Final answer:
The basic parts of the nail unit include the nail bed, the nail matrix, the nail plate, and the cuticle (eponychium), with the cuticle being the missing part in the question. These components are responsible for nail growth, protection, and support.
Step-by-step explanation:
The nail unit consists of several basic parts which are the nail bed, the matrix bed (also known as the nail matrix), the nail plate, and what is missing in the description is the cuticle, also called the eponychium. The nail plate, or nail body, is the hard part of the nail that is most visible. The nail bed is the skin beneath the nail plate that is rich in blood vessels, giving it a pink appearance. The nail matrix is a deep layer of epidermal tissue at the proximal end of a nail, which is responsible for nail growth, and this is where the nail body forms. Surrounding the nail matrix is the lunula, a crescent-shaped layer of thick epithelium. At the sides of the nail body are the nail folds that help to anchor the nail, and the cuticle, which overlaps the nail bed at the base of the nail body. Finally, there is also the hyponychium, which is the area beneath the free edge of the nail, furthest from the cuticle.