Chapter 6 The Skeletal System: Bone Tissue

Chapter 6 The Skeletal System: Bone Tissue Questions and Answers Make certain that you can define, and use in context, each of the terms listed below, and that you understand the significance of each of the concepts. 1. Describe the main functions of bones. • functions of the skeletal system -A bone is composed of several different tissues working together: bone or osseous tissue, cartilage, dense connective tissues, epithelium, adipose tissue, and nervous tissue. -Bone tissue makes up about 18% of the weight of the human body. The skeletal system performs several basic functions: 1. Support. The skeleton serves as the structural framework for the body by supporting soft tissues and providing attachment points for the tendons of most skeletal muscles. 2. Protection. The skeleton protects the most important internal organs from injury. For example, cranial bones protect the brain, vertebrae (backbones) protect the spinal cord, and the rib cage protects the heart and lungs. 3. Assistance in movement. Most skeletal muscles attach to bones; when they contract, they pull on bones to produce movement. This function is discussed in detail in Chapter 10. 4. Mineral homeostasis (storage and release). Bone tissue stores several minerals, especially calcium and phosphorus, which contribute to the strength of bone. Bone tissue stores about 99% of the body's calcium. On demand, bone releases minerals into the blood to maintain critical mineral balances (homeostasis) and to distribute the minerals to other parts of the body. 5. Blood cell production. Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis (hēm‐ō‐poy‐Ē‐sis; hemo‐ = blood; ‐poiesis = making). Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers. It is present in developing bones of the fetus and in some adult bones, such as the hip (pelvic) bones, ribs, sternum (breastbone), vertebrae (backbones), skull, and ends of the bones of the humerus (arm bone) and femur (thigh bone). In a newborn, all bone marrow is red and is involved in hemopoiesis. With increasing age, much of the bone marrow changes from red to yellow. Blood cell production is considered in detail in Section 19.2. 6. Triglyceride storage. Yellow bone marrow consists mainly of adipose cells, which store triglycerides. The stored triglycerides are a potential chemical energy reserve. • support • protection • assistance in movement • mineral homeostasis • blood cell production • red bone marrow • hemopoiesis • triglyceride storage • yellow bone marrow 2. Identify the parts of a long bone. • structure of a long bone 1. The diaphysis (dī‐AF‐i‐sis = growing between) is the bone's shaft or body—the long, cylindrical, main portion of the bone. 2. The epiphyses (e‐PIF‐i‐sēz = growing over; singular is epiphysis) are the proximal and distal ends of the bone. 3. The metaphyses (me‐TAF‐i‐sēz; meta‐ = between; singular is metaphysis) are the regions between the diaphysis and the epiphyses. In a growing bone, each metaphysis contains an epiphyseal (growth) plate (ep′‐i‐FIZ‐ē‐al), a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length (described later in the chapter). When a bone ceases to grow in length at about ages 18–21, the cartilage in the epiphyseal plate is replaced by bone; the resulting bony structure is known as the epiphyseal line. 4. The articular cartilage is a thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation (joint) with another bone. Articular cartilage reduces friction and absorbs shock at freely movable joints. Because articular cartilage lacks a perichondrium and lacks blood vessels, repair of damage is limited. 5. The periosteum (per‐ē‐OS‐tē‐um; peri‐ = around) is a tough connective tissue sheath and its associated blood supply that surrounds the bone surface wherever it is not covered by articular cartilage. It is composed of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer that consists of cells. Some of the cells enable bone to grow in thickness, but not in length. The periosteum also protects the bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons. The periosteum is attached to the underlying bone by perforating (Sharpey's) fibers, thick bundles of collagen that extend from the periosteum into the bone extracellular matrix. 6. The medullary cavity (MED‐ū‐lar‐ē; medulla‐ = marrow, pith), or marrow cavity, is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels in adults. This cavity minimizes the weight of the bone by reducing the dense bony material where it is least needed. The long bones' tubular design provides maximum strength with minimum weight. 