In episode 32 when Renji is going through the story of how he and Rukia met and became Soul Reapers we notice that when he was a kid and when they first entered the Soul Reaper Academy his forehead was clear.

When Renji runs to tell Rukia that he passed his Second Exam only to see Rukia meeting with Byakuya Kuchiki (where she was offered to be adopted by him) we can see that Renji now has a pair lines on his forehead, but it is still less than what we see in the present day.

So i am wondering what is the meaning behind these tattoos?