The structure indicated is the epiphyseal line.

The epiphyseal plate is a plate of hyaline cartilage found in children and adolescents, located in the metaphysis at the ends of each long bone. Long bones consist of a diaphysis, metaphysis and epiphysis. The diaphysis is the main long section of the bone, the epiphysis is the rounded end of the long bone, and the metaphysis is the section of bone between the diaphysis and metaphysis.  In adults, the epiphyseal plate is replaced with the epiphyseal line and marks the point of union were the epiphysis meets the diaphysis.

The epiphyseal plates permit growth and lengthening of the bone, as the cartilage reproduces and ossifies. The epiphyseal plate has a zonal arrangement, with cartilaginous proliferation occurring closer to the epiphysis, and ossification occurring closer to the diaphysis. The zonal arrangement of the epiphyseal plate, from epiphysis to diaphysis, is as follows:

• Zone of reserve
• Zone of proliferation
• Zone of maturation and hypertrophy
• Zone of calcification
• Zone of ossification