Appendageal skin structures

In discussing skin structure, we limit ourselves to those features of the membrane which are pertinent to drag delivery in particular, to the stratum corneum (SC), the outermost layer wherein skin s barrier function principally resides. Macroscopically, skin comprises two main layers the epidermis and the dermis ( 0.1 and 1 mm in thickness, respectively) (see Figure 8.1). The dermal-epidermal junction is highly convoluted ensuring a maximal contact area. Other anatomical features of the skin of interest are the appendageal structures the hair follicles, nails and sweat glands. 

Appendageal structures commonly found within the skin are the hairs, hair follicles, associated sebaceous glands, apocrine and eccrine sweat glands, and arrector pili muscles. Hairs are formed by epidermal invaginations. These keratinized structures traverse the dermis and may extend into the hypodermis. The free part of the hair above the surface of the skin is the hair shaft, and the part deep within the dermis is the hair root, which forms an expanded knob-like structure called the hair bulb. This is composed of a matrix of epithelial cells in different stages of differentiation. Hair is composed of three concentric epithelial cell layers the outermost thin cuticle, a densely packed keratinized cortex, and a central medulla of cuboidal cells. The hair follicle consists of four major components (1) internal root sheath (internal root sheath cuticle, granular layer, pale epithelial layer) (2) external root sheath (several layers similar to the epidermis) (3) dermal papilla (connective tissue) and (4) hair matrix (comparable to the stratum basale of the epidermis). 

In general, rat skin has been proven more permeable than human skin (ECETOC, 1993). This is likely to be caused by the different structure of the skin in both species (e.g. number of appendages, intercellular lipid composition of the stratum corneum and corneocyte surface area). It should be noted, however, that some cases have been reported where rat skin was found to be less permeable than human skin (Hotchkiss et al., 1992). Since the availability of human skin is limited, pig skin is often used. Because of its similar morphology and barrier function (Bronaugh et al., 1982), pig skin is often considered a good alternative to human skin. 

Phenol is applied up to the lower limits set before the start of the peel this forms the demarcation line (Figure 34.14). The skin on the neck is structurally different to that on the face it has fewer appendages and pilosebaceous imits. After a deep peel, the skin regenerates from these appendages. A phenol peel on the neck carries an increased risk of scarring. What is more, the usual post-peel facial edema tends to extend downwards to the neck, and if the neck is treated... 

The complexity of skin may be better understood by its structure, the various strata epidermis is waterproof and acts as a barrier to infection dermis has the appendages of the skin hypodermis has the subcutaneous adipose tissue. Epidermis comprises the physical, the chemical/ biochemical [antimicrobial, innate immunity], and the acquired immunological barriers. The main cells in epidermis are keratinocytes, melanocytes, and Langerhans and Merkel cells. The dermis consists of connective tissue, and it is connected to the epidermis by a basement membrane. It contains nerve endings, hair follicles, sweat glands, sebaceous glands, apocrine glands. 

These include nearly all the structural proteins which have molecules with a high axial (length/ width) ratio. Most of them are very insoluble and metabolically unreactive. They include the various keratins found in the skin and its appendages, the fibrin of blood clots, and collagen and elastin the protein constituents of connective tissues. Myosin the fibrous protein of muscle contains both fibrous and globular regions and is both metabolically reactive and relatively soluble. 

The appendages of the skin include hair follicles, sebaceous glands, eccrine and apocrine sweat glands, and the nails. All of these structures, except the nails, originate in the dermis. Because a detailed discussion of the skin appendages is beyond the scope of this chapter, we refer the readers to several reviews [1,2]. 

Four important layers comprise normal skin. The epidermis, outside, is a 0.1-mm-thick sheet, comprising about 10 layers of keratinocytes at levels of maturation which increase from the inside out. The dermis, inside, is a 2-5-mm-thick layer of vascularized and innervated connective tissue with very few cells, mostly quiescent fibroblasts. Interleaved between the epidermis and the dermis is the basement membrane, an approximately 20-nm-thick multilayered membrane (Figure 82.1). A fourth layer, the subcutis, underneath the dermis and 0.4-mm in thickness, comprises primarily fat tissue. In addition to these basic structural elements, skin contains several appendages (adnexa) including hair follicles, sweat glands, and sebaceous glands. The latter are mostly embedded in the dermis, although they are derived from epidermal elements. 

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