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The importance of various unsaturated fatty acid triglycerides to the repair of faulty skin barrier function was studied in essential fatty acid-deficient rats. Following cutaneous application of the pure triglycerides for up to 5 days, the hitherto high rate of transepidermal water loss, characteristic of essential fatty acid deficiency in rats, was reduced by the triglycerides of linoleic and gamma-linolenic acids. Incorporation of the applied fatty acids into the lecithin of the epidermis accompanied these changes in water loss, indicating that cutaneously applied triglycerides may be metabolized by the skin and incorporated into complex lipids. Other fatty acid triglycerides, including alpha-linolenic, dihomo-gamma-linolenic, arachidonic and omega-7-heneicosatrienoic acid, did not lower the rate of transepidermal water loss, although all were incorporated into epidermal structural lipids. The non-essential oleic acid also had no effect upon the rate of transepidermal water loss. These data suggest that of the two main essential fatty acids that occur in skin, linoleic acid and arachidonic acid, the former specifically plays an important role in regulating barrier function whereas the later may have a separate function, such as serving as a precursor of prostaglandins.

Epidermal barrier function in rats was experimentally impaired by two separate means, namely, by rendering the animals deficient in essential fatty acids and by evoking a primary cutaneous irritant response by treating with a solution of sodium laurate. Impaired barrier function was manifested by a greatly increased rate of transepidermal water loss. Application to the skin of sunflower seed oil, which is rich in linoleic acid, rapidly restored to normal the abnormally high rates of transepidermal water loss in both experimental cases, and it was shown with the essential fatty acid-deficient rats that there was a concomitant incorporation of linoleic acid of the sunflower seed oil into epidermal lipids. Cutaneous application of olive oil, which is low in linoleic acid but rich in the non-essential oleic acid, did not influence epidermal barrier function. A close relationship of barrier function and essential fatty acids is indicated.

Rat dorsal skin was fractionated by means of an electrokeratotonie into epidermis and dermis containing the sebaceous glands. These fractions were separately cultured in the presence of ^^P-orthophosphate, '^C-acetate, 'C-glycerol and '^C-mevalonate, and the radioactivity incorporated into different classes of Upids was measured. The nature of the labelled products was different in the epidermis compared with the sehaceous glands: in the former, phospholipids and sterols were the major radioactive lipids synthesized, whereas, in the sebaceous glands labelled triglycerides, sterol esters and wax esters predominated.The nature of individual fatty acids labelled from "C-acetate was studied and it was found that the sterol/wax esters fractions of both epidermis and sebaceous glands contained more radioactive odd-numbered normal chain and iso-and anteiso-branched chain fatty acids than the phospholipids and triglycerides. Isolated epidermal cells were prepared, and lipogenesis studies with radioactive precursors showed labelling patterns very similar to those of the cultured intact epidermis fractions. The presence of biochemically active sebaceous glands in the dermal residues, after excision of the epidermis fractions, was confirmed by treating skin in vivo with methyl testosterone, a hormone which stimulates the function of sebaceous glands, when a higher rate of lipogenesis was observed in the subsequently cultured dermal residue fractions, compared with untreated controls. No such differences were seen in the epidermis after testosterone treatment.MEASUREMENT of lipid synthesis in skin in vitro has been described fornumerous speeies, including human (Vroinan H aL, 1909), guinea-pig (Wheatley ci al., 1970 and mouse (Brooks et al., 1963;1966; Prottey and Ferguson. 1972), but this technique has not been extended to rat skin in such detail, despite considerable data on the cheniieal nature of rat skin lipids (Clayton et al., 1963;Horlick and Avigan. 1963: Nikkari and ilaahti, 1964; Nikkari, 1965). Wilson (1963) described tiie synthesis of sterols from '•*(.'-acetate by rat skin cultured for short times. Similarly, Ziboh and Hsia (1969) demonstrated aetive metabolism of both i*C-acetate and ^^C-glycerophosphate, although the precise identities of the lipid products were not described. Grigor et al. (1970) have demonstrated the synthesis of iso-and anteiso-branched fatty acids in vivo after injection of specific radioactive precursors.Recently, in a comprehensive study of lipogenesis in cultured human skin, Hsia et-al. (1970) have shown that various fractions of the whole tissue possessed difFerent capacities for lipid synthesis: in partic^dar, the sebaceous glands manufa(ttured sqnalene and triglycerides, whereas greater proportions of sterols and polar lipids were synthesized in the epidermis. On a weiglit basis, the epidermis was several times more active than the dermis for lipogenesis. Also, Summerly