Hairs are composed of keratinized cells that develop from hair follicles. Hairs are present over almost the entire body, being conspicuously absent only from the sides and palmar surfaces of the hands, from the sides and plantar surfaces of the feet, from the lips, and from the skin around the urogenital orifices. Coloration of the hair is due to the content and type of melanin that it contains. The follicle varies in appearance, depending on whether it is in a growing or a resting phase; the growing follicle is the more elaborate.
The skin appendages (adnexa), especially hair follicles and sweat glands, are particularly important in healing of skin wounds. They serve as the source of new epithelial cells when there is extensive loss of epidermis, as in deep abrasions and second-degree burns.
Figure 1, skin, human, H&E X300; inset x440.
The growing end of a hair follicle consists of an expanded bulb of epithelial cells that is invaginated by a papilla (HP) of connective tissue. The epithelial cells surrounding the papilla at the very tip of the follicle are not yet specialized; they constitute the matrix, the region of the hair follicle where cell division occurs. As the cells leave the matrix, they form cell layers that will become the shaft of the hair and the inner and outer root sheaths of the hair follicle.
The cells that will develop into the shaft of the hair are seen just to the right of the expanded bulb. They constitute the cortex (C), medulla (M), and cuticle (asterisks) of the hair. The cells of the cortex become keratinized. This layer will come to constitute most of the hair shaft as a thick cylinder. The medulla forms the centrally located axis of the hair shaft; it does not always extend through the entire length of the hair and is absent from some hairs. The cuticle consists of overlapping cells that ultimately lose their nuclei and become filled with keratin. The cuticle covers the hair shaft like a layer of overlapping shingles.
The root sheath (RS) has two parts: the outer root sheath, which is continuous with the epidermis of the skin, and the inner root sheath, which extends only as far as the level at which sebaceous glands enter the hair follicle. The inner root sheath is further divided into three layers: Henle's layer, Huxley's layer, and the cuticle of the inner root sheath. These layers are seen in the growing hair follicle and are shown at higher magnification in the inset with numbers 1 to 5: 1, cells of the outer root sheath; 2, Henle's layer; 3, Huxley's layer; 4, cuticle of the inner root sheath; and 5, future cuticle of the hair.
Many of the cells of the growing hair follicle contain pigment that contributes to the color of the hair. Most of this pigment is inside the cell (inset); however, in very dark hair some pigment is also extracellular.
The connective tissue surrounding the hair follicle forms a distinct layer referred to as the sheath, or dermal sheath (DS), of the hair follicle.
A nail is a keratinized plate located on the dorsal aspect of the distal phalanges. A section through a nail plate is shown here. The nail itself (N) is difficult to stain. Under the free edge of the nail is a boundary layer, the hyponych-ium (Hypon), which is continuous with the stratum corneum of the adjacent epidermis. The proximal end of the nail is overlapped by skin; here, the junctional region is called the eponychium (Epon) and is also continuous with the stratum corneum of the adjacent epidermis. Under the nail is a layer of epithelium, the posterior portion of which is referred to as the nail matrix (NM). The cells of the nail matrix function in the growth of the nail. Together, the ep ithelium under the nail and the underlying dermis (D) constitute the nail bed. The posterior portion of the nail, covered by the fold of the skin, is the root of the nail (NR).
The relationship of the nail to other structures in the fingertip is also shown in this figure. The bone (B) in the specimen represents a distal phalanx. Note that in this bone there is an epiphyseal growth plate (EP) at the proximal extremity of the bone but not at the distal extremity. Numerous Pacinian corpuscles (PC) are present in the connective tissue of the palmar side of the finger. Also seen to advantage in this section is the stratum lucidum (SL) in the epidermis of the thick skin of the fingertip.
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