of porcine epidermis and oral epithelium. Arch Oral Biol 31 741-747... 

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. 

The principal pigment of human skin, hair, and eyes is melanin, which is synthesized in specialized cells, the melanocytes. They lie between the epidermis (outer layer) and the dermis (inner layer) as shown in Box 8-F. Melanocytes originate from embryonic nervous tissue and migrate into the skin by the third month of fetal life. They retain the highly branched morphology of neurons. Persons of different races all have the same numbers of melanocytes but the numbers and sizes of the pigmented melanosomes (Box 8-F) vary as does the content and chemical composition of the melanin.a d Melanosomes not only are found in the dendrites of the melanocytes but are transferred from them into adjacent epithelial cells.6 1... 

Microscopically, the skin is a multilayered organ composed of many histological layers. It is generally subdivided into three layers the epidermis, the dermis, and the hypodermis [1]. The uppermost nonviable layer of the epidermis, the stratum corneum, has been demonstrated to constitute the principal barrier to percutaneous penetration [2,3]. The excellent barrier properties of the stratum corneum can be ascribed to its unique structure and composition. The viable epidermis is situated beneath the stratum corneum and responsible for the generation of the stratum corneum. The dermis is directly adjacent to the epidermis and composed of a matrix of connective tissue, which renders the skin its elasticity and resistance to deformation. The blood vessels that are present in the dermis provide the skin with nutrients and oxygen [1]. The hypodermis or subcutaneous fat tissue is the lowermost layer of the skin. It supports the dermis and epidermis and provides thermal isolation and mechanical protection of the body. 

The composition of the free fatty acids is also unique. In both human and pig stratum cornea, the free fatty acid fraction consists mainly of long and saturated hydrocarbon chains [44,45], Oleic and linoleic acid are the only unsaturated free fatty acids detected in the stratum corneum. There are various sterols present in human stratum corneum, of which cholesterol predominates. Cholesterol is the only major lipid class that is present in both plasma membranes and the intercellular lipid lamellae. Cholesterol is synthesized in the epidermis and this synthesis is independent of the hepatic one. A minor fraction is sulfated to... 

Freinkel, R.K., and T.N. Traczyk. 1985. Lipid composition and acid hydrolase content of lamellar granules of fetal rat epidermis. J Invest Dermatol 85 295. 

Lanolin has stood the test of time as an emulsifier and skin emollient. Its complex nature has been a rich resource of derivatives formed from fractionation and chemical reactions. Although the composition of lanolin is different from the lipids found on the surface of human skin, lanolin has been demonstrated to be equivalent in its ability to restore barrier function. In addition to the beneficial effects attributable to its physical properties, lanolin may also have a pharmacological effect on the epidermis. 

Yardley, H.J., and Summerly, R., Lipid composition and metabolism in normal and diseased epidermis, Pharmacol. Then, 13, 357, 1981. 

We examine the detailed structure and composition of skin below inasmuch as the ECM in skin is involved in sensing of external forces applied to the epidermis and transducing these forces into changes in skin thickness and composition. It is well known that increased friction on the surface of skin leads to thickening of the epidermis. 

In order to have a basic understanding and appreciation of how chemicals may interact with skin, its anatomy, physiology, and chemical composition must be fully grasped. All of the aforementioned biological functions and structural adaptations have a substantial impact on the skin s barrier properties and the rate and extent of percutaneous absorption. When a compound or toxicant is applied topically, it must penetrate through several cell layers of the skin in order to be absorbed by the capillaries for systemic distribution. Alternatively, it may have a direct effect on the keratinocytes themselves. Skin can be anatomically divided into two principal components, the outermost epidermis and the underlying dermis. 

Gray, G. M. and Yardley, H. J., Lipid compositions of cells isolated from pig, human, and rat epidermis. Journal of Lipid Research 76 343-440,1975. 

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