Gartner_BRS Cell Biology & Histology, 9e

Chapter 12 Skin

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VI. NAILS (Fig. 12.8)

Nails ( nail plates ) are hard keratinized plates on the dorsal surface of the distal phalanx of each digit. The invagination of the epidermis forms the nail matrix where the nail is rooted and the nail bed , which underlies and supports the nail plate . Nails grow as the result of mitoses of cells in the nail matrix that add onto the nail root. The proximal end of the nail plate is covered by a fold of epidermis called the cuticle ( eponychium ), which corresponds to the stratum corneum. The cuticle overlies the crescent-shaped whitish lunula , corresponding to the nail matrix. The distal (free) edge of the nail plate is underlain by the hyponychium , which is also composed of stratum corneum. PEARLS Warts ( verrucae ) are common benign skin and mucosal proliferative lesions caused by the human papillomavirus (HPV) . Histologic features of warts include marked epidermal hyperplasia, eosinophilic cytoplasmic inclusions, and deeply basophilic nuclei. By electron microscopy, many intranuclear viral particles can be observed in the keratinocytes. Androgenic alopecia ( pattern alopecia , male-pattern baldness ) is a common genetic condition affecting all sexes. The incidence is thought to be much greater in individuals with a 46,XY karyotype; however, some studies suggest that the difference in the incidence is due to different ways in which this condition is expressed in individuals with a 46,XX karyotype. In typical males, androgenic alopecia is char acterized by loss of hair at both sides of the temple and at the crown of the head, with an age of onset varying between the early 30s and 50s. Up to 50% of the individuals with the 46,XX karyotype older than age 40 years are thought to be affected by this condition, which is characterized by significant thinning of hair without developing truly bald areas. Clinical

FIGURE 12.8. A micrograph of a fingernail on the dorsal surface of a distal phalanx (Ph). The highly keratinized nail plate (NP) extends deep into the dermis (D) to form the nail root (NR). The epidermis of the distal phalanx forms a continuous fold, resulting in the eponychium (Ep), or cuticle, the nail bed (NB) underlying the NP, and the hyponychium (Hy). The epithelium (arrow) surrounding the NR forms the nail ma trix and is responsible for the continuous elon gation of the nail. The dermis between the NB and the bone (Bo) of the distal phalanx is tightly secured to the fibrous periosteum (FP). The presence of hyaline cartilage (HC) and endo chondral osteogenesis (arrowheads) indicates that this is a developing finger.

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