Epidermal turnover

Tretinoin 0.05-0.1% which is the most critical component of this regimen as it results in decreased stratum corneum thickness, increases the kinetics of epidermal turnover, and decreases corneocyte adhesion... 

Generally used at a concentration of 0.05-1% with minimal side effects (erythema and desquamation), retinoic acid has to be applied once a day, usually nightly. It does not suppress melanogenesis, but accelerates epidermal turnover. 

Seleninm snlfide (8.101) is used dermatologically as an antiseborrheal agent. Selenium acts as a cytostatic agent to decrease epidermal turnover rates in the skin. 

Pyrithione zinc, 1-2% shampoo, apply once or twice weekly Denorex Daily, Head Shoulders, Sebulon, various generic Both selenium sulfide and zinc pyrithione are cytostatic agents that decrease epidermal turnover rates. Massage into wet scalp for 2-3 minutes. Rinse thoroughly and repeat application. Selenium sulfide can be irritating to the eyes and skin. 

An application of dansyl chloride accumulates in the epidermis and stains the skin. It is possible to measure the rate of disappearance of this coloration after the application of different hydroxy acid solutions at a constant pH. The rate at which the color disappears is correlated with the rate of cell replacement, and therefore provides information on the stimulation of epidermal turnover and on the capacity of the acid to stimulate skin regeneration. TCA (which is not a hydroxy acid), at 0.5%, stimulates skin renewal up to 50%, lactic acid (at 4%, pH 3) stimulates up to 34%, acetic acid (3%, pH 3) up to 30%, and pyruvic acid (4%, pH 3) up to 23%. Partial neutralization of these solutions, to pH 5, quite clearly reduces the rate of renewal, and neutralizing these same solutions to pH 7 takes away all of their clinical effectiveness. 

Glycolic acid (8%) softens the skin, prevents pores clogging up, stimulates epidermal turnover (pH 4)... 

The effect of age on percutaneous absorption has been examined in vivo in man with variable results. It was postulated (Roskos et al. 1989) that reduced hydration levels and lipid content of older skin may be responsible for a demonstrated reduction in skin permeability where the permeants were hydrophilic in nature (no reduction was seen for model hydrophobic compounds) (Table 14.2). The reduced absorption of benzoic acid demonstrated in the elderly (Rougier 1991) was in line with this suggestion, but not the reduction in absorption of testosterone (lipophilic) (Roskos et al. 1986), or lack of change in the absorption of methyl nicotinate (more hydrophilic) with age (Guy et al. 1983). There are a number of potential physiological changes which may be responsible for age-related alterations, including an increase in the size of individual stratum corneum corneocytes, increased dehydration of the outer layers of the stratum corneum with age, decreased epidermal turnover and decreased microvascular clearance (reviewed in Roskos and Maibach 1992). The issue of age-related variability, however, is far from resolved. 

The stratum comeum in the dandruff scalp is thinner [161] than in the normal scalp. In addition, the epidermal turnover rate is increased in dandruff [159,162]. It has been suggested that this rapid transfer of cells to the... 

Dandruff is age-related [163], rarely seen before puberty, but common with the onset of puberty. It peaks in the early twenties and declines in middle and old age. Dandruff appears to be seasonal, being most severe in the winter months (October through December) and milder in the summer [163]. Dandruff occurs equally among males and females [164], but the cause(s) of dandruff has not been widely accepted (i.e., the cause of the increased rate of epidermal turnover in the dandruff scalp is still a controversial subject). 

Reddy, M B., Guy, R.H., and Bunge, A.L. (2000). Does epidermal turnover reduce percutaneous penetration Pharmaceutical Research, 17, 1414—1419. 

LHA up to 10% is widely used in Europe. A derivative of salicylic acid (SA) with an additional fatty chain, its lipophilicity is increased. Even with a lowered concentration, its keratolytic effect is three times more potent than SA. It possesses a superficial desquamating effect and it increases the ration of cycling keratinocytes as part of an overall boosting of epidermal turnover [11, 93]. 

It is not proved that in the skin of hypophysectomized rats prostaglandin synthesis is prevented, neither is it proved that this is the cause of scahness, but the above data support these ideas. It is thus suggested that the function of arachidonic acid in the skin is to serve as precursor to prostaglandins, which, in turn, participate in the regulation of epidermal turnover, and, indirectly, skin scaliness. When prostaglandin synthesis in the skin is prevented, as in either EFA-deficiency or after hypophysectomy, skin scaliness results. 

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