Moisturization and skin barrier function
描写菊花的诗ABSTRACT： Over the past decade，great progress has been made toward elucidating the structure and function of the stratum corneum （SC)，the outermost layer of the epidermis. SC cells (corneocytes） protect against desiccation and environmental by the SC is largely dependent on veral factors. First， intercellular lamellar lipids， organized predominantly in an orthorhombic gel pha， provide an effective barrier to the passage of water through the tissue。 Secondly， the diffusion path length also retards water loss, since water must traver the tortuous path created by the SC layers and corneocyte envelopes。 Thirdly， and equally important, is natural moisturizing factor （NMF）, a complex mixture of low-molecular—weight， water—soluble compounds first formed within thee corneocytes by degradation of the histidine-rich protein known as filaggrin。 Each maturation step leading to the formation of an effective moisture barrier－including corneocyte strengthening， lipid processing， and NMF generation－is influenced by the level of SC hydration。 The process, as well as the final step of corneodesmolysis that mediates exfoliations, are often disturbed upon environmental challenge, resulting in dry, fla
ky skin conditions。 T林微因he prent paper reviews our current understanding of the biology of the SC， particularly its homeostatic mechanisms of hydration。
Keywords: corneocyte, corneodesmolysis, filaggrin, natural moisturizing factor, stratum corneum。2003年属啥
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For humans to survive in a terrestrial environment, the loss of water from the skin must be carefully regulated by the epidermis， a function dependent on the complex nature of its outer layer, the stratum corneum (SC） (1)。 赞美老师的名言警句The SC is a lectively permeable, heterogeneous, composite outer layer of the epidermis that protects against desiccation and environmental challenge, and retains sufficient water to allow it to function in arid environments. The small amount of water loss that does occur hydrates the outer layers of the SC， maintaining its flexibility and facilitating the enzymatic reactions that drive the SC’s maturation （2－4）. The water-retaining capacity of the SC is highly dependent upon the phenotype of the corneocytes， their spatial arrangement， the preci compos
ition and physical packing of extracellular lipids, and the prence of highly hygroscopic compounds found largely within the corneocytes （Fig。 1）.
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Under conditions of normal humidity （〉 80％）， a steep water gradient exists in the SC, which can be viewed by a variety of elegant techniques (Fig。 2) (5，6)。 The gradient is established, in part， by a discontinuity in the water—binding capacities between different corneocyte cell layers, as demonstrated via cryo-scanning electron microscopy （Fig. 3) (7). Briefly, corneocytes do not appear to be swollen at low hydration levels (18–26％ w/w)， suggesting that only bound water is prent in the SC. However， at higher hydration levels （57– 87％ w/w）, the corneocytes are more swollen in the central portion of the SC compared with the superficial and deeper layers。 The corneocyte cell thickness is shown to increa linearly in a direction perpendicular to the skin surface with increasing hydration. Extracellular pools of water are only obrved at very high hydration levels （> 300% w/w). The explanation of the differential hydration levels is rooted in the mechanisms involved in SC moisturization。
The prent review describes the most recent rearch that reveals the complexity and intricacy of the everyday functioning of the SC to maintain hydration in relation to ever—changing environmental conditions。 It shows that skin is involved in a constant battle to maintain optimal moisture barrier and protective functions。 As discusd throughout this supplement， different elements of fundamental skin care exercid as a daily routine truly help the skin to achieve its healthiest state.
Stratum corneum moisturization at the molecular level
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Stratum corneum
The SC consists of terminally differentiated keratinocytes （corneocytes） and the creted contents of lamellar bodies （2,8）. Each corneocyte originates from an actively proliferating keratinocyte in the epidermis beneath the SC. Corneocyte proteins are generally questered to the cytosol, while ceramides and other lipids are enriched in the extracellular space， where they form a continuous pha （8）。 The physical packing of the corneocytes creates a tortuous path for molecules to traver, effectively increasin
步字的笔顺g the diffusion length and thereby improving the SC barrier function. Extracellular lipids, tightly arranged as a covalently bound matrix in a crystalline pha called an orthorhombic packing state further reduce the rate of water flux through the tissue (9）。 The constitution， and thus the effectiveness, of the lipid barrier are dependent on the absolute concentrations and the relative proportions of the different lipids (e。g。, ceramides， cholesterol， and fatty acids）。 Lectins and desmosomes found in the SC also help to maintain the structural cohesiveness of the SC. The reduction of water flux and loss through the tissue is not the sole cau of the apparent discontinuity in hydration between different corneocyte layers， however。 Selective retention of water is required as well, central to which is SC natural moisturizing factor (NMF）.
Natural moisturizing factor
Found exclusively in the SC， NMF consists primarily of amino acids or their derivatives, such as pyrrolidone carboxylic acid (PCA) and urocanic acid， together with lactic acid, urea， citrate, and sugars （10) （Table 1). Natural moisturizing factor compounds are pr
ent in high concentrations within corneocytes and reprent up to 20–30％ of the dry weight of the SC （11）. By absorbing atmospheric water and dissolving in their own water of hydration， hygroscopic NMF components act as very efficient humectants （2). Biologically, this humectancy allows the outermost layers of the SC to remain hydrated despite the desiccating action of the environment. Corneocytes that posss the highest concentration of NMF retain more water and appear more swollen when viewed under cryo—scanning electron microscopy （7）. However, NMF is much more important than this. As our understanding of the terminal differentiation and SC maturation process has incread, it has become clear that, by maintaining free water in the SC， NMF also facilitates critical biochemical events。 The most striking example of this is the regulation of veral corneocyte proteas that are ultimately responsible for the very generation of NMF itlf.
