One source says...
"...the germinative (growing) zone of the nail bed consists functionally of two parts. The part beneath the root of the nail and the lunule, called the GERMINAL MATRIX, is thicker and actively proliferative (dividing), and is concerned with the growth of the nail, the epidermal cells being gradually converted into the nail substance. On the other hand, the part beneath the rest of the nail, called the STERILE MATRIX, is thinner and is not concerned with nail growth but provides a surface over which the growing nail glides."