Final answer:
A large region between the primary and secondary ossification centers signifies a young individual because this indicates ongoing bone growth. The epiphyseal plates are areas of cartilage that allow bones to lengthen until skeletal maturity, and their presence indicates an individual's bones are still growing. As these plates ossify and disappear, it shows growth has ceased, which typically occurs after puberty.
Step-by-step explanation:
If there is a large region between the primary and secondary ossification centers in a bone, it is indicative that the person is young. During bone development, ossification starts at the primary ossification center within the diaphysis of a long bone. This forms the shaft of the bone. The secondary ossification centers appear in the epiphyses and contribute to bone lengthening until skeletal maturity is reached. The presence of a large region of cartilage, known as the epiphyseal plate or growth plate, signifies ongoing bone growth and that the individual has not yet reached skeletal maturity. Conversely, as a person ages and reaches skeletal maturity, these areas ossify and the epiphyseal plates ultimately disappear, indicating that growth in bone length has ceased.
The process of endochondral ossification begins with the formation of a cartilage "model" which continues to grow as ossification occurs. In long bones such as the femur, the primary ossification center appears during fetal development, rapidly expanding to ossify the shaft of the bone before birth. Secondary ossification centers develop postnatally at various times in different locations, including the epiphyses, and contribute to both bone growth and shaping.
Eventually, after puberty, these secondary ossification centers undergo fusion with the diaphysis, and the epiphyseal plates close. The timing of when these epiphyseal plates disappear is crucial for determining skeletal age, particularly in the clinical interpretation of pediatric radiographs where the plates appear darker due to their lower density compared to ossified bone. Therefore, a prominent epiphyseal plate can be used as an indicator of youth in skeletal analysis.