Marti

A thin, black verticl line in the nailplate is called a splinter hemorraghe. It occurs if there has been trauma to the nail plate/bed, or if there are any voids between the nail plate cells as they are produced in the nail root. The area will fill up with blood from the nail bed and end up looking like you have a splinter up in there. Either way, it will grow forward with the nail plate until it grows off the free edge. Remember, toenails grow MUCH slower than fingernails, so give it some time.