Epidermal layer/cells

Emulsion components enter the stratum corneum and other epidermal layers at different rates. Most of the water evaporates, and a residue of emulsifiers, Hpids, and other nonvolatile constituents remains on the skin. Some of these materials and other product ingredients may permeate the skin others remain on the surface. If the blend of nonvolatiles materially reduces the evaporative loss of water from the skin, known as the transepidermal water loss (TEWL), the film is identified as occlusive. AppHcation of a layer of petrolatum to normal skin can reduce the TEWL, which is normally about 4—8 g/(m h), by as much as 50 to 75% for several hours. The evaporated water is to a large extent trapped under the occlusive layer hydrating or moisturizing the dead cells of the stratum corneum. The flexibiHty of isolated stratum corneum is dependent on the presence of water dry stratum corneum is britde and difficult to stretch or bend. Thus, any increase in the water content of skin is beHeved to improve the skin quaHty. 

Air pollutants may enter plant systems by either a primary or a secondary pathway. The primary pathway is analogous to human inhalation. Figure 8-2 shows the cross section of a leaf. Both of the outer surfaces are covered by a layer of epidermal cells, which help in moisture retention. Between the epidermal layers are the mesophyll cells—the spongy and palisade parenchyma. The leaf has a vascular bundle which carries water, minerals, and carbohydrates throughout the plant. Two important features shown in Fig. 8-2 are the openings in the epidermal layers called stomates, which are controlled by guard cells which can open and close, and air spaces in the interior of the leaf. 

O Exposure to ultraviolet radiation from the sun is recognized as one of the primary triggers for skin carcinogenesis. Based on their wavelengths, UV radiation is divided into three components UVA (320 00 nm), UVB (280-320 nm), and UVC (200-280 nm).15 UVB accounts for only 5% of the solar radiation that reaches the earth, but it is the primary carcinogenic component in the UV spectrum.15 The following sequence of events describes the process in which UV radiation causes skin cancer (1) UV radiation reaches the earth, and on the skin, it reaches the cells in the epidermal layer (i.e., squamous cells, basal cells, and melanocytes),16 (2) the UV radiation (specifically... 

Plants were probably the first to have polyester outerwear, as the aerial parts of higher plants are covered with a cuticle whose structural component is a polyester called cutin. Even plants that live under water in the oceans, such as Zoestra marina, are covered with cutin. This lipid-derived polyester covering is unique to plants, as animals use carbohydrate or protein polymers as their outer covering. Cutin, the insoluble cuticular polymer of plants, is composed of inter-esterified hydroxy and hydroxy epoxy fatty acids derived from the common cellular fatty acids and is attached to the outer epidermal layer of cells by a pectinaceous layer (Fig. 1). The insoluble polymer is embedded in a complex mixture of soluble lipids collectively called waxes [1], Electron microscopic examination of the cuticle usually shows an amorphous appearance but in some plants the cuticle has a lamellar appearance (Fig. 2). 

OCT images of plant tissue in the process of its saturation with water are shown in Fig. 3b-3i. It can be seen from this figure that the thickness of the epidermal and palisade layers increase linearly with the time of the plant being in water. The layers reach their maximum thickness as soon as 40 minutes after the plant has been placed in water (Fig. 3g). Correspondingly, during the same time an increase in the volume of cells of the upper epidermal layer situated closer to the surface is observed. Most significantly the cells and the epidermis increase within the first 5-25 minutes (Fig. 3b-3f)... 

Capsaicinoids start to accumulate 20 days post anthesis and synthesis usually persists through Suit development [59, 62,73], The site of synthesis and accumulation of the capsaicinoids is the epidermal cells of the placenta in the fmit (Fig. 8.5) [60, 72, 73], Ultimately, the capsaicinoids are secreted extracellularly into receptacles between the cuticle layer and the epidermal layer of the placenta [74]. These receptacles of accumulated capsaicinoids are macroscopically visible as pale yellow to orange droplets or blisters on the placenta of many chile types (Fig. 8.5). 

