The proximal part of the nail, the nail root, is buried in a fold of epidermis and covers the cells of the germinative zone, or matrix. The matrix contains a variety of cells, including stem cells, epithelial cells, melanocytes, Merkel’s cells, and Langerhans’ cells. The stem cells of the matrix regularly divide, migrate toward the root of the nail, and there differentiate and produce the keratin of the nail. Nail keratin is a hard keratin, like that of the hair cortex. Unlike the soft keratin of the epidermis, it does not desquamate. It consists of densely packed keratin filaments embedded in a matrix of amorphous keratin with a high sulfur content, which is responsible for the hardness of the nail. The process of hard keratin formation, as with the hair cortex, does not involve keratohyalin granules. In addition, a cornified cell envelope contains proteins similar to those found in the epidermis.
FIGURE 15.18 • Photomicrograph of a sagittal section of distal phalanx with a nail. A nail is a keratinized plate located on the dorsal aspect of the distal phalanges. Under the free edge of the nail is a boundary layer, the hyponychium, which is continuous with the stratum corneum of the adjacent epidermis. The proximal end, the root of the nail, is overlapped by skin, the eponychium, which is also continuous with the stratum corneum of the adjacent epidermis. Deep to the nail is a layer of epithelium with underlying dermis. The proximal portion of this epithelium is referred to as the nail matrix. The bone in this section represents a distal phalanx. Numerous Pacinian corpuscles are present in the connective tissue of the palmar side of the finger. Note that even at this low magnification, the stratum lucidum is visible in the epidermis of the fingertip. 10.
The constant addition of new cells at the root and their keratinization account for nail growth. As the nail plate grows, it moves over the nail bed. On the microscopic level, the nail plate contains closely packed interdigitating corneocytes lacking nuclei and organelles.
The crescent-shaped white area near the root of the nail, the lunula, derives its color from the thick, opaque layer of partially keratinized matrix cells in this region. When the nail plate becomes fully keratinized, it is more transparent and takes on the coloring of the underlying vascular bed. The edge of the skin fold covering the root of the nail is the eponychium, or cuticle. The cuticle is also composed of hard keratin; therefore, it does not desquamate. Because of its thinness, it tends to break off or, as with many individuals, it is trimmed and pushed back. A thickened epidermal layer, the hyponychium, secures the free edge of the nail plate at the fingertip.
 FOLDER 15.6 Clinical Correlation: Skin Repair