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. 

In general, the basic architecture of the integument is similar in all mammals. However, differences exist in the thickness of the epidermis and dermis, the number of cell layers, and the blood flow patterns between species. Additionally, differences exist within the same species at different body sites. Skin is thickest over the dorsal and lateral surfaces and thinnest on the ventral and medial surfaces of the body. The stratum corneum is thickest in glabrous skin regions such as the palmar and 

Schematic of mammalian skin. Depicts structures found in both animal (apocrine sweat glands) and human skin (eccrine sweat glands). (From Monteiro-Riviere, 1991.) 

Meissner s corpuscle Melanocyti Merkel cells Langerhans cell Stratum corneum Stratum lucidum  

Stratum granulosum Stratum spinosum Stratum basale 

Sebaceous gland Arrector p muscle Connectr tissue sheath External root sheath Internal root sheath Apocri sweat gland Matri  

Free nerve endings Opening of sweat duct 

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Psyllium Hydrophilic Mucciloid. Metamucil, or psyllium hydrophilic mucciloid, is a white-to-cream colored, slightly granular powder having Htde or no odor and a slightly acidic taste. It is made from the mucilagiaous portion, ie, outer epidermis, of blond psylHum seeds (Plantago ovatd). 

Leather. Natural leather is made from hides, which are salted and cured, then taimed. Through the preparing process, useless matter which caimot be taimed, such as outerskin (epidermis) and flesh, are removed, leaving the tme skin (corium). In the tanning process, the fluid matter which maintains the skin in a flexible and moist condition is removed, and there remains nothing but the fibrous portion to be acted on by the tanning chemicals (1-3). 

Hydroxy vitamin D pools ia the blood and is transported on DBF to the kidney, where further hydroxylation takes place at C-1 or C-24 ia response to calcium levels. l-Hydroxylation occurs primarily ia the kidney mitochondria and is cataly2ed by a mixed-function monooxygenase with a specific cytochrome P-450 (52,179,180). 1 a- and 24-Hydroxylation of 25-hydroxycholecalciferol has also been shown to take place ia the placenta of pregnant mammals and ia bone cells, as well as ia the epidermis. Low phosphate levels also stimulate 1,25-dihydtoxycholecalciferol production, which ia turn stimulates intestinal calcium as well as phosphoms absorption. It also mobilizes these minerals from bone and decreases their kidney excretion. Together with PTH, calcitriol also stimulates renal reabsorption of the calcium and phosphoms by the proximal tubules (51,141,181—183). 

Products that are allowed to remain on the skin are differentiated from those that are meant to be rinsed off. Components of products left on the skin can be expected to penetrate the viable epidermis and to be systematically absorbed. Products that are rinsed off shordy after skin contact, such as shampoos, can, if propedy labeled, contain preservatives that might eUcit adverse reactions if left on the skin. Typical examples of such preservatives are formaldehyde, formaldehyde releasers such as Quatemium 15 or MDM hydantoin, and the blend of methylchloroisothia2olinone and methylisothia olinone. 

Studies of the interactions between water and the Hpid constituents of the stratum corneum suggest that the supply of water per se is not responsible for skin quaHty and condition. Water vapor from lower layers provides a constant supply of moisture to the epidermis. Instead, the abiHty of the skin to retain the moisture is critical, and this abiHty depends on the HpidlameUar bilayers that occupy the spaces between the ceUs of the stratum corneum (44—46). 

Hydrogen fluoride Tip and margin bums, dwarfing, leaf abscission narrow brown-red band separafes necrotic from Youngest leaves most sensitive Epidermis and mesophyll cells 0.1 (ppb) 0.08 wi. i... 

Mature leaves most sensitive Epidermis and mesophyll cells 0.10 290 2 hr... 

The skin is the largest organ in the human being. In particular, the surface layer of the outer epidermis, the stratum corneum, usually provides quite good protection against chemical compounds. Nevertheless, the skin is an important entry route for chemical compounds into the body. 

Skin has several protective mechanisms in addition to its thick epidermis that prevent many chemical compounds from penetrating it. Eccrine (sweat)... 

Light and Toxic Reactions In many individuals, exposure to ultraviolet radiation from the sun causes skin reactions such as erythema, thickening of the epidermis, and darkening of existing pigment. Exposure to ultraviolet light also increases the risk of different forms of skin cancers, especially malignant melanoma. ... 

Epidermis The outermost skin of the body, through which some dangerous chemicals can be absorbed into the body. 

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). 

Ober-hand, /. back of the hand upper hand advantage, -haut,/. epidermis. 

Schleim-saure /. mucic acid, -schicht /. layer of sHme or mucilage or mucus specif., mucous layer, Malpighian layer (of the epidermis). -stoff m. sHmy substance mucin. 

In the skin, ENaC is expressed in keratinocytes of the epidermis and in hair follicles. It could play a role in terminal differentiation by modulating keratinocyte calcium signaling. The skin expresses MR, GR, and 11 3HSD2, but the role of aldosterone and glucocorticoids on ENaC activity and keratinocyte differentiation is not yet understood. 

For the topical treatment of some chronic inflammatory skin diseases (like atopic dermatitis) immunosuppressive macrolides (like TRL and pimecrolimus) that permeate the inflamed epidermis are of benefit for patients. Severe side effects comparable to those after systemic application of TRL in transplanted patients (see above) have not been observed so far. For the treatment of psoriasis vulgaris these drugs are less effective. The CD2 antagonist alefacept may be a suitable alternative to allergic reactions. 

Epidermal Maturation and Wound Healing. All three PPAR isotypes are expressed during epidermal maturation each isotype has a specific pattern of expression in regard to development and the various layers of the epidermis. An important role for PPARS in the development and/or maintenance of normal skin health is indicated by the presence of defective wound healing in the PPARS-null mouse. 

The efficacy of chitosan in the treatment of leg ulcers stems from its antiinflammatory action and stimulation of epithehahzation. Chitosans stimulate the granulation process and the epidermis formation, thus accelerating heahng, even in the elderly suffering from chronic venous insufficiency, the... 

In vitro studies on the absorption of dioctyltin dichloride and dioctyltin EHMA through rat and human epidermis (occluded and unoccluded) were performed by... 

Ward RJ (2003) Dioctyltin dichioride In vitro absorption through human and rat epidermis. Central Toxicology Laboratory,... 

Profiling of wheat class III peroxidases genes derived from powdery mildew-attacked epidermis reveals distinct sequence-associated expression patterns / / Molecular Plant-Microbe Interactions. V. 18. P. 730-741. 

Photosynthesis and gas exchange of leaves are affected by many stresses including drought, flooding, salinity, chilling, high temperature, soil compaction and inadequate nutrition. Many, but not all, of these stresses have symptoms in common. For example, stomatal conductance and the rate of assimilation of CO2 per unit leaf area often decrease when stress occurs. Further, it is possible that several of the stresses may exert their effects, in part, by increasing the levels of the hormone abscisic acid (ABA) in the leaf epidermis. This hormone is known to close stomata when applied to leaves. 

Fig. 9. Leaf water potential and turgor, abaxial stomatal conductance, and ABA content of abaxial epidermis of leaves of Commelina plants which were grown with their root systems divided between two pots. Water was either applied daily to both halves of the root system (A) or was withheld from one half of the root system after day 1 of the experimental period (A). Points for water relations and conductance are means s.e. Modified from Zhang, Schurr Davies (1987).

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