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CHAPTER 6 The Skeletal System: Bone Tissue Bone Tissue and Homeostasis Bone tissue is continuously growing, remodeling, and repairing itself. It contributes to homeostasis of the body by providing s...

CHAPTER 6 The Skeletal System: Bone Tissue Bone Tissue and Homeostasis Bone tissue is continuously growing, remodeling, and repairing itself. It contributes to homeostasis of the body by providing support and protection, producing blood cells, and storing minerals and triglycerides. Bone tissue is a complex and dynamic living tissue. It continually subject their bones to great forces, which place significant strain on the engages in a process called bone remodeling—the building of new bone bone tissue. Accomplished athletes show an increase in overall bone tissue and breaking down of old bone tissue. In the early days of space density. How is bone capable of changing in response to the different exploration, young, healthy men in prime physical shape returned from mechanical demands placed on it? Why do high activity levels that strain their space flights only to alarm their physicians. Physical examinations bone tissue greatly improve bone health? This chapter surveys the various of the astronauts revealed that they had lost up to 20% of their total bone components of bones to help you understand how bones form, how they density during their extended stay in space. The zero-gravity (weightless) age, and how exercise affects their density and strength. environment of space, coupled with the fact that the astronauts traveled in small capsules that greatly limited their movement for extended Q Did you ever wonder why more females than males are periods of time, placed minimal strain on their bones. In contrast, athletes affected by osteoporosis? 171 172 CH APTE R 6 The Skeletal System: Bone Tissue 3. Which bones contain red bone marrow? 6.1 Functions of Bone and 4. How do red bone marrow and yellow bone marrow differ in the Skeletal System composition and function? OBJECTIVE Describe the six main functions of the skeletal system. 6.2 Structure of Bone OBJECTIVE A bone is an organ made up of several different tissues working together: bone (osseous) tissue, cartilage, dense connective tissue, Describe the structure and functions of each part of a long epithelium, adipose tissue, and nervous tissue. The entire framework bone. of bones and their cartilages constitute the skeletal system. The study of bone structure and the treatment of bone disorders is referred to as osteology (os-tē-OL-o-jē; osteo- = bone; -logy = study of). We will now examine the structure of bone at the macroscopic level. The skeletal system performs several basic functions: Macroscopic bone structure may be analyzed by considering the 1. Support. The skeleton serves as the structural framework for the parts of a long bone, such as the humerus (the arm bone) shown in body by supporting soft tissues and providing attachment points Figure 6.1a. A long bone is one that has greater length than width. for the tendons of most skeletal muscles. A typical long bone consists of the following parts: 2. Protection. The skeleton protects the most important internal 1. The diaphysis (dī-AF-i-sis = growing between) is the bone’s shaft organs from injury. For example, cranial bones protect the brain, or body—the long, cylindrical, main portion of the bone. and the rib cage protects the heart and lungs. 2. The epiphyses (e-PIF-i-sēz = growing over; singular is epiphysis) 3. Assistance in movement. Most skeletal muscles attach to bones; are the proximal and distal ends of the bone. when they contract, they pull on bones to produce movement. This 3. The metaphyses (me-TAF-i-sēz; meta- = between; singular is function is discussed in detail in Chapter 10. metaphysis) are the regions between the diaphysis and the epi- 4. Mineral homeostasis (storage and release). Bone tissue makes physes. In a growing bone, each metaphysis contains an epiphy- up about 18% of the weight of the human body. It stores several seal (growth) plate (ep′-i-FIZ-ē-al), a layer of hyaline cartilage that minerals, especially calcium and phosphorus, which contribute to allows the diaphysis of the bone to grow in length (described later the strength of bone. Bone tissue stores about 99% of the body’s in the chapter). When a bone ceases to grow in length at about calcium. On demand, bone releases minerals into the blood to ages 14–24, the cartilage in the epiphyseal plate is replaced by maintain critical mineral balances (homeostasis) and to distribute bone; the resulting bony structure is known as the epiphyseal line. the minerals to other parts of the body. 4. The articular cartilage is a thin layer of hyaline cartilage covering 5. Blood cell production. Within certain bones, a connective tissue the part of the epiphysis where the bone forms an articulation (joint) called red bone marrow produces red blood cells, white blood with another bone. Articular cartilage reduces friction and absorbs cells, and platelets, a process called hemopoiesis (hēm-ō-poy-ē-sis; shock at freely movable joints. Because articular cartilage lacks a hemo- = blood; -poiesis = making). Red bone marrow consists of perichondrium and lacks blood vessels, repair of damage is limited. developing blood cells, adipocytes, fibroblasts, and macrophages 5. The periosteum (per-ē-OS-tē-um; peri- = around) is a tough con- within a network of reticular fibers. It is present in developing bones nective tissue sheath and its associated blood supply that surrounds of the fetus and in some adult bones, such as the hip (pelvic) bones, the bone surface wherever it is not covered by articular cartilage. It is ribs, sternum (breastbone), vertebrae (backbones), skull, and ends composed of an outer fibrous layer of dense irregular connective tis- of the bones of the humerus (arm bone) and femur (thigh bone). In sue and an inner osteogenic layer that consists of cells. Some of the a newborn, all bone marrow is red and is involved in hemopoiesis. cells enable bone to grow in thickness, but not in length. The perios- With increasing age, much of the bone marrow changes from red to teum also protects the bone, assists in fracture repair, helps nourish yellow. Blood cell production is considered in detail in Section 19.2. bone tissue, and serves as an attachment point for ligaments and 6. Triglyceride storage. Yellow bone marrow consists mainly of adi- tendons. The periosteum is attached to the underlying bone by per- pose cells, which store triglycerides. The stored triglycerides are a forating fibers or Sharpey’s fibers, thick bundles of collagen that potential chemical energy reserve. extend from the periosteum into the bone extracellular matrix. 6. The medullary cavity (MED-ul-er-ē; medulla- = marrow, pith), or Checkpoint marrow cavity, is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels in 1. How does the skeletal system function in support, protection, adults. This cavity minimizes the weight of the bone by reducing the movement, and storage of minerals? dense bony material where it is least needed. The long bones’ tubu- 2. Describe the role of bones in blood cell production. lar design provides maximum strength with minimum weight.  6.2 Structure of Bone 173 FIGURE 6.1 Parts of a long bone. The spongy bone tissue of the epiphyses and metaphyses contains red bone marrow, and the medullary cavity of the diaphysis contains yellow bone marrow (in adults). A long bone is covered by articular cartilage at the articular surfaces of its proximal and distal epiphyses and by periosteum around all other parts of the bone. Functions of Bone Tissue 4. Stores and releases minerals. 