CHAPTER 2

FUNCTIONS OF THE SKIN

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The skin has very important vital functions for keeping the physiological and biochemical conditions of the body in its optimum state. The most important functions of the skin are: 

  1. Regulates body temperature.

  2. Prevents loss of essential body fluids, and penetration of toxic substances.

  3. Protection of the body from harmful effects of the sun and radiation.

  4. Excretes toxic substances with sweat.

  5. Mechanical support.

  6. Immunological function mediated by Langerhans cells.

  7. Sensory organ for touch, heat, cold, socio-sexual and emotional sensations.

  8. Vitamin D synthesis from its precursors under the effect of sunlight and introversion of steroids.

The epidermis is the outer most layer of skin that acts as a barrier preventing toxic chemical and other materials from penetrating deeper into the skin. This is relatively porous and undergoes changes in thickness in response to different factors such as trauma or pressure.

The layers of the epidermis differentiate and gradually develop to a more rigid structure, which provides a barrier to excessive loss of body fluids and the penetration of noxious substances. The basal layer is the precursor of the different cells of the epidermis which divide, pushed further upwards, loosing much of their metabolic function and enzymatic activity. The spinous layer is characterized by growth of keratin fibrils where these are present also in the cells of the basal layer.

Epidermal cells as they are pushed up away from the basal layer, begin to dehydrate and become filled with cross-linked keratin, which gives the cells a granular appearance. Lamellar bodies containing structured lipids play an important role in skin protection. The intercellular lipids, the corneocytes, amino acids, and other salts from sweat, sebaceous secretions, degradation products from corneal proteins besides lipids and others all have an important barrier effect preventing loss of water and keep the skin pH in its optimum condition (5.5).

The stratum corneum provides most of the barrier function.

The skin acts as a two-way barrier to prevent the inward or outward passage of water and electrolytes. The epidermis largely represents the barrier; whereas once the epidermis is removed the residual dermis is almost completely permeable.

There are two possible routes for the passage of drugs through the epidermis, through the transcellular, which is probably the major pathway for polar substances, and through the intercellular.

FACTORS AFFECTING SKIN PENETRATION

The penetration of substances through the skin surface depends upon different factors:


Fig. 8. Stratum corneum with 
the intercellular lipid layers

  1. Age - penetration is more in newborn and children than in adults.

  2. Skin condition - penetration is more on injured or abraded skin surfaces. Chemicals may cause injury and increase penetration.

  3. Hydration of the skin - penetration is more in hydrated skin than dry skin. Hydration increases the permeability of the stratum corneum. Water is an effective penetration enhancer.

  4. Fat content of the epidermis has no much effect on penetration.

  5. Type of vehicles: vehicles may increase penetration and absorption of the drug from the skin surface. This depends on the type of vehicle and the condition of the skin. Certain vehicles that may cause injury to the skin even minimal injury predispose to more penetration of the drugs or other materials applied topically to the skin surface.

  6. Hyperemia - vasodilatation of the blood vessels in response to different stimuli either local or generalized increases the penetration.

  7. Physiological and pharmacological factors

    The penetration in vivo of topically applied substances can be assessed by physiological or pharmacological signs or analyzed by chemical or histological techniques:

  • Vasoconstriction has been utilized for corticosteroids.

  • Vasodilatation for nicotinates.

  • Whealing for histamines.

  • Sweating for pilocarpine.

  • Anesthesia for local anesthetics.

  1. Lipoid soluble substances facilitate penetration of substances applied to the skin surface. Steroid hormones and vitamin D, salts such as chloride and sulfate can penetrate the skin surface. Gases and volatile substances can pass through the skin.

REFERENCES

  1. Abraham W, Downing DT. Preparation of model membranes for skin permeability studies using stratum corneum lipids. J Invest Dermatol 1989; 93: 809-13.

  2. Breathnach AS. Embryology of human skin. A review of ultrastructural studies. The Herman Beerman Lecture. J Invest Dermatol 1971; 57: 133-43.

  3. Breathnach AS. An Atlas of the Ultrastructure of Human Skin. London: J. & A. Churchill, 1971.

  4. Biochemistry and Physiology of the Skin Vol 2. New York and Oxford: Oxford University Press, 1983: 1255-95.

  5. Blank IH. Cutaneous barriers. J Invest Dermatol 1965; 45: 249-56.

  6. Elias PM. Epidermal lipids, membranes, and keratinization. Int J Dermatol 1981; 20: 1-19.

  7. Deutsch TA, Esterly NB. Elastic fibers in fetal dermis. J Invest Dermatol 1975; 65: 320-3.

  8. Farmer ER, Hood AF, eds. Pathology of the Skin. London: Prentice Hall International, 1990.

  9. Goldsmith LA, ed. Biochemistry and Physiology of the Skin 2nd edn. New York: Oxford University Press, 1991.

  10. Holbrook KA, Odland GF. Regional development of the epidermis in the first trimester embryo and the second trimester fetus (ages related to the timing of amniocentesis and fetal biopsy). J Invest Dermatol 1980; 74: 161-8.

  11. Holbrook KA, Hoff MS. Structure of the developing human embryo and fetal skin. Semin Dermatol 1984; 3: 185-202.

  12. Hashimoto K, Gross BG, Lever WF. The ultrastructure of the skin of human embryos. I. The intraepidermal eccrine sweat duct. J Invest Dermatol 1965; 45: 139-51.

  13. Lever WF, Schaumburg-Lever G. Histopathology of the Skin 7th edn.

  14. Briggaman RA, Wheeler CE. Epidermal-dermal interactions in adult human skin. II. The nature of the dermal influence. J Invest Dermatol 1971; 56: 18-26.

  15. Montagna W, Yun JS. The skin of primates. XVI The skin of Lemur mongoz. Amer J Phys Anthrop 1963; 21: 371-81. Philadelphia: Lippincott, 1990.

  16. McKee PH. Pathology of the Skin. Philadelphia: Lippincott, 1989.

  17. Scheuplein RJ, Bronaugh RL. Percutaneous absorption. In: Goldsmith LA, ed.

  18. Smith JG, Jr, Fischer RW, Blank H. The epidermal barrier: a comparison between scrotal and abdominal skin. J Invest Dermatol 1961; 36: 337-41.

  19. Scheuplein RJ, Bronaugh RI. Percutaneous absorption. In: Goldsmith LA, ed. Biochemistry and Physiology of the Skin Vol II. New York and Oxford: Oxford University Press, 1983: 1255-95.

  20. Wertz PW. Lipids of keratinizing tissues. In: Bereiter-Hahn J, Matoltsy AG, Richards KS, eds. Biology of the Integument. Vol 2: Vertebrates. Berlin: Springer-Verlag, 1986; 815-23.

  21. Yardley HJ. Epidermal lipids. In: Goldsmith LA, ed. Biochemistry and Physiology of the Skin. New York: Oxford University Press, 1983: 363-81.

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