Chapter 4 The Skeletal System 127 Over the course of the life span from birth through old age, some parts of the human body grow, while others tend to shrink. Knowledge of the processes by which bones grow and develop is key to understanding why these phenomena occur. Osteoblasts and Osteoclasts Specialized bone cells called osteoblasts (AHS-tee- oh-blasts) carry out the work of building new bone tissue. When there is a need to resorb or eliminate weakened or damaged bone tissue, that work is accomplished by other specialized cells called osteoclasts (AHS-tee-oh-klasts). Bone growth clearly 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. MEMORY TIP Osteoblast and osteoclast are similar-sounding names for these specialized bone cells. An easy way to avoid confusing them is to remember the “b” in osteoblast is the same as the “b” in build. Similarly, the “cl” in osteoclast is the same as the “cl” in clear. Osteoblasts build bone and osteoclasts clear away old or damaged bone. Bone Formation Bone modeling is the process in which new bone is created through osteoblast activity during the formation and growth of immature bones. The skeleton of early- developing embryos is composed mainly of a flexible tissue called hyaline cartilage. By the end of the eighth week there is a rapid replacement of cartilage with bone in the developing fetus. The process of bone formation is called ossification (ahs-i-fi-KAY-shun). Before birth, this occurs in two phases. During the first phase, a bone matrix shell covers the hyaline cartilage through the activity of osteoblasts. Next, osteoclasts resorb the enclosed hyaline cartilage, creating a medullary cavity within the bony superstructure. Longitudinal Growth Bones grow in length at the epiphyseal (ehp-i-FIZ-ee-al) plates, located close to the ends of long bones (Figure 4.5). During childhood growth, osteoblasts on the central side of the epiphyseal plate produce new bone cells, 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, ending the longitudinal growth of the bone. Circumferential Growth Although most bone growth occurs during childhood, bones actually grow in diameter, or width, throughout most of life (Figure 4.6). Osteoblasts in the internal layer of the periosteum build concentric layers of new bone on top of existing ones. To understand the process, it helps to visualize the way in which the rings on a cross-cut tree stump reveal the tree’s growth. MicroScape/Science Source Figure 4.5 Bones grow in length at the epiphyseal plates (the dark blue area). When will this plate dissolve? LIFE SPAN DEVELOPMENT: Bones © Body Scientific International Figure 4.6 Cross section of a long bone showing normal bone throughout life. Is it the circumference or the length of bones that continues to grow throughout life? Medullary cavity Osteoblast activity builds new bone Osteoclast activity resorbs bone – – – – – – – – Periosteum Cortical bone Endosteum + + + + + + + + + + + + + + + + + + (continued) Copyright Goodheart-Willcox Co., Inc.