1. Supports soft tissue and provides 5. Contains red bone marrow, which attachment for skeletal muscles. produces blood cells. 2. Protects internal organs. 6. Contains yellow bone marrow, which 3. Assists in movement, along with stores triglycerides (fats). skeletal muscles. Articular cartilage Spongy bone Proximal (contains red epiphysis bone marrow) Red bone marrow Metaphysis Proximal epiphysis Epiphyseal line Spongy Epiphyseal line bone Metaphysis Compact bone Compact Endosteum (lines bone medullary cavity) Nutrient artery Medullary cavity Diaphysis Medullary cavity in diaphysis (contains yellow bone marrow in adults) Periosteum (b) Partially sectioned humerus Humerus Metaphysis Distal epiphysis Articular cartilage (a) Partially sectioned humerus (arm bone) Q What is the functional significance of the periosteum? 174 CH APTE R 6 The Skeletal System: Bone Tissue 7. The endosteum (end-OS-tē-um; endo- = within) is a thin mem- salt is calcium phosphate [Ca3(PO4)2]. It combines with another min- brane that lines the medullary cavity. It contains a single layer of eral salt, calcium hydroxide [Ca(OH)2], to form crystals of hydroxyapa- bone-forming cells and a small amount of connective tissue. tite [Ca10(PO4)6(OH)2] (hī-drok-sē-AP-a-tīt). As the crystals form, they combine with still other mineral salts, such as calcium carbonate Checkpoint (CaCO3), and ions such as magnesium, fluoride, potassium, and sul- fate. As these mineral salts are deposited in the framework formed by 5. Diagram the parts of a long bone, and list the functions of each part. the collagen fibers of the extracellular matrix, they crystallize and the - tissue hardens. This process, called calcification (kal′-si-fi-KA-shun), is initiated by bone-building cells called osteoblasts (described shortly). It was once thought that calcification simply occurred when 6.3 Histology of Bone Tissue enough mineral salts were present to form crystals. We now know that the process requires the presence of collagen fibers. Mineral salts first begin to crystallize in the microscopic spaces between collagen fibers. After the spaces are filled, mineral crystals accumulate around OBJECTIVES the collagen fibers. The combination of crystallized salts and collagen fibers is responsible for the characteristics of bone. Explain why bone tissue is classified as a connective tissue. Although a bone’s hardness depends on the crystallized inorganic Describe the cellular composition of bone tissue and the mineral salts, a bone’s flexibility depends on its collagen fibers. Like functions of each type of cell. reinforcing metal rods in concrete, collagen fibers and other organic Compare the structural and functional differences between molecules provide tensile strength, resistance to being stretched or compact and spongy bone tissue. torn apart. Soaking a bone in an acidic solution, such as vinegar, dissolves its mineral salts, causing the bone to become rubbery and flexible. As you will see shortly, when the need for particular minerals We will now examine the structure of bone at the microscopic level. arises or as part of bone formation or breakdown, bone cells called Like other connective tissues, bone, or osseous tissue (OS-ē-us), con- osteoclasts secrete enzymes and acids that break down both the min- tains an abundant extracellular matrix that surrounds widely sepa- eral salts and the collagen fibers of the extracellular matrix of bone. rated cells. The extracellular matrix is about 15% water, 30% collagen Four types of cells are present in bone tissue: osteoprogenitor fibers, and 55% crystallized mineral salts. The most abundant mineral cells, osteoblasts, osteocytes, and osteoclasts (Figure 6.2). FIGURE 6.2 Types of cells in bone tissue. Osteoprogenitor cells undergo cell division and develop into osteoblasts, which secrete bone extracellular matrix. From bone cell lineage From white blood cell lineage Ruffled border Osteoprogenitor Osteoblast Osteocyte Osteoclast cell (develops into (forms bone (maintains (functions in resorption, the an osteoblast) extracellular matrix) bone tissue) breakdown of bone extracellular matrix) Steve Gschmeissner/Science Steve Gschmeissner/Science SPL/Science Source Source Images Source Images SEM 800x SEM 4000x SEM 2700x Q Why is bone resorption important?  6.3 Histology of Bone Tissue 175 1. Osteoprogenitor cells (os′-tē-ō-prō-JEN-i-tor; -genic = producing) Compact bone tissue is composed of repeating structural units are unspecialized bone stem cells derived from mesenchyme, the called osteons, or haversian systems (ha-VER-shan). Each osteon con- tissue from which almost all connective tissues are formed. They sists of concentric lamellae arranged around an osteonic (haversian are the only bone cells to undergo cell division; the resulting cells or central) canal. Resembling the growth rings of a tree, the concen- develop into osteoblasts. Osteoprogenitor cells are found along tric lamellae (la-MEL-ē) are circular plates of mineralized extracellu- the inner portion of the periosteum, in the endosteum, and in the lar matrix of increasing diameter, surrounding a small network of canals within bone that contain blood vessels. blood vessels and nerves located in the central canal (Figure 6.3a). 2. Osteoblasts (OS-tē-ō-blasts′; -blasts = buds or sprouts) are These tubelike units of bone generally form a series of parallel cylin- bone-building cells. They synthesize and secrete collagen fibers and ders that, in long bones, tend to run parallel to the long axis of the other organic components needed to build the extracellular matrix bone. Between the concentric lamellae are small spaces called lacu- of bone tissue, and they initiate calcification (described shortly). nae (la-KOO-nē = little lakes; singular is lacuna), which contain oste- As osteoblasts surround themselves with extracellular matrix, they ocytes. Radiating in all directions from the lacunae are tiny canaliculi become trapped in their secretions and become osteocytes. (Note: (kan-a-LIK-ū-lī = small channels), which are filled with extracellular The ending -blast in the name of a bone cell or any other connective fluid. Inside the canaliculi are slender fingerlike processes of osteo- tissue cell means that the cell secretes extracellular matrix.) cytes (see inset at right of Figure 6.3a). Neighboring osteocytes com- municate via gap junctions (see Section 4.2). The canaliculi connect 3. Osteocytes (OS-tē-ō-sīts′; -cytes = cells), mature bone cells, are the lacunae with one another and with the central canals, forming an main cells in bone tissue and maintain its daily metabolism, such intricate, miniature system of interconnected canals throughout the as the exchange of nutrients and wastes with the blood. Like osteo- bone. This system provides many routes for nutrients and oxygen to blasts, osteocytes do not undergo cell division. (Note: The ending reach the osteocytes and for the removal of wastes. -cyte in the name of a bone cell or any other tissue cell means that Osteons in compact bone tissue are aligned in the same direction the cell maintains and monitors the tissue.) and are parallel to the length of the diaphysis. As a result, the shaft of 4. Osteoclasts (OS-tē-ō-klasts′; -clast = break) are huge cells derived a long bone resists bending or fracturing even when considerable