Nails

Structure of the nail bed

Histologically, the structure upon which the horny plate rests is called the nail bed. It is clearly subdivided into two key zones: deep and superficial.

The deep part, or dermal nail bed, is formed by dense connective tissue. This zone is characterized by an abundant blood supply and a high content of collagen fibers. A prominent feature of the dermal bed relief is the presence of pronounced longitudinal folds and high papillae that project into the overlying layers.

The superficial part is called the nail bed plate, or hyponychium. By its tissue nature, this is the epidermis, but it is represented only by viable layers: the stratum basale and stratum spinosum. The most important zone of the hyponychium is its posterior part — the nail matrix (Matrix unguis). This is where the main processes ensuring growth take place: cells actively divide and undergo keratinization.

The keratinization process in the matrix follows the so-called "shortened" pathway. This means that the stratum granulosum and stratum lucidum are completely absent here—cells transform directly into cornified scales with the formation of hard keratin. Cells in other areas of the hyponychium are also capable of keratinization, but this process proceeds much slower and is less pronounced.

Nail plate

From a histogenetic standpoint, the nail plate represents a massive horny layer of the hyponychium. It is formed primarily as a result of the continuous keratinization of nail matrix cells and partially by the rest of the hyponychium. Structurally, the plate consists of tightly adhered cornified scales filled with hard keratin. The direction of growth is always distal.

Anatomically, the nail plate is divided into three parts:

• Root (Radix): the proximal (posterior) part, which is completely hidden under the posterior nail fold. The root includes the lunula (Lunula) — a whitish crescent-shaped zone visible from beneath the skin fold.
• Body (Corpus): the main, most extensive part of the plate, which lies openly on the nail bed.
• Edge (Margo): the distal part, projecting freely beyond the nail bed.

Nail folds and eponychium

Posteriorly and laterally, the nail is bounded by skin folds — nail folds. In this region, a separation of the epidermal layers (germinative and cornified) is observed, forming a specific "pocket" for the immersion of the nail root and lateral edges.

The topography of the living (germinative) epidermal layers, which include the stratum basale and stratum spinosum, is continuous. From the outer surface of the skin folds, they transition to their inner surface (which is tightly fused with the dorsal surface of the nail). These layers then curve around the nail root and seamlessly continue into the hyponychium of the bed.

The behavior of the stratum corneum of the folds leads to the formation of the eponychium (cuticle). At the edge of the nail folds, the superficial horny layer separates from the underlying cells and "creeps" onto the dorsal surface of the nail plate as a narrow transparent strip. The most important histochemical feature of the eponychium is that its cornified scales are formed by soft keratin, distinguishing it from the nail plate itself.

Class and exam questions

What else to study on this topic

• Histochemical differences between hard and soft keratin
• Mechanisms of formation of longitudinal folds in the dermal bed
• Features of vascularization in the high papillae of the dermal nail bed
• Regulation of cell division in the nail matrix

Other topics in "Skin and its derivatives"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.