Hyponychium

The hyponychium is the anatomical region of the nail unit located distal to the nail bed and beneath the free edge of the nail plate. It is composed of epidermal tissue and serves as a natural barrier at the point where the nail plate separates from the nail bed, protecting the nail bed from external pathogens and irritants.

Histologically, the hyponychium is characterized by a thickened stratum corneum and exhibits epidermal-type keratinization, distinct from the specialized keratinization of the nail bed and nail isthmus.

Functionally, the hyponychium marks the transition from the nail bed to the surrounding fingertip skin and is involved in sealing the distal nail unit. It is supported by a vascular and innervated dermis, and its structure can vary in thickness and appearance among individuals.

The hyponychium is clinically relevant as it is a site prone to disorders such as subungual hyperkeratosis and pterygium inversum unguis.