Skin appendages follicles

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

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... 

Deficiencies of current commercially available in vitro RhE models include the lack of fatty layer, lack of skin appendages such as hair follicles and sweat glands, and lack of immune cells, innervation, or blood perfusion. However,... 

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]. 

Arteries entering the skin arise from more substantial vessels located in the subcutaneous connective tissue. These offshoots form a plexus just beneath the dermis [11]. Branches from this subcutaneous network directly supply blood to the hair follicles, the glandular appendages, and the subcutaneous fat. Branches to the upper skin from this deep plexus divide again within... 

The appendages of skin are hair follicles, sebaceous glands, eccrine and apocrine sweat glands, hair, nails, and arrector pili muscles. 

The face is the most common site for cosmetic reactions, particularly in the eyelid area.4 5 9 Facial skin is highly permeable, due to a thinner stratum corneum and a greater density of appendages (e.g., sweat glands, hair follicles). Moreover, facial skin contains an elaborate network of sensory nerves. The frequency of cosmetic application is also increased at this body site. Although mild inflammatory changes are often masked on the face, in the event that eruptions do occur, they are readily noticed by the consumer. 

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). 

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