There are two main reasons for the concavity in the center of the nipple: one is congenital, mostly due to abnormal development of the nipple during the embryonic period, which leads to the concavity in the center of the nipple. For acquired causes, most of them are due to poor hormone level secretion in the body and endocrine disruption, which leads to poor development of the mammary ducts near the nipple, or not getting sufficient development and appearing concave. Cracks on the nipple are mainly due to the lack of vitamins and water and other nutrients in the body, resulting in dry skin in the nipple area, and the nipple is dehydrated and thus cracks appear. If you like to wear rough clothes or too tight bra, it will also rub the skin of the nipple area for a long time, resulting in cracks on the nipple. In addition, if you do not pay attention to the hygiene of the nipple area, resulting in bacterial infections invading the nipple, it will also cause nipple depression and cracks.