128 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. Fetal Development Bone modeling is the process in which new bone is created through osteoblast activity during the for- mation and growth of immature bones. 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, there is a rapid replacement of cartilage with bone even within the developing fetus. Ossification, the process of bone formation, begins early in fetal development. 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. Childhood Development During childhood, bones grow in length at the epiphyseal plates, located close to the ends of long bones (Figure 5.8). 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, during or soon after adolescence, the plate dissolves and the bone on either side of the plate fuses, effectively ending the longitudinal growth of the bone. Most epiphyses close around age 18, but some may be present through about age 25. O ver the course of the lifespan from birth through old age, some parts of the body grow, while others tend to shrink. Bone is a tissue that follows a normal course of development and growth but is also strongly affected by the influence of the forces to which it is subjected. Bone Growth and Development Specialized cells called osteoblasts 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. Bone growth clearly involves more osteoblast activity than osteoclast activity. However, both osteo- blasts 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. Understanding Medical Terminology 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. Sharpey’s fibers tiny connective tissue fibers that join together to firmly bind the periosteum to the underlying cortical bone trabecular bone interior, spongy bone with a porous, honeycomb structure Review Questions 1. List each of the five functions of the skeletal system and describe how they benefit the body. 2. Explain the differences between cortical bone and trabecular bone. 3. Describe where the diaphysis of a long bone is in relation to the epiphysis, and where the periosteum is in relation to the endosteum. SECTION 5.2 Development of Bones Objectives • Explain the roles of osteoblasts and osteoclasts. • Describe the processes involved in longitudinal an circumferential growth of bone. Key Terms epiphyseal plates modeling ossification osteoblasts osteoclasts remodeling