Skin appendages

Skin appendages can be distinguished into hair follicles with their associated sebaceous glands, eccrine sweat glands, apocrine sweat glands, and nails. 

Eccrine glands can be found on the entire body surface of humans, except for the lips, external ear canal, clitoris, and labia minora. These glands play an important role in thermoregulation, which is necessary for fluid and electrolyte homeostasis. They secrete a milky or oily odorless liquid which produces the characteristic body smell after metabolism through surface bacteria of the skin. 

Apocrine glands produce a viscous secretion that contains compounds related to communication between individuals of a species, by acting as a sex attractant or as territorial marker. In humans, these glands are located only in the axillary, pubic, and perianal region. 

The nails are composed of flattened, keratinized cells, fused into a dense and hard, yet slightly elastic plate. Their thickness varies from 0.5 to 1.0 mm. In contrast to the stratum corneum (10%), the total lipid content of the nails lies between 0.1% and 1%, and the keratin domain is harder, due to higher sulfur content (cystine). Moreover, the water content is only 7% to 12%, in comparison to 25% in the stratum corneum. The relative water gain may not exceed 25% at 100% relative humidity, in sharp contrast to 200-300% as found in the stratum corneum. 

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Skin absorption pathways can be divided into the transport (a) across the intact stratum corneum and (b) along using skin appendages. The physicochemical properties of the compound, as well as the used formulation, are the main factors influencing the choice of pathway. 

The transcellular pathway has been discredited as a major pathway, although some polar substances can penetrate the outer surface of the protein filaments of hydrated stratum comeum. The transfollicularpathway is really an invagination of the epidermis into the dermis, and the chemical still has to penetrate the epidermis to be absorbed into the blood stream. This is also a regarded as minor route. Sweat pores are not lined with the stratum comeum layer, but the holes are small, and this route is still considered a minor route for chemical absorption. In general, the epidermal surface is 100 to 1000 times the surface area of skin appendages, and it is likely that only very small and/or polar molecules penetrate the skin via these appendages. 

Wang, C., Tammi, M., and Tammi, R., Distribution of hyaluronan and its CD44 receptor in the epithelia of human skin appendages, Histochem., 98, 105, 1992. 

Conte, L., Ramis, J., Mis, R. et al. Percutaneous absorption and skin distribution of [14C]flutrimzaole in mini-pigs. Arzneimittelforschung 42(6) 847-853, 1992. Kao, J., Hall, J. and Helman, G. In vitro percutaneous absorption in mouse skin Influence of skin appendages. Tox. App. Pharm. 94 93-103, 1988. 

Hair follicles and their associated sebaceous glands (pilosebaceous glands), eccrine glands, apocrine glands, and finger and toenails are all considered skin appendages. Hair follicles are found everywhere within the skin except for the soles of the feet, the palms of the hand, the red portion (vermilion border) of the lips, and the external genitalia. All are formed from fetal epidermal cells. Hair differs markedly in its prominence from place to place over the body. Delicate primary hair is found on the fetus secondary hair or down covers the adult forehead terminal hair ordinarily blankets the scalp and is found as pubic... 

Integumentary system—Skin, subcutaneous tissue, and skin appendages. 

Hashimoto T, Inamato N, Nakamura K, et al. Involucrin expression in skin appendage tumors. Br J Dermatol. 1987 117 325-332. 

Tsuhura A, Senzaki H, Sasaki M, et al. Immunohistochemical demonstrated of hreast-derived and/or carci-noma-associated glycoproteins in normal skin appendages and their tumors. / Cutan Pathol. 1992 19 73-79. 

Shikata N, Kurokawa I, Andachi H, et al. Expression of androgen receptors in skin appendage tumors an immunohistochemical study. / Cutan Pathol. 1995 22 149-153. 

Lloveras B, Googe PB, Goldberg DE, et al. Estrogen receptors in skin appendage tumors and extramammary Paget s disease. Mod Pathol. 1991 4 487-490. 

Seelentag WK, Gunthert U, Saremaslani P, et al. CD44 standard and variant isoform expression in normal human skin appendages and epidermis. Histochem Cell Biol. 1996 106 283-289. 

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