Sebum, Cosmetics, And Skin Care

In situations of perfect balance between sebum production and requirements, the skin feels, looks, and smells good. It is comfortable, well moisturized, clean, and easy to decorate and not so easy to irritate. In situations of lipid deficit, the skin feels itchy, dry, and rough and looks lusterless, erythematous, and scaly. Makeup preparations spread unevenly, and irritation is common. In situations of lipid excess, the skin feels uncomfortable, greasy, shiny, and unclean and smells rancid, and color makeup smears. Under this condition, skin may be affected by acne, rosacea, or both. In short, excess oil is cosmetically and dermatologically undesirable. As a result of advances in the understanding of the physiology of sebaceous glands, epidermal lipid biochemistry and metabolism, and function of skin surface lipids, strategies to balance the needs of the skin to its optimal lipid requirements are being developed.

SEBACEOUS GLANDS AND THE PATHOGENESIS OF ACNE

Sebaceous glands are present over most of the body surface, occurring in highest concentration in the midline of the body, on the scalp, forehead, and central face. This pattern of medial oiliness and lateral dryness, known as the T of the face, is the basis of combination skin, a term used by marketers. Below the head sebaceous glands are more numerous on the lower back and chest, sparse toward the dorsa of the hands and feet, and normally absent on the palms and soles.

Sebaceous glands are holocrine, meaning that their secretion is the result of the total disintegration of its glandular cells. The periphery of each of their multiple lobes is made of undifferentiated sebaceous cells, resting on a basement membrane interconnected by desmosomes. Over a period of little more than 1 week, these cells differentiate fully, becoming lipid laden with many large sebum vacuoles, ready to be discharged through the lobule duct into a main sebaceous duct, lined by keratinizing squamous epithelium.'j The lobule duct, in turn, empties into a pilary canal also lined by keratinizing squamous epithelium continuous with that of the epidermis. Excessive sebum production and hyperkeratinization of that distal segment of the pilary canal, known as the infundibulum, leads to seborrhea and acne, which are exclusively human diseases.

The pathogenesis of acne involves an intraductal calculus, the sebolith, which results from the crystallization of sebum components. The sebolith punctures and erodes the follicular lining, leading to eventual rupture and inflammation. Excessive sebum production is one of the major factors contributing to the formation of primary acne lesions. Agents that suppress sebaceous gland activity either topically or endogenously also alleviate acne.

Previous concepts suggested that free fatty acids, found to be abundant in acne lesions, are a product of triglyceride metabolism induced by follicular bacteria; however, human sebaceous glands and sebocytes in culture have been shown to synthesize low amounts of free fatty acids. Interleukin-la (IL-a), a proinflammatory cytokine, has been detected in vivo, whereas abundant IL-a was found in comedonal extracts; this suggests that sebocytes initiate acne lesions by an intrinsic mechanism not necessarily involving bacteria.

The epidermal surface lipid film contains lipids produced by the sebaceous glands, lipids derived from epidermal keratinocytes, lipids secretions from eccrine glands, and some lipids accrued from the environment and cosmetics. Chemical analysis has shown that wax esters, squalene, triglycerides, and small proportions of cholesterol and its esters are produced by the sebaceous gland only. These sebaceous ducts are where bacterial lipases, particularly from Propionibacterium acnes, generate free fatty acids, monoglycerides, and diglycerides.'O The lipids extracted from the viable epidermis consist principally of cholesterol, phospholipids, and glucosylceramides, with traces of free fatty acids, triglycerides, and ceramides. The lipids extractable from the stratum comeum contain a mixture of ceramides; cholesterol; free fatty acids; and a low proportion of triglycerides, cholesterol sulfate, sphingosine, and phospholipid. Moisturizers attempt to mimic the effect of these lipids on the skin.

Surface lipids have been known to serve as a homeostatic and permeability barrier. The lipids found in the stratum corneum are well suited for this task because of their high melting points and polarity, which result in the formation of water-resistant lipid bilayer

Friction coefficients are affected by the presence of sebum, sweat, and hydrati~n.~ Documentation is accumulating to support a role for surface lipids and their generating cells in immune modulation. Sebocytes express tumor necrosis factor-a and certain neuropeptides (such as those of proopiomelanocortin) known to antagonize proinflammatory cytokines and induce immunosuppressive cytokines, modulate costimulatory molecule expression, and suppress macrophage-derived nitric oxide interaction with their target cells. Keratinocytes are able to express and release a plethora of potent cytokines and other mediators of inflammation and modulate the expression of immune-related cell surface receptors, adhesion molecules, and extracellular matrix factors that orchestrate responses to irritants and antigens. Ceramides, produced by keratinocytes, modulate the proliferation of natural killer and other T cells, induce adhesion molecule expression, induce early B cell activation, induce nonapoptotic killing of B lymphocytes, and inhibit antigen uptake and presentation by dendritic cells.

On the sebaceous gland-rich areas, such as the forehead, sebum constitutes more than 95% of the surface lipids during the neonatal period and after sexual maturity. The androgens produced at puberty develop and maintain the activity of the sebaceous glands. After puberty, the sebaceous glands enlarge, and their output of sebum production increases significantly, remaining high until the eighth decade in men and the fifth decade in women.

There is a variable degree of sensitivity to androgen stimulation between sebaceous glands. Within a given area of skin, for example, the forehead, there may be minor variations in the amount of sebum excreted from one gland to the next.13 Acne patients do not need increased circulating androgens; their problem is most likely the result of end-organ hyperresponse.