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. 

Cutaneous biotransformation is mostly associated with the stratum basale layer where there can be phase I and phase II metabolism. However, the skin is not very efficient, compared to the liver. The epidermal layer accounts for the major portion of biochemical transformations in skin, although the total skin activity is low (2-6% that of the liver). Where activity is based on epidermis alone, that layer is as active as the liver or, in the case of certain toxicants, several times more active. For some chemicals, metabolism can influence absorption, and transdermal delivery systems of drugs utilize this activity. For example prodrug such as lipid esters are applied topically, and cutaneous esterases liberate the free drug. These basal cells and extracellular esterases have been shown to be involved in detoxification of several pesticides and bioactivation of carcinogens such as benzo(a)pyrene. For rapidly penetrating substances, metabolism by the skin is not presently considered to be of major significance, but skin may have an important first-pass metabolic function, especially for compounds that are absorbed slowly. 

A number of studies with widely diverse species have established that the major site of cuticular hydrocarbon synthesis is within the cells associated with the epidermal layer or the peripheral fat body, particularly the oenocytes. In Schistocerca gregaria, Diehl (1973, 1975) separated the oenocyte-rich peripheral fat body from the central fat body tissue and observed the highest rate of hydrocarbon synthesis in the oenocyte-rich peripheral fat body. In Tenebrio molitor, Romer (1980) demonstrated that isolated oenocytes efficiently and specifically incorporated [14C]acetate into hydrocarbon. Similar studies in Periplaneta americana (Nelson, 1969), Sarcophaga bullata (Arnold and Regnier, 1975), and Musca domestica (Dillwith et al., 1981) demonstrated that hydrocarbon synthesis occurs primarily in the epidermal tissue. 

Dysplasia is a special type of cell adaptation that literally means disordered growth, and it is usually associated with neoplasia or preneoplastic changes. Dysplasia implies a loss in the uniformity of the individual cells and a loss in the normal tissue growth pattern. For example, a dysplastic area involving the epidermal layers of the skin might contain jumbled, atypical epithelial cells with loss of their normal polarity and orderly differentiation. 

Epidermal cells are on the upper and lower surfaces of a leaf. They have two features which prevent evaporative water loss they are packed densely together and they are covered by a cuticle, a waxy layer secreted by the cells. The epidennis usually consists of a single layer of cells, although the specialized leaves of some desert plants have epidermal layers which are several cells thick. Epidermal cells often have large vacuoles which contain flavonoid pigments. Elavonoids generally absorb ultraviolet radiation, and may act as a sort of natural sunscreen for the internal layers of the leaf, by filtering out harmful ultraviolet radiation from the sun. 

DNA alkylation may not completely account for sulfur mustard s toxicity, however. Sulfur mustard may also affect intracellular enzymes and the structural components of cell membranes, causing dissolution of intracellular attachments and a cleft in the basal epidermal layer. In turn, inflammation develops, followed by edema and blisters (3). Although nitrogen mustard is less potent than sulfur, its mechanism of action is similar. 

Phenol is not considered to be carcinogenic. On the contrary, long-term histological studies show that phenol (even when the formula contains croton oil) has a protective effect against skin cancers. A logical explanation of this protective effect may lie in the fact that the cells that take the brunt of UV rays are the keratinocytes closest to the stratum corneum, the ones that form, for example, the top of the dermal papillae. On the other hand, the keratinocytes that are in the lower regions of the dermal papillae are relatively protected compared with those higher up the UV photons have been partially absorbed or diffracted by the epidermal layers above them. In addition, the sebo-cytes or keratinocytes that make up the shaft of the hair are anchored more deeply in the skin, at a depth where the harmful effects of the sun are not as severe. 

The epidermal layers are dissolved down to the basal layer in 24—36 hours under an occlusive mask. Thereafter, the architecture of the cells returns to normal and the histological coloring evens out. After a peel, the palisade structure of the keratinocytes is more even. 

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