Epidermis detailed

The concentration of 4-oxo-flavonoids depends on the plant, environmental conditions, the part of the plant consumed, the degree of ripeness as well as on the food processing. Flavonoids are preferentially located in the epidermis solubilized in the vacuolar sap (especially flavones and flavonols glycosides) or in the epicuticular zone [20]. More detailed information about the occurrence of the different compounds is given in each subchapter. 

Particle probe analysis and, in particular, proton probe analysis, which is sensitive to trace element levels in tissue sections have been demonstrated to reveal important details about cellular physiology in the differentiating epidermis of normal and pathological skin. Such a physiological approach will serve to complement data from other techniques. A future collective approach of this kind will make it possible to understand how a dry and eczematous skin develops and also what the mechanisms of subsequent healing are. 

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. 

A significant amount of 7-dehydrochoIesteroI is also found in the deeper dermis layer, but because most UVB photons are absorbed by the epidermis, very little production of vitamin D3 occurs in the dermal layer. There is some evidence that formation of 7-dehydrocholesterol is under the regulation of the vitamin D3 endocrine system [39] however, details of this regulation are not fully understood. 

The mechanism of action of sulfur mustard is multifaceted and complex, and has been reviewed in some detail by Papirmeister et al. (1991), Hurst and Smith (2008), and Smith et al. (2008). Efforts to understand the mechanisms of sulfur mustard toxicity are ongoing. Basically, sulfur mustard disrupts the interface of the epidermis and basement membrane causing blistering between the epidermis and dermis. Both immediate (immediate cell membrane damage) and delayed phases (secondary effects resulting from inflammatory responses, DNA damage, vascular leakage) have been described for sulfur mustard-induced dermal effects (Somani and Babu, 1989). Many of the toxic effects of sulfur mustard can be attributed to oxidative stress. 

A poorly understood aspect of lipophorin metabolism is tissue-specific delivery of lipids. At present mechanistic details are lacking, but some properties of the system are apparent. Lipid seems to be transferred from lipophorin to tissue only in those cases in which the tissue can carry out some additional reaction with the lipid, e.g., in the fat body DG is converted to TG, and in the epidermis hydrocarbon is secreted onto the outer surface of cuticle. In other cases there may be intracellular lipidbinding proteins that, by binding the lipid, drive lipid uptake into the tissue. These or other processes would drive the equilibrium of lipid transfer in favor of the tissue, irrespective of the mechanism by which lipid delivery occurs. 

The epidermis contains five histologically distinct layers which, from the inside to the outside, are the stratum germinativum, stratum spinosum, stratum granulosum, stratum lucidum and the stratum comeum (Figure 2). The stratum corneum, comprising anucleate (dead) cells, provides the main barrier to topical drug delivery and hence is considered in further detail here. 

The stratum corneum is a 10-20 pun thick non-viable epidermis, consisting of 15 to 25 flattened and stacked cornified cells. Each stratum corneum cell is composed mainly of insoluble bundled keratins (—70 percent) and lipids ( 20 percent) encased in a cell envelope. The intercellular region consists mainly of lipids together with desmosomes which provide cor-neocyte cohesion, as described later. There is continuous desquamation of the outermost layer of the stratum corneum, with a total turnover occurring once every two to three weeks. The most important function of the viable epidermis is the generation of the stratum corneum, which is described in detail below. Other functions include metabolism and the synthesis of melanin from melanocytes for skin pigmentation and sun protection. 

Secretion of ecdysone starts the many biochemical processes that are necessary for the molting. The cells in the epidermis are stimulated to produce a new cuticle, and when ready, the insect will creep out of its old skin. The molecular mechanism of ecdysone has been studied in some detail. The molecular target of ecdysone and other ecdysteroids consists of at least two proteins, the ecdysteroid receptor (EcR) and ultraspiracle (USP). Both EcR and USP are members of the steroid hormone receptor superfamily with characteristic ligand-binding domains. An EcR-USP-ecdysteroid complex is formed, which activates several genes that code for transcription factors, i.e., proteins that activate or repress the activity of other genes, and the appropriate amounts of proteases and other enzymes necessary to degrade old structures and rebuild new ones are formed in a time-controlled sequence. 

Fig. 4. Effects of high (10 ppm and 100 ppm) ethylene treatment (24 h) on MTs in cells of cv. LG-11 maize roots. 10 ppm treatment (a-e) has no effects in cells of the quiescent centre (QC) (a) and in meristematic cells of root cap (RC) (a.b), but it depletes MTs from developmentally older root cap cells (b). In addition this ethylene treatment randomizes CMTs in all postmitotic cells of the root proper. For the outer cortex (CXI) and epidermis (EP) cells, this effect is only slight (c). However, cells of the inner cortex (d,e) are strongly affected in this respect and the transverse CMTs are replaced by completely random CMTs. 100 ppm ethylene treatment (f,g) disintegrates MTs from most root cells with the exception of the distal part of meristem and root cap initials and quiescent centre cells (not shown). In the root proper, the epidermis (EP) and outer cortex (OC) cells are the only ones to preserve their MTs which, in the case of epidermis, are still transverse and well-ordered (f). In contrast, the inner cortex cells (IC) are devoid of MTs (g). For more details on ethylene treatments see [157]. Bar represents 10 pm.

Prohibitins isolated so far include eleven flavonoid derivatives, such as albanins A-E (64-68) (71-73), F (25) (74) [ = kuwanon G (55)=mor-acenin B (75)], G (26) (74) [(=kuwanon H (5 )=moracenin A (76)], H (69) (7, morusin (24) (72), kuwanons C (33) (72), and E (42) (73), and three 2-arylbenzofuran derivatives such as albafurans A-C (70-72) (78, 79) (Fig. 16). These prohibitins are obtained from the epidermis and not from the xylem tissue of the shoot, which suggests that the epidermis of the mulberry shoot plays an important role not only as a physical defense structure but also as a chemical defense against pathogens. Details concerned with the Diels-Alder type adducts albanins F (25), G (26), and H (69), will be presented later in this article. 

Hyaluronic acid (HA), discussed in detail in Section IILA.l.e, is a component of human skin that occurs in the interstitial fluids of the epidermis and, because of its water-absorbing capacity, lends turgidity, viscolasticity, and firmness to the skin. Changes in HA concentrations have been noted in skin as tlie body ages (196). In aging skin free of chronic sun... 

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