Final answer:
The epiphyseal plate allows the diaphysis of the bone to increase in length during childhood and contributes to shaping the articular surfaces. It is replaced by osseous tissue as growth ceases, leading to the formation of the epiphyseal line in early adulthood.
Step-by-step explanation:
The structure that allows the diaphysis of the bone to increase in length until early childhood, as well as shaping the articular surfaces, is the epiphyseal plate. This plate is a layer of hyaline cartilage located at the ends of long bones, functioning as a growth area where bones can lengthen. The process involves chondrocytes dividing on the epiphyseal side, with one cell remaining undifferentiated and the other moving toward the diaphysis. As these cells are pushed away from the epiphysis, they mature, calcify, and are in time replaced by bone on the diaphyseal side, which results in the lengthening of the bone. Once a person reaches early adulthood (approximately 18-21 years), these plates ossify and form what is known as the epiphyseal line, marking the end of the bone's lengthening.
In addition to the role of the epiphyseal plate in lengthening, bones also grow in diameter through a process called appositional growth. This involves the addition of new bone tissue beneath the periosteum, as well as the resorption of old bone lining the medullary cavity, both of which contribute to the increasing diameter of the bone.