Structure of the Nail Bed
Histologically, the structure underlying the nail plate is called the nail bed. It is clearly divided into two key zones: deep and superficial.
The deep layer, or dermal nail bed, is formed by dense connective tissue. This zone features a rich vascular network and a high concentration of collagen fibers. The surface relief of the dermal bed is characterized by prominent longitudinal folds and tall dermal papillae that project upward into the overlying layers.
The superficial layer is termed the subungual plate or hyponychium. Histologically, it is epidermis, but it consists exclusively of viable layers: the stratum basale and stratum spinosum. The most critical functional zone of the hyponychium is its posterior region—the nail matrix (Matrix unguis). This is the primary site driving nail growth, where cells divide rapidly and undergo keratinization.
Keratinization in the matrix follows an "abbreviated" pathway. This means that the stratum granulosum and stratum lucidum are entirely absent; cells transition directly into cornified squames containing hard keratin. Cells in other regions of the hyponychium are also capable of keratinization, but this process is significantly slower and less pronounced.
Nail Plate
From a histogenetic standpoint, the nail plate represents a massive stratum corneum of the subungual plate. It is formed primarily by the continuous keratinization of nail matrix cells and partially by the rest of the hyponychium. Structurally, the plate consists of densely packed cornified squames filled with hard keratin. The direction of growth is strictly distal.
Anatomically, the nail plate is divided into three parts:
- Root (Radix): The proximal (posterior) part, which is entirely concealed beneath the posterior nail fold. The root includes the lunula, a whitish, crescent-shaped area visible from beneath the skin fold.
- Body (Corpus): The main, most extensive part of the plate that rests openly on the nail bed.
- Free margin (Margo): The distal portion that projects freely beyond the nail bed.
Nail Folds and Eponychium
Posteriorly and laterally, the nail is bounded by skin folds known as nail folds. In this region, the epidermal layers (germinative and cornified layers) diverge to form a specialized "pocket" that anchors the root and lateral edges of the nail.
The topography of the viable (germinative) epidermal layers, comprising the stratum basale and stratum spinosum, is continuous. From the outer surface of the nail folds, they reflect onto the inner surface (which is firmly adherent to the dorsal surface of the nail). These layers then curve around the nail root and transition smoothly into the subungual plate of the bed.
The behavior of the cornified layer of the nail folds results in the formation of the eponychium (cuticle). At the edge of the nail folds, the superficial stratum corneum separates from the underlying cells and extends over the dorsal surface of the nail plate as a narrow, transparent band. A key histochemical feature of the eponychium is that its cornified squames are composed of soft keratin, distinguishing it from the nail plate itself.