Final answer:
In juveniles, the epiphysis of long bones is filled with spongy bone and contains a layer of hyaline cartilage called the epiphyseal plate, allowing for further growth. In mature adults, the cartilage in the epiphyseal plate is replaced by bone, and the epiphysis fuses with the diaphysis, marking the end of bone growth.
Step-by-step explanation:
Long bones in juveniles and mature adults have different characteristics in their epiphysis. In juveniles, the epiphysis is filled with spongy bone and contains a layer of hyaline cartilage called the epiphyseal plate, which allows for further growth in length.
However, in mature adults, the cartilage in the epiphyseal plate is replaced by bone, and the epiphyseal plate becomes an epiphyseal line. The epiphysis and diaphysis fuse, marking the end of bone growth.
Therefore, the epiphysis of long bones in juveniles is still actively growing and contains cartilage, while in mature adults, it has stopped growing and is replaced by bone.