Root sheath, epithelial

The hair follicle consists of four major components (1) internal root sheath (internal epithehal root sheath), (2) external root sheath (external epithelial root sheath), (3) dermal papilla, and (4) hair matrix. The cells covering the dermal papilla and composing most of the hair bulb are the hair matrix cells. These are comparable to stratum basale cells of regular epidermis except that they are more lipid deficient and produce harder keratin than their epidermal counterparts. 

The innermost layer, next to the hair root, is the internal epithelial root sheath, which is composed of three layers (1) internal root sheath cuticle, (2) middle granular epithelial layer (Huxley s layer), and (3) outer pale epithelial layer (Henle s... 

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). 

Autologous keratinocytes and fibroblasts have generally been derived from biopsies of the patient s uninjured skin [71 73]. Recently, keratinocytes have been isolated from the outer root sheath of plucked hair follicles [41,42]. The cells can be expanded in culture and used to populate skin substitutes in vitro. Grafted as epithelial sheets or as composites of biopolymers and cells, these are theoretically permanent skin substitutes once engraffment is achieved [25-27,33,41]. 

Anagen is followed by a brief involutional phase called catagen. Here the follicular cells cease division and the hair root shrivels to a small group of cells ascending toward the surface, pushing the inert hair shaft ahead of it. Another characteristic feature in catagen is the phenomenon of cell death by apoptosis. Apoptosis is observed in the outer sheath cells at the level of the bulb and of the epithelial column. 

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