Dihydrotestosterone, testosterone, and androstenediol, in order of potency, stimulate sebaceous gland activity. In this gland, the presence of the type 1 isoenzyme of 5a-reductase is of paramount importance as the necessary catalyst and rate-limiting factor for androgenic action. A second enzyme, 17Phydroxysteroid dehydrogenase, is responsible for the interconversion of androstenedione and testosterone, and its activity may regulate the local concentrations of testosterone in tissues.I6 Androgens stimulate sebocyte receptors acting on specific stromal components, binding to nuclear receptors, and interacting with retinoic acid to regulate differentiation. In women, the sebaceous glands are stimulated by adrenal and ovarian androgens and possibly by progesterone.

Measuring true excretory rates of sebum production is not easy to do with precision. A delay of 8 days between sebum synthesis stimulation and its appearance on the skin has been recognized. The follicular reservoir may cause apparent differences in sebum output from a given gland between collections. Sebum output may increase in the summer and in oily-complexioned women the week before menstruation.

Sebutape (CuDerm Corp, Dallas, TX) can be used to measure sebum output. This is a white porous tape that traps oil and becomes translucent. It is applied to the skin and after a standardized period, usually 1 hour, removed and, within half an hour, placed on a black background. A pattern of dots of variable sizes and density develops. This pattern is matched to a grading scale to generate a numerical report. A photocopy of the resulting patterns can be made to preserve the results. More sophisticated methods of image analysis may add to the exactness of the technique." Other instruments measure the sebum content of given areas by using photocells to measure the transparency of a ground-glass slide applied to the skin for 30 seconds. Increases in transparency are in direct proportion to the sebum present. Sequential measurements may be used to estimate sebum production and the effectiveness of products intended to reduce oiline~

METHODS OF INCREASING OR DECREASING SEBUM ON THE SKIN

In cases of deficit, sebum production formulations may be applied that mimic its effect, whereas in cases of excess sebum production, formulations may be applied to remove it from the skin surface. To stimulate production, the dermatologist could prescribe medications with androgenic effects; however, pharmacologic doses of testosterone given over several weeks to men cause only a small rise in secretion rates. Retinoids in vitro manifest a biphasic effect, stimulating sebocyte proliferation at low concentrations, while inhibiting it at higher ones.

To replace missing moisturizing and barrier properties in cases of deficit sebum production, preparations with optimal molar ratios of skin surface lipids, which mimic synthetic molecules, such as pseudoceramides, are under development. These products should have a better profile than currently available preparations formulated with glycerin, petrolatum, lanolin, alpha-hydroxy acids, urea, and an array of other ingredients intended to please marketers more than patients.

To inhibit production of excessive amounts of sebum, the most powerful drug currently available for clinical use is systemic isotretinoin. At a dose of 0.3 to 0.5 mg/kg, isotretinoin reduces sebum excretion rates by 90% in 1 month.7 Other retinoids decrease sebum secretion to a lesser extent or not at all. The antiandrogens cyproterone acetate and chlormadinone in combinations with estrogens have been well established in the treatment of acne for European women; however, they are not available for prescription in the United States at this time. Spironolactone, a diuretic with peripheral action by competitive inhibition, antagonizes the stimulatory effects of testosterone and 5a-dihydrotestosterone. Other more specific 5a-reductase type 1 inhibitors still under development have shown down-regulating effects. Topically applied progesterone, spironolactone, corticosteroids, ketoconazole, hydroxychloride, retinol, salicylic acid, and a few other molecules may have a minor or questionable sebosuppressive effect. A product containing elubiol was shown to decrease sebum production by 11% to 15% in 6 weeks, with a negligible sideeffect profile.

To remove excess sebum from the skin surface, the market offers an array of lipid solvents and surfactants as well as absorbing lotions, papers, and powders. Lipid solvents and surfactants carry a minor but not negligible risk for irritancy. A gel containing an oilimbibing methacrylate copolymer was shown to decrease facial oiliness to a statistically significant level, while showing an excellent profile of safety and patient acceptance in comparison with a traditional perfluoroalkyl surfactant solution., This product is made of spherical microparticles that are able to absorb and bind sebum 20 times their volume. In contrast with other microspongebased systems, which release the oil imbibed when crushed, the acrylate copolymer binds with sebum so strongly that the microparticles do not release oil even when cut in half. A study using optical densitometry to quantify the amount of sebum adsorbed to a ground glass showed that the copolymercontaining gel reduces facial oiliness and shine for 8 hour.

These new copolymer products represent the future of oil control for those with excessive sebum in need of better cosmetics. Further research is required before it can be shown that oil control improves acne, however. Conversely, dry skin with too little sebum is better controlled by synthetic ceramides that mimic more accurately the effect of sebum on the skin. The incorporation of these new technologies into skin care products, coupled with an understanding of physiology of sebum production, promises overall better skin care. 

Fore L&L Skin home use skincare instruments, their MIO2 face lifting massage beauty device  provide 3 modes for daily skin care: 45 ℃ mild heat opens the pores. Supplemented by vibration massage, it promotes blood circulation. The micro-current lifts and helps the skincare products rapidly penetrate the muscle, increasing the skin’s elasticity and activating skin cells.

References:

L&L Skin MIO2 ： https://www.llskin.jp/products/mio2/