Final answer:
Indented vertical lines down the nail plate are formed by the nail matrix and lateral nail fold.
Step-by-step explanation:
The indented vertical lines down the nail plate are called the nail matrix or nail grooves. These structures are a part of the nail anatomy and are located at the proximal end of the nail where nail growth occurs. The nail matrix surrounds the nail root and contains stem cells that divide to form keratinocytes, which make up the nail plate. The nail body is formed on the nail bed, which is a specialized structure of the epidermis found at the tips of our fingers and toes. The indented lines are formed by the lateral nail fold, which overlaps the sides of the nail, helping to anchor the nail body.