116 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. resulting in an increase in bone length. At the end of the growth period, occurring during or shortly after adolescence, the plate dissolves and the bone on either side of the plate fuses, effectively ending the longitudinal growth of the bone. Circumferential Growth Although most bone growth occurs during childhood, bones actually grow in diameter, or width, throughout the life span (Figure 5.9). Osteoblasts in the internal layer of the periosteum build concentric layers of new bone on top of existing ones. To understand the process, you might visualize the way in which the rings on a cross-cut tree stump reveal the tree’s growth. At the same time that the osteoblasts are doing their work, the osteoclasts resorb layers of bone inside the medullary cavity, causing the diameter of the cavity to be progressively enlarged. This beautifully engineered process, known as appositional growth, occurs in such a way that a healthy bone remains optimally functional, lightweight, and strong enough to resist daily stresses. Adult Bone Development While osteoblast and osteoclast activity tends to maintain bones at functional sizes and shapes through- out life, age-related changes in the proportion of mineralized to nonmineralized bone do occur. As people age, there is a progressive loss of collagen (which provides elasticity) and an increase in bone brittleness. This means that children are often able to sustain falls and other accidents without harm, while older adults tend to be increasingly vulnerable to bone fractures. Bone growth involves more osteoblast activity than osteoclast activity. However, both osteoblasts and osteoclasts remain extremely busy over the course of a normal person’s life. In healthy adult bone, the activity of osteoblasts and osteoclasts is balanced. As a result of this balance, the bones stay strong and optimally designed for their various functions. Bone Formation Bone modeling is the process in which new bone is created through osteoblast activity. The skeleton of early-developing embryos is composed mainly of a flexible tissue called hyaline cartilage. Although cartilage forms parts of the adult nose, ribs, and some joints, cartilage is rapidly replaced with bone even within the developing fetus. The process of bone formation is called ossification. Before birth, ossification occurs in two phases. During the first phase, a bone matrix shell covers the hyaline cartilage through the activity of osteo- blasts. Next, osteoclasts resorb the enclosed hyaline cartilage, creating a medullary cavity within the bony superstructure. Longitudinal Growth Bones grow in length at the epiphyseal plates, which are located close to the ends of long bones (Figure 5.8). During childhood growth, osteoblasts on the central side of the epiphyseal plate produce new bone cells, Epiphyseal plate Jose Luis Calvo/Shutt erstock.com Figure 5.8 Bones grow in length at the epiphyseal plates (the dark pink area). Medullary cavity Osteoblast activity builds new bone Osteoclast activity resorbs bone – – – – – – – – Periosteum Cortical bone Endosteum + + + + + + + + + + + + + + + + + + © Body Scientifi c International Figure 5.9 Cross section of a long bone showing normal bone throughout life.