The structure of the fingernail is similar to the structure of the bones, 3 coats fix it:

 

The superficial coat is formed by cells that have lost its vitality and they accumulate fibrous material like keratins.

 

The intermediate coat is thicker, and it has the same origin than the superficial one but with alive close-knit cells among them and a less density of fibers  of keratin.

 

The deep coat, finally, are only two coats of cells that come from the bed of the skin.