from the fusion of as many as 50 monocytes (a type of white blood force is applied from either end. Compact bone tissue tends to be cell) and are concentrated in the endosteum. On the side of the cell thickest in those parts of a bone where stresses are applied in rela- that faces the bone surface, the osteoclast’s plasma membrane is tively few directions. The lines of stress in a bone are not static. They deeply folded into a ruffled border. Here the cell releases powerful lys- change as a person learns to walk and in response to repeated strenu- osomal enzymes and acids that digest the protein and mineral com- ous physical activity, such as weight training. The lines of stress in a ponents of the underlying extracellular bone matrix. This breakdown bone also can change because of fractures or physical deformity. of bone extracellular matrix, termed bone resorption (rē-SORP- Thus, the organization of osteons is not static but changes over time shun), is part of the normal development, maintenance, and repair of in response to the physical demands placed on the skeleton. bone. (Note: The ending -clast means that the cell breaks down extra- The areas between neighboring osteons contain lamellae called cellular matrix.) As you will see later, in response to certain hormones, interstitial lamellae (in′-ter-STISH-al), which also have lacunae with os- osteoclasts help regulate blood calcium level (see Section 6.7). They teocytes and canaliculi. Interstitial lamellae are fragments of older osteons are also target cells for drug therapy used to treat osteoporosis (see that have been partially destroyed during bone rebuilding or growth. Disorders: Homeostatic Imbalances at the end of this chapter). Blood vessels and nerves from the periosteum penetrate the com- pact bone through transverse interosteonic (Volkmann’s or perforat- You may find it convenient to use an aid called a mnemonic ing) canals. The vessels and nerves of the interosteonic canals connect device (ne-MON-ik = memory) to learn new or unfamiliar informa- with those of the medullary cavity, periosteum, and central canals. tion. One such mnemonic that will help you remember the difference Arranged around the entire outer and inner circumference of between the function of osteoblasts and osteoclasts is as follows: the shaft of a long bone are lamellae called circumferential lamellae osteoBlasts Build bone, while osteoClasts Carve out bone. (ser′-kum-fer-EN-shē-al). They develop during initial bone formation. Bone is not completely solid but has many small spaces between The circumferential lamellae directly deep to the periosteum are its cells and extracellular matrix components. Some spaces serve as called external circumferential lamellae. They are connected to the channels for blood vessels that supply bone cells with nutrients. periosteum by perforating (Sharpey’s) fibers. The circumferential Other spaces act as storage areas for red bone marrow. Depending on lamellae that line the medullary cavity are called internal circumferen- the size and distribution of the spaces, the regions of a bone may be tial lamellae (Figure 6.3a). categorized as compact or spongy (see Figure 6.1). Overall, about 80% of the skeleton is compact bone and 20% is spongy bone. Spongy Bone Tissue Compact Bone Tissue In contrast to compact bone tissue, spongy bone tissue, also referred Compact bone tissue contains few spaces (Figure 6.3a) and is the to as trabecular or cancellous bone tissue, does not contain osteons strongest form of bone tissue. It is found beneath the periosteum of (Figure 6.3b, c). Spongy bone tissue is always located in the interior of all bones and makes up the bulk of the diaphyses of long bones. Com- a bone, protected by a covering of compact bone. It consists of lamel- pact bone tissue provides protection and support and resists the lae that are arranged in an irregular pattern of thin columns called stresses produced by weight and movement. trabeculae (tra-BEK-ū-lē = little beams; singular is trabecula). 176 CH APTE R 6 The Skeletal System: Bone Tissue FIGURE 6.3 Histology of compact and spongy bone. (a) Sections through the diaphysis of a long bone, from the surrounding periosteum on the right, to compact bone in the middle, to spongy bone and the medullary cavity on the left. The inset at the upper right shows an osteocyte in a lacuna. (b, c) Details of spongy bone. See Table 4.7 for a photo-micrograph of compact bone tissue and Figure 6.11a for a scanning electron micrograph of spongy bone tissue. Bone tissue is organized in concentric lamellae around an osteonic canal in compact bone and in irregularly arranged lamellae in the trabeculae in spongy bone. Compact bone Medullary Spongy cavity bone Interstitial Periosteum lamellae External Osteocyte circumferential Concentric lamellae Canaliculi lamellae See Figure 6.3b, c Lacuna for details Blood vessels Medullary cavity Osteon Trabeculae Internal circumferential Periosteal vein lamellae Periosteal artery Periosteum: Outer fibrous layer Inner osteogenic layer Osteonic canal Interosteonic (Volkmann’s Spongy bone or perforating) canal Perforating (Sharpey’s) Compact bone fibers (a) Osteons (haversian systems) in compact bone and trabeculae in spongy bone Lacuna Lamellae Canaliculi Osteocyte Space for red bone marrow Osteoclast Trabeculae Osteoblasts aligned along trabeculae of new bone (b) Enlarged aspect of spongy bone trabeculae (c) Details of a section of a trabecula Q As people age, some osteonic (haversian) canals may become blocked. What effect would this have on the surrounding osteocytes?  6.4 Blood and Nerve Supply of Bone 177 Between the trabeculae are spaces that are visible to the unaided eye. bones from the periosteum. We will consider the blood supply of a These macroscopic spaces are filled with red bone marrow in bones long bone such as the mature tibia (shin bone) shown in Figure 6.4. that produce blood cells, and yellow bone marrow (adipose tissue) in Periosteal arteries (per-ē-OS-tē-al), small arteries accompanied other bones. Both types of bone marrow contain numerous small by nerves, enter the diaphysis through many interosteonic blood vessels that provide nourishment to the osteocytes. Each tra- (Volkmann’s or perforating) canals and supply the periosteum and becula consists of concentric lamellae, osteocytes that lie in lacunae, outer part of the compact bone (see Figure 6.3a). Near the center of the and canaliculi that radiate outward from the lacunae. diaphysis, a large nutrient artery passes through a hole in compact Spongy bone tissue makes up most of the interior bone tissue of bone called the nutrient foramen (foramina is plural). On entering short, flat, sesamoid, and irregularly shaped bones. In long bones it the medullary cavity, the nutrient artery divides into proximal and forms the core of the epiphyses beneath the paper-thin layer of com- distal branches that course toward each end of the bone. These pact bone, and forms a variable narrow rim bordering the medullary branches supply both the inner part of compact bone tissue of the cavity of the diaphysis. Spongy bone is always covered by a layer of diaphysis and the spongy bone tissue and red bone marrow as far as compact bone for protection. the epiphyseal plates (or lines). Some bones, like the tibia, have only At first glance, the trabeculae of spongy bone tissue may appear one nutrient artery; others, like the femur (thigh bone), have several. to be less organized than the osteons of compact bone tissue. How- The