7. The endosteum (end‐OS‐tē‐um; endo‐ = within) is a thin membrane that lines the medullary cavity. It contains a single layer of bone‐ forming cells and a small amount of connective tissue • diaphysis • epiphysis (plural is epiphyses) • metaphysis (plural is metaphyses) • epiphyseal plate and line • articular cartilage • periosteum • medullary cavity or marrow cavity • endosteum 3. Describe the histological features of bone tissue. • histology of bone tissue • hydroxyapatite- a crystal unit that is composed of two entities: calcium phosphate, and calcium hydroxide. -Like other connective tissues, bone, or osseous tissue (OS‐ē‐us), contains an abundant extracellular matrix that surrounds widely separated cells. The extracellular matrix is about 15% water, 30% collagen fibers, and 55% crystallized mineral salts. The most abundant mineral salt is calcium phosphate [Ca3(PO4)2]. It combines with another mineral salt, calcium hydroxide [Ca(OH)2], to form crystals of hydroxyapatite [Ca10(PO4)6(OH)2] (hī‐drok‐sē‐AP‐a‐tīt). • Calcification -As the crystals form, they combine with still other mineral salts, such as calcium carbonate (CaCO3), and ions such as magnesium, fluoride, potassium, and sulfate. As these mineral salts are deposited in the framework formed by the collagen fibers of the extracellular matrix, they crystallize and the tissue hardens. This process, called calcification (kal′‐si‐fi‐KĀ‐shun), is initiated by bone‐building cells called osteoblasts (described shortly). -It was once thought that calcification simply occurred when 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 the collagen fibers. The combination of crystallized salts and collagen fibers is responsible for the characteristics of bone. -a bone's hardness depends on the crystallized inorganic mineral salts, a bone's flexibility depends on its collagen fibers - collagen fibers and other organic molecules provide tensile strength, resistance to being stretched or torn apart. -when the need for particular minerals arises or as part of bone formation or breakdown, bone cells called osteoclasts secrete enzymes and acids that break down both the mineral salts and the collagen fibers of the extracellular matrix of bone. -Four types of cells are present in bone tissue: osteogenic cells, osteoblasts, osteocytes, and osteoclasts (Figure 6.2). • osteogenic cell • osteoblast • osteocyte • osteoclast 1. Osteogenic cells (os′‐tē‐ō‐JEN‐ik; ‐genic = producing) are unspecialized bone stem cells derived from mesenchyme, the tissue from which almost all connective tissues are formed. They are the only bone cells to undergo cell division; the resulting cells develop into osteoblasts. Osteogenic cells are found along the inner portion of the periosteum, in the endosteum, and in the canals within bone that contain blood vessels. 2. Osteoblasts (OS‐tē‐ō‐blasts′; ‐blasts = buds or sprouts) are bone‐building cells. They synthesize and secrete collagen fibers and other organic components needed to build the extracellular matrix of bone tissue, and they initiate calcification (described shortly). As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes. (Note: The ending ‐blast in the name of a bone cell or any other connective tissue cell means that the cell secretes extracellular matrix.) 3. Osteocytes (OS‐tē‐ō‐sīts′; ‐cytes = cells), mature bone cells, are the main cells in bone tissue and maintain its daily metabolism, such as the exchange of nutrients and wastes with the blood. Like osteoblasts, osteocytes do not undergo cell division. (Note: The ending ‐cyte in the name of a bone cell or any other tissue cell means that the cell maintains the tissue.) 4. Osteoclasts (OS‐tē‐ō‐klasts′; ‐clast = break) are huge cells derived from the fusion of as many as 50 monocytes (a type of white blood cell) and are concentrated in the endosteum. On the side of the cell that faces the bone surface, the osteoclast's plasma membrane is deeply folded into a ruffled border. Here the cell releases powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying extracellular bone matrix. This breakdown of bone extracellular matrix, termed resorption (rē‐SORP‐shun), is part of the normal development, maintenance, and repair of bone. (Note: The ending ‐clast means that the cell breaks down extracellular matrix.) As you will see later, in response to certain hormones, osteoclasts help regulate blood calcium level (see Section 6.7). They are also target cells for drug therapy used to treat osteoporosis (see Disorders: Homeostatic Imbalances at the end of this chapter). Remember: osteoBlasts Build bone, while osteoClasts Carve out bone. • compact bone tissue-Compact bone tissue contains few spaces (Figure 6.3a) and is the strongest form of bone tissue. It is found beneath the periosteum of all bones and makes up the bulk of the diaphyses of long bones. Compact bone tissue provides protection and support and resists the stresses produced by weight and movement. • osteon or Haversian system-Compact bone tissue is composed of repeating structural units called osteons