Final answer:
Long bones lengthen through the addition of bone tissue at the epiphyseal plate, which stops growing in early adulthood. They also increase in diameter via appositional growth, wherein new bone tissue is added beneath the periosteum, and this growth can continue even after lengthening has ceased.
Step-by-step explanation:
How Long Bones Grow-
Growth in long bones occurs in two main ways: lengthening and increasing in diameter. Lengthening of long bones happens through a process at the epiphyseal plate, also known as the growth plate. This plate is located at the end of long bones. Here, new bone tissue is added as chondrocytes (cartilage cells) divide. One cell stays near the epiphysis (the end part of the long bone), while the other moves towards the diaphysis (the shaft of the bone), matures, and is eventually replaced by bone tissue, contributing to the lengthening of the bone.
In addition to lengthening, bones also grow in diameter through appositional growth. This process involves osteoclasts, which resorb old bone lining the medullary cavity, and osteoblasts, which form new bone tissue beneath the periosteum. As this new bone is added to the outer surface, the bone increases in thickness, a process known as modeling.
Growth in length typically stops in early adulthood when the epiphyseal plate closes and is replaced by an epiphyseal line, signifying the end of the bone's growth in length. However, bones can still increase in diameter in response to factors such as muscle activity.