The region above the line is calcified cartilage. It is easy to see that the the space below the line is occupied by another tissue: it is the bone tissue that has been deposited over the calcified cartilage septa.
Lower image
A higher magnification of this transitional area.
In the final region of the calcified cartilage zone, the chondrocytes have undergone cell death, and their nuclei and cytoplasm have disintegrated.
Many sites previously occupied by chondrocytes are now empty (highlighted in green).
In the lower part of the image: the walls of extracellular matrix that contained cartilage cells is covered by a thin layer of bone cells: osteoblasts, osteocytes (highlighted in pink). The newly produced bone matrix is highlighted in a darker tone of blue.
Observe at the center of the image a blood vessel that penetrated the cartilage. Cartilages normaly do not contain blood vessels.
Epiphyseal disc. Staining: HE. Magnification: small.
Epiphyseal disc. Staining: HE. Magnification: medium.