ends of long bones are supplied by the metaphyseal and epiphy- ever, they are precisely oriented along lines of stress, a characteristic seal arteries, which arise from arteries that supply the associated that helps bones resist stresses and transfer force without breaking. joint. The metaphyseal arteries (met-a-FIZ-ē-al) enter the metaphy- Spongy bone tissue tends to be located where bones are not heavily ses of a long bone and, together with the nutrient artery, supply the stressed or where stresses are applied from many directions. The tra- red bone marrow and bone tissue of the metaphyses. The epiphyseal beculae do not achieve their final arrangement until locomotion is arteries (ep′-i-FIZ-ē-al) enter the epiphyses of a long bone and supply completely learned. In fact, the arrangement can even be altered as the red bone marrow and bone tissue of the epiphyses. lines of stress change due to a poorly healed fracture or a deformity. Veins that carry blood away from long bones are evident in three Spongy bone tissue is different from compact bone tissue in two places: (1) One or two nutrient veins accompany the nutrient artery respects. First, spongy bone tissue is light, which reduces the overall and exit through the diaphysis; (2) numerous epiphyseal veins and weight of a bone. This reduction in weight allows the bone to move more readily when pulled by a skeletal muscle. Second, the trabecu- lae of spongy bone tissue support and protect the red bone marrow. FIGURE 6.4 Blood supply of a mature long bone. Spongy bone in the hip bones, ribs, sternum (breastbone), vertebrae, and the proximal ends of the humerus and femur is the only site where Bone is richly supplied with blood vessels. red bone marrow is stored and, thus, the site where hemopoiesis (blood cell production) occurs in adults. Articular cartilage Epiphysis Epiphyseal artery Checkpoint Epiphyseal vein 6. Why is bone considered a connective tissue? Epiphyseal line 7. What factors contribute to the hardness and tensile strength of bone? Metaphysis 8. List the four types of cells in bone tissue and their functions. Metaphyseal artery 9. What is the composition of the extracellular matrix of bone tissue? Metaphyseal vein 10. How are compact and spongy bone tissues different in microscopic appearance, location, and function? 11. What is a bone scan and how is it used clinically? Medullary cavity Compact bone Nutrient vein Nutrient artery 6.4 Blood and Nerve Supply Diaphysis Periosteal artery Periosteal vein of Bone Periosteum Nutrient foramen OBJECTIVE Describe the blood and nerve supply of bone. Bone is richly supplied with blood. Blood vessels, which are especially Partially sectioned tibia (shin bone) abundant in portions of bone containing red bone marrow, pass into Q Where do periosteal arteries enter bone tissue? 178 CH APTE R 6 The Skeletal System: Bone Tissue metaphyseal veins accompany their respective arteries and exit in the general shape of bones, is the site where cartilage formation through the epiphyses and metaphyses, respectively; and (3) many and ossification occur during the sixth week of embryonic develop- small periosteal veins accompany their respective arteries and exit ment. Bone formation follows one of two patterns. through the periosteum. The two patterns of bone formation, which both involve the re- Nerves accompany the blood vessels that supply bones. The peri- placement of a preexisting connective tissue with bone, do not lead to osteum is rich in sensory nerves, some of which carry pain sensations. differences in the structure of mature bones, but are simply different These nerves are especially sensitive to tearing or tension, which methods of bone development. In the first type of ossification, called explains the severe pain resulting from a fracture or a bone tumor. For intramembranous ossification (in′-tra-MEM-bra-nus; intra- = within; the same reason, there is some pain associated with a bone marrow -membran- = membrane), bone forms directly within mesenchyme, needle biopsy. In this procedure, a needle is inserted into the middle which is arranged in sheetlike layers that resemble membranes. In the of the bone to withdraw a sample of red bone marrow to examine it second type, endochondral ossification (en′-dō-KON-dral; endo- = for conditions such as leukemias, metastatic neoplasms, lymphoma, within; -chondral = cartilage), bone forms within hyaline cartilage Hodgkin’s disease, and aplastic anemia. As the needle penetrates the that develops from mesenchyme. periosteum, pain is felt. Once it passes through, there is little pain. Intramembranous Ossification Intramembranous ossifi- cation is the simpler of the two methods of bone formation. The flat Checkpoint bones of the skull, most of the facial bones, mandible (lower jawbone), 12. Explain the location and roles of the nutrient arteries, nutrient and the medial part of the clavicle (collar bone) are formed in this way. foramina, epiphyseal arteries, and periosteal arteries. Also, the “soft spots” that help the fetal skull pass through the birth 13. Which part of a bone contains sensory nerves associated canal later harden as they undergo intramembranous ossification, with pain? which occurs as follows (Figure 6.5): 14. Describe one situation in which these sensory neurons are 1 Development of the ossification center. At the site where the important. bone will develop, specific chemical messages cause the cells 15. How is a bone marrow needle biopsy performed? What of the mesenchyme to cluster together and differentiate, first conditions are diagnosed through this procedure? into osteoprogenitor cells and then into osteoblasts. The site of such a cluster is called an ossification center. Osteoblasts secrete the organic extracellular matrix of bone until they are surrounded by it. 6.5 Bone Formation 2 Calcification. Next, the secretion of extracellular matrix stops, and the cells, now called osteocytes, lie in lacunae and extend their narrow cytoplasmic processes into canaliculi that radiate in OBJECTIVES all directions. Within a few days, calcium and other mineral salts are deposited and the extracellular matrix hardens or calcifies Describe the steps of intramembranous and endochondral (calcification). ossification. 3 Formation of trabeculae. As the bone extracellular matrix forms, Explain how bone grows in length and thickness. it develops into trabeculae that fuse with one another to form Describe the process involved in bone remodeling. spongy bone around the network of blood vessels in the tissue. Connective tissue associated with the blood vessels in the trabeculae differentiates into red bone marrow. - The process by which bone forms is called ossification (os′-i-fi-KA- 4 Development of the periosteum. In conjunction with the formation shun; ossi- = bone; -fication = making) or osteogenesis (os′-tē-ō-JEN- of trabeculae, the mesenchyme condenses at the periphery of e-sis). Bone formation occurs in four principal situations: (1) the initial the bone and develops into the periosteum. Eventually, a thin formation of bones in an embryo and fetus, (2) the growth of bones layer of compact bone replaces the surface layers of the spongy during infancy, childhood, and adolescence until their adult sizes are bone, but spongy bone remains in the center. Much of the newly reached, (3) the remodeling of bone (replacement of old bone by new formed bone is remodeled (destroyed and reformed) as the bone bone tissue throughout life), and (4) the repair of fractures (breaks in is transformed into its adult size and shape. bones) throughout life. Endochondral Ossification The replacement of cartilage Initial Bone Formation in an Embryo by bone is called endochondral ossification. Although most bones of the body are formed in this way, the process is best observed in a long and Fetus bone. It proceeds as follows (Figure 6.6): We will first consider the initial formation of bone in an embryo and 1 Development of the cartilage model. At the site where the bone fetus. The embryonic “skeleton,” initially composed of mesenchyme is going to form, specific chemical messages cause the cells in  6.5 Bone Formation 179 FIGURE 6.5 Intramembranous ossification. Refer to this figure as you read the corresponding numbered paragraphs in the text. Illustrations 1 and 2 show a smaller field of vision at higher magnification than illustrations 3 and 4. Intramembranous ossification involves the formation of bone within mesenchyme arranged in sheetlike layers that resemble membranes. Blood capillary Flat bone of skull Ossification center Mesenchyme Osteoblast Mandible Collagen fiber 1 Development of ossification center: osteoblasts secrete organic extracellular matrix. Periosteum Osteocyte in Compact bone lacuna tissue Canaliculus Spongy bone tissue Osteoblast Compact bone Newly calcified bone tissue extracellular matrix 4 Development of the periosteum: 2 Calcification: calcium and other mineral mesenchyme at the periphery of the salts are deposited and extracellular matrix bone develops into the periosteum. calcifies (hardens). Mesenchyme condenses Blood vessel Spongy bone trabeculae Osteoblast 3 Formation of trabeculae: extracellular matrix develops into trabeculae that fuse to form spongy bone. Q Which bones of the body develop by intramembranous ossification? mesenchyme to crowd together in the general shape of the future 2 Growth of the cartilage model. Once chondroblasts become bone, and then develop into chondroblasts. The chondroblasts deeply buried in the cartilage extracellular matrix, they are called secrete cartilage extracellular matrix, producing a cartilage model chondrocytes. The cartilage model grows in length by continual (future diaphysis) consisting of hyaline cartilage. A covering cell division of chondrocytes, accompanied by further secretion called the perichondrium (per′-i-KON-drē-um) develops around of the cartilage extracellular matrix. This type of cartilaginous the cartilage model. growth, called interstitial (endogenous) growth (growth from 180 CH APTE R 6 The Skeletal System: Bone Tissue FIGURE 6.6 Endochondral ossification. During endochondral ossification, bone gradually replaces a cartilage model. Perichondrium Proximal epiphysis Periosteum Calcified Cartilage extracellular model matrix (Hyaline Diaphysis cartilage) Primary Periosteum Calcified Nutrient ossification extracellular artery center matrix Spongy Medullary Distal bone cavity epiphysis Nutrient artery and vein 1 Development of cartilage 2 Growth of 3 Development of primary 4 Development of the model: Mesenchymal cells cartilage model: ossification center: In this region medullary (marrow) develop into chondroblasts, Growth occurs by of the diaphysis, bone tissue has cavity: Bone breakdown which form the cartilage cell division of replaced most of the cartilage. by osteoclasts forms the model. chondrocytes. medullary cavity. Articular cartilage Epiphyseal Secondary artery and ossification vein Spongy bone center Epiphyseal plate Calcified Uncalcified Scott Camazine/Science Source 5 Development of secondary 6 Formation of articular cartilage (b) Twelve-week-old fetus. The red areas ossification centers: These and epiphyseal plate: Both represent bones that are forming occur in the epiphyses of structures consist of hyaline (calcified). Clear areas represent the bone. cartilage. cartilage (uncalcified). (a) Sequence of events Q Where in the cartilage model do secondary ossification centers develop during endochondral ossification?  6.5 Bone Formation 181 within), results in an increase in length. In contrast, growth of Bone Growth during Infancy, Childhood, the cartilage in thickness is due mainly to the deposition of extracellular matrix material on the cartilage surface of the model and Adolescence by new chondroblasts that develop from the perichondrium. This During infancy, childhood, and adolescence, bones throughout the process is called appositional (exogenous) growth (a-pō-ZISH-o- body grow in thickness by appositional growth, and long bones nal), meaning growth at the outer surface. Interstitial growth and lengthen by the addition of bone material on the diaphyseal side of appositional growth of cartilage are described in more detail in the epiphyseal plate by interstitial growth. Section 4.5. As the cartilage model continues to grow, chondrocytes in Growth in Length The growth in length of long bones involves its midregion hypertrophy (increase in size) and the surrounding the following two major events: (1) interstitial growth of cartilage on cartilage extracellular matrix begins to calcify. Other chondrocytes the epiphyseal side of the epiphyseal plate and (2) replacement of within the calcifying cartilage die because nutrients can no longer cartilage on the diaphyseal side of the epiphyseal plate with bone by diffuse quickly enough through the extracellular matrix. As these endochondral ossification. chondrocytes die, the spaces left behind by dead chondrocytes To understand how a bone grows in length, you need to know merge into small cavities called lacunae. some of the details of the structure of the epiphyseal plate. The 3 Development of the primary ossification center. Primary epiphyseal (growth) plate (ep-i-FIZ-ē-al) is a layer of hyaline carti- ossification proceeds inward from the external surface of the lage in the metaphysis of a growing bone that consists of four zones bone. A nutrient artery penetrates the perichondrium and the (Figure 6.7b): calcifying cartilage model through a nutrient foramen in the 1. Zone of resting cartilage. This layer is nearest the epiphysis and midregion of the cartilage model, stimulating osteoprogenitor consists of small, scattered chondrocytes. The term “resting” is cells in the perichondrium to differentiate into osteoblasts. used because the cells do not function in bone growth. Rather, Once the perichondrium starts to form bone, it is known as the they anchor the epiphyseal plate to the epiphysis of the bone. periosteum. Near the middle of the model, periosteal capillaries grow into the disintegrating calcified cartilage, inducing growth 2. Zone of proliferating cartilage. Slightly larger chondrocytes in this of a primary ossification center, a region where bone tissue will zone are arranged like stacks of coins. These chondrocytes undergo replace most of the cartilage. Osteoblasts then begin to deposit interstitial growth as they divide and secrete extracellular matrix. bone extracellular matrix over the remnants of calcified cartilage, The chondrocytes in this zone divide to replace those that die at forming spongy bone trabeculae. Primary ossification spreads the diaphyseal side of the epiphyseal plate. - from this central location toward both ends of the cartilage 3. Zone of hypertrophic cartilage (hī-per-TRO-fik). This layer consists model. of large, maturing chondrocytes arranged in columns. 4 Development of the medullary (marrow) cavity. As the primary 4. Zone of calcified cartilage. The final zone of the epiphyseal plate is ossification center grows toward the ends of the bone, osteoclasts only a few cells thick and consists mostly of chondrocytes that are break down some of the newly formed spongy bone trabeculae. dead because the extracellular matrix around them has calcified. This activity leaves a cavity, the medullary (marrow) cavity, in the Osteoclasts dissolve the calcified cartilage, and osteoblasts and diaphysis (shaft). Eventually, most of the wall of the diaphysis is capillaries from the diaphysis invade the area. The osteoblasts lay replaced by compact bone. down bone extracellular matrix, replacing the calcified cartilage by 5 Development of the secondary ossification centers. When the process of endochondral ossification. Recall that endochondral branches of the epiphyseal artery enter the epiphyses, ossification is the replacement of cartilage with bone. As a result, secondary ossification centers develop, usually around the the zone of calcified cartilage becomes the “new diaphysis” that is time of birth. Bone formation is similar to what occurs in firmly cemented to the rest of the diaphysis of the bone. primary ossification centers. However, in the secondary The activity of the epiphyseal plate is the only way that the di- ossification centers spongy bone remains in the interior of the aphysis can increase in length. As a bone grows, chondrocytes prolif- epiphyses (no medullary cavities are formed here). In contrast to erate on the epiphyseal side of the plate. New chondrocytes replace primary ossification, secondary ossification proceeds outward older ones, which are destroyed by calcification. Thus, the cartilage is from the center of the epiphysis toward the outer surface of the replaced by bone on the diaphyseal side of the plate. In this way the bone. thickness of the epiphyseal plate remains relatively constant, but the 6 Formation of articular cartilage and the epiphyseal (growth) bone on the diaphyseal side increases in length (Figure 6.7c). If a plate. The hyaline cartilage that covers the epiphyses becomes bone fracture damages the epiphyseal plate, the fractured bone may the articular cartilage. Prior to adulthood, hyaline cartilage be shorter than normal once adult stature is reached. This is because remains between the diaphysis and epiphysis as the epiphyseal damage to cartilage, which is avascular, accelerates closure of the (growth) plate, the region responsible for the lengthwise growth epiphyseal plate due to the cessation of cartilage cell division, thus of long bones that you will learn about next. inhibiting lengthwise growth of the bone. 182 CH APTE R 6 The Skeletal System: Bone Tissue (a) Radiograph showing the epiphyseal plate FIGURE 6.7 Epiphyseal (growth) plate. The epiphyseal (growth) plate of the femur of a 3-year-old appears as a dark band between whiter calcified areas in the radiograph Femur (x-ray) shown in part (a). The Bergman Collection/Project Masters, Inc. Epiphyseal The epiphyseal (growth) plate allows the diaphysis of a bone to increase plate in length. When adolescence comes to an end (at about age 18 in females and age 21 in males), the epiphyseal plates close; that is, the epiphy- seal cartilage cells stop dividing and bone replaces all remaining Tibia cartilage. The epiphyseal plate fades, leaving a bony structure called the epiphyseal line. With the appearance of the epiphyseal line, bone Diaphyseal side growth in length stops completely. Developing bone Closure of the epiphyseal plate is a gradual process and the of diaphysis degree to which it occurs is useful in determining bone age, predicting adult height, and establishing age at death from skeletal remains, Epiphyseal plate: especially in infants, children, and adolescents. For example, an open Zone of calcified epiphyseal plate indicates a younger person, while a partially closed cartilage epiphyseal plate or a completely closed one indicates an older person. Zone of hypertrophic It should also be kept in mind that closure of the epiphyseal plate, on cartilage average, takes place 1–2 years earlier in females. Growth in Thickness Like cartilage, bone can grow in Zone of proliferating thickness (diameter) only by appositional growth (Figure 6.8a): cartilage 1 At the bone surface, periosteal cells differentiate into osteoblasts, which secrete the collagen fibers and other organic molecules that form bone extracellular matrix. The osteoblasts become surrounded by extracellular matrix and develop into osteocytes. This process forms bone ridges on either side of a periosteal blood vessel. The ridges slowly enlarge and create a groove for the periosteal blood vessel. 2 Eventually, the ridges fold together and fuse, and the groove becomes a tunnel that encloses the blood vessel. The former periosteum now becomes the endosteum that lines the tunnel. Mark Nielsen Zone of resting cartilage Epiphyseal side LM 400x (b) Histology of the epiphyseal plate Articular cartilage Epiphysis New chondrocytes are formed Epiphyseal (growth) plate: Zone of resting cartilage Zone of proliferating cartilage Zone of hypertrophic cartilage Old chondrocytes are replaced by bone Zone of calcified cartilage New diaphysis Diaphysis (c) Lengthwise growth of bone at epiphyseal plate Q How does the epiphyseal (growth) plate account for the lengthwise growth of the diaphysis?  6.5 Bone Formation 183 FIGURE 6.8 Bone growth in thickness. As new bone is deposited on the outer surface of bone by osteoblasts, the bone tissue lining the medullary cavity is destroyed by osteoclasts in the endosteum. Periosteal ridges Periosteum Endosteum Periosteal blood vessel Interosteonic (perforating) canal Tunnel Groove 1 Ridges in periosteum create 2 Periosteal ridges fuse, forming groove for periosteal blood vessel. an endosteum-lined tunnel. Osteonic Circumferential (haversian) lamellae canal Periosteum Endosteum New osteon 3 Osteoblasts in endosteum build 4 Bone grows outward as osteoblasts in new concentric lamellae inward toward periosteum build new circumferential center of tunnel, forming a new osteon. lamellae. Osteon formation repeats as new periosteal ridges fold over blood vessels. (a) Microscopic details Bone formed by osteoblasts Bone destroyed by osteoclasts Medullary cavity Infant Child Young adult Adult (b) Macroscopic changes Q How does the medullary cavity enlarge during growth in thickness? 184 CH APTE R 6 The Skeletal System: Bone Tissue 3 Osteoblasts in the endosteum deposit bone extracellular matrix, During the process of bone resorption, an osteoclast attaches forming new concentric lamellae. The formation of additional tightly to the bone surface at the endosteum or periosteum and forms concentric lamellae proceeds inward toward the periosteal a leakproof seal at the edges of its ruffled border (see Figure 6.2). blood vessel. In this way, the tunnel fills in, and a new osteon is Then it releases protein-digesting lysosomal enzymes and several created. acids into the sealed pocket. The enzymes digest collagen fibers and 4 As an osteon is forming, osteoblasts under the periosteum other organic substances while the acids dissolve the bone minerals. deposit new circumferential lamellae, further increasing the Working together, several osteoclasts carve out a small tunnel in the thickness of the bone. As additional periosteal blood vessels old bone. The degraded bone proteins and extracellular matrix miner- become enclosed as in step 1 , the growth process continues. als, mainly calcium and phosphorus, enter an osteoclast by endo- cytosis, cross the cell in vesicles, and undergo exocytosis on the side Recall that as new bone tissue is being deposited on the outer opposite the ruffled border. Now in the interstitial fluid, the products surface of bone, the bone tissue lining the medullary cavity is de- of bone resorption diffuse into nearby blood capillaries. Once a small stroyed by osteoclasts in the endosteum. In this way, the medullary area of bone has been resorbed, osteoclasts depart and osteoblasts cavity enlarges as the bone increases in thickness (Figure 6.8b). move in to rebuild the bone in that area. Clinical Connection Remodeling of Bone Like skin, bone forms before birth but continually renews itself Paget’s Disease thereafter. Bone remodeling is the ongoing replacement of old A delicate balance exists between the actions of osteoclasts and osteoblasts. bone tissue by new bone tissue. It involves bone resorption, the Should too much new tissue be formed, the bones become abnormally removal of minerals and collagen fibers from bone by osteoclasts, thick and heavy. If too much mineral material is deposited in the bone, the and bone deposition, the addition of minerals and collagen fibers surplus may form thick bumps, called spurs, on the bone that interfere with to bone by osteoblasts. Thus, bone resorption results in the destruc- movement at joints. Excessive loss of calcium or tissue weakens the bones, and they may break, as occurs in osteoporosis, or they may become too flex- tion of bone extracellular matrix, while bone deposition results in ible, as in rickets and osteomalacia. In Paget’s disease, there is an excessive the formation of bone extracellular matrix. At any given time, about proliferation of osteoclasts so that bone resorption occurs faster than bone 5% of the total bone mass in the body is being remodeled. Remod- deposition. In response, osteoblasts attempt to compensate, but the new eling also takes place at different rates in different regions of the bone is weaker because it has a higher proportion of spongy to compact body. The distal portion of the femur is replaced about every four bone, mineralization is decreased, and the newly synthesized extracellular months. By contrast, bone in certain areas of the shaft of the femur matrix contains abnormal proteins. The newly formed bone, especially that will not be replaced completely during an individual’s life. Even after of the pelvis, limbs, lower vertebrae, and skull, becomes enlarged, hard, and bones have reached their adult shapes and sizes, old bone is con- brittle and fractures easily. tinually destroyed and new bone is formed in its place. Remodeling also removes injured bone, replacing it with new bone tissue. Remodeling may be triggered by factors such as exercise, sedentary lifestyle, and changes in diet. Factors Affecting Bone Growth Remodeling has several other benefits. Since the strength of and Bone Remodeling bone is related to the degree to which it is stressed, if newly formed bone is subjected to heavy loads, it will grow thicker and therefore be Normal bone metabolism—growth in the young and bone remodeling stronger than the old bone. Also, the shape of a bone can be altered in the adult—depends on several factors. These include adequate for proper support based on the stress patterns experienced during dietary intake of minerals and vitamins, as well as sufficient levels of the remodeling process. Finally, new bone is more resistant to frac- several hormones. ture than old bone. 1. Minerals. Large amounts of calcium and phosphorus are needed while bones are growing, as are smaller amounts of magnesium, fluoride, and manganese. These minerals are also necessary during Clinical Connection bone remodeling. 2. Vitamins. Vitamin A stimulates activity of osteoblasts. Vitamin Remodeling and Orthodontics C is needed for synthesis of collagen, the main bone protein. As you will soon learn, vitamin D helps build bone by increasing the Orthodontics (or-thō-DON-tiks) is the branch of dentistry concerned with the prevention and correction of poorly aligned teeth. The movement of absorption of calcium from foods in the gastrointestinal tract into teeth by braces places a stress on the bone that forms the sockets that the blood. Vitamins K and B12 are also needed for synthesis of bone anchor the teeth. In response to this artificial stress, osteoclasts and osteo- proteins. blasts remodel the sockets so that the teeth align properly. 3. Hormones. During childhood, the hormones most important to bone growth are the insulin-like growth factors (IGFs), which are  6.6 Fracture and Repair of Bone 185 produced by the liver and bone tissue (see Section 18.6). IGFs stim- bone. One way that estrogens slow resorption is by promoting apop- ulate osteoblasts, promote cell division at the epiphyseal plate and tosis (programmed death) of osteoclasts. As you will see shortly, in the periosteum, and enhance synthesis of the proteins needed to parathyroid hormone, calcitriol (the active form of vitamin D), and build new bone. IGFs are produced in response to the secretion of calcitonin are other hormones that can affect bone remodeling. growth hormone (GH) from the anterior lobe of the pituitary gland Moderate weight-bearing exercises maintain sufficient strain on (see Section 18.6). Thyroid hormones (T3 and T4) from the thyroid bones to increase and maintain their density. gland also promote bone growth by stimulating osteoblasts. In addition, the hormone insulin from the pancreas promotes bone Checkpoint growth by increasing the synthesis of bone proteins. 16. What are the major events of intramembranous ossification and At puberty, the secretion of hormones known as sex hormones endochondral ossification, and how are they different? causes a dramatic effect on bone growth. The sex hormones include 17. Describe the zones of the epiphyseal (growth) plate and their estrogens (produced by the ovaries) and androgens such as testoster- functions, and the significance of the epiphyseal line. one (produced by the testes). Although females have much higher levels 18. Explain how bone growth in length differs from bone growth in of estrogens and males have higher levels of androgens, females thickness. also have low levels of androgens, and males have low levels of estro- 19. How could the metaphyseal area of a bone help determine the gens. The adrenal glands of both sexes produce androgens, and other age of a skeleton? tissues, such as adipose tissue, can convert androgens to estrogens. These hormones are responsible for increased osteoblast activity, 20. Define remodeling, and describe the roles of osteoblasts and synthesis of bone extracellular matrix, and the sudden “growth spurt” osteoclasts in the process. that occurs during the teenage years. Estrogens also promote changes 21. What factors affect bone growth and bone remodeling? in the skeleton that are typical of females, such as widening of the pelvis. Ultimately sex hormones, especially estrogens in both sexes, shut down growth at epiphyseal (growth) plates, causing elongation of the bones to cease. Lengthwise growth of bones typically ends ear- 6.6 Fracture and Repair of Bone lier in females than in males due to their higher levels of estrogens. During adulthood, sex hormones contribute to bone remodeling by slowing resorption of old bone and promoting deposition of new OBJECTIVES Clinical Connection Describe several common types of fractures. Explain the sequence of events involved in fracture repair. Hormonal Abnormalities That Affect Height Excessive or deficient secretion of hormones that normally control bone growth can cause a person to be abnormally tall or short. Oversecretion of A fracture (FRAK-choor) is any break in a bone. Fractures are named growth hormone (GH) during childhood produces giantism, in which a per- according to their severity, the shape or position of the fracture line, son becomes much taller and heavier than normal. Dwarfism is a condi- or even the physician who first described them. tion of small stature in which the height of an individual is typically under 4 In some cases, a bone may fracture without visibly breaking. A feet 10 inches, usually averaging 4 feet. Generally, there are two types of stress fracture is a series of microscopic fissures in bone that forms dwarfism: proportionate and disproportionate. In proportionate dwarf- without any evidence of injury to other tissues. In healthy adults, ism, all parts of the body are small but they are proportionate to each stress fractures result from repeated, strenuous activities such as run- other. One cause of proportionate dwarfism is a hyposecretion of GH dur- ning, jumping, or aerobic dancing. Stress fractures are quite painful ing childhood and the condition is appropriately called pituitary dwarf- ism. The condition can be treated medically with administration of GH and also result from disease processes that disrupt normal bone cal- until epiphyseal plate closure. In disproportionate dwarfism, some parts cification, such as osteoporosis (discussed in Disorders: Homeostatic of the body are normal size or larger than normal while others are smaller Imbalances at the end of this chapter). About 25% of stress fractures than normal. For example, the trunk can be average size while the limbs are involve the tibia. Although standard x-ray images often fail to reveal short and the head may be large in relation to the rest of the body, with a the presence of stress fractures, they show up clearly in a bone scan. prominent forehead and flattened nose at the bridge. The most common The repair of a bone fracture involves the following phases cause of this type of dwarfism is a condition called achondroplasia (a-kon- (Figure 6.9): - drō-PLA-zē-a; a = without; chondro = cartilage; -plasai = to mold), an in- herited condition in which the conversion of hyaline cartilage to bone is 1 Reactive phase. This phase is an early inflammatory phase. Blood abnormal and the long bones of the limbs stop growing in childhood. Other vessels crossing the fracture line are broken. As blood leaks from bones are unaffected, and thus the person has short stature but a normal the torn ends of the vessels, a mass of blood (usually clotted) size head and trunk. This type of dwarfism is called achondroplastic forms around the site of the fracture. This mass of blood, called - dwarfism. The condition is essentially untreatable, although some indi- a fracture hematoma (hē′-ma-TO-ma; hemat- = blood; -oma = viduals opt for limb-lengthening surgery. tumor), usually forms 6 to 8 hours after the injury. Because the circulation of blood stops at the site where the fracture hematoma 186 CH APTE R 6 The Skeletal System: Bone Tissue FIGURE 6.9 Steps in repair of a bone fracture. Bone heals more rapidly than cartilage because its blood supply is more plentiful. Periosteum Fracture hematoma Bony (hard) callus New blood vessel Healed Spongy bone fracture trabeculae Fibrocartilaginous (soft) callus 1 Reactive phase: Fibrocartilaginous 2 Reparative phase Bony callus 3 Bone remodeling formation of callus formation formation phase fracture hematoma Q Why does it sometimes take months for a fracture to heal? forms, nearby bone cells die. Swelling and inflammation occur in bone replaces spongy bone around the periphery of the fracture. response to dead bone cells, producing additional cellular debris. Sometimes, the repair process is so thorough that the fracture line is Phagocytes (neutrophils and macrophages) and osteoclasts undetectable, even in a radiograph (x-ray). However, a thickened area begin to remove the dead or damaged tissue in and around the on the surface of the bone remains as evidence of a healed fracture. fracture hematoma. This stage may last up to several weeks. 2a Reparative phase: Fibrocartilaginous callus formation. The reparative phase is characterized by two events: the formation of Clinical Connection a fibrocartilaginous callus, and a bony callus to bridge the gap between the broken ends of the bones. Treatments for Fractures Blood vessels grow into the fracture hematoma and phago- Treatments for fractures vary according to age, type of fracture, and the cytes begin to clean up dead bone cells. Fibroblasts from the bone involved. The ultimate goals of fracture treatment are realignment periosteum invade the fracture site and produce collagen fibers. of the bone fragments, immobilization to maintain realignment, and In addition, cells from the periosteum develop into chondroblasts restoration of function. For bones to unite properly, the fractured ends must and begin to produce fibrocartilage in this region. These events be brought into alignment. This process, called reduction, is commonly referred to as setting a fracture. In closed reduction, the fractured ends lead to the development of a fibrocartilaginous (soft) callus (fi- of a bone are brought into alignment by manual manipulation, and the brō-kar-ti-LAJ-i-nus), a mass of repair tissue consisting of collagen skin remains intact. In open reduction, the fractured ends of a bone are fibers and cartilage that bridges the broken ends of the bone. brought into alignment by a surgical procedure using internal fixation Formation of the fibrocartilaginous callus takes about 3 weeks. devices such as screws, plates, pins, rods, and wires. Following reduction, 2b Reparative phase: Bony callus formation. In areas closer to a fractured bone may be kept immobilized by a cast, sling, splint, elastic well-vascularized healthy bone tissue, osteoprogenitor cells bandage, external fixation device, or a combination of these devices. develop into osteoblasts, which begin to produce spongy bone trabeculae. The trabeculae join living and dead portions of the original bone fragments. In time, the fibrocartilage is converted Although bone has a generous blood supply, healing sometimes to spongy bone, and the callus is then referred to as a bony (hard) takes months. The calcium and phosphorus needed to strengthen and callus. The bony callus lasts about 3 to 4 months. harden new bone are deposited only gradually, and bone cells generally 3 Bone remodeling phase. The final phase of fracture repair is bone grow and reproduce slowly. The temporary disruption in their blood remodeling of the callus. Dead portions of the original fragments supply also helps explain the slowness of healing of severely fractured of broken bone are gradually resorbed by osteoclasts. Compact bones. Some of the common types of fractures are shown in Table 6.1.  6.6 Fracture and Repair of Bone 187 TA B L E 6.1 Some Common Fractures FRACTURE DESCRIPTION ILLUSTRATION RADIOGRAPH Open (Compound) The broken ends of the bone Humerus protrude through the skin. Conversely, a closed (simple) fracture does not break the skin. Courtesy Dr. Brent Layton Radius Ulna Comminuted The bone is splintered, crushed, (KOM-i-noo-ted; or broken into pieces at the site com- = together; of impact, and smaller bone -minuted = fragments lie between the two crumbled) main fragments. Courtesy Per Amundson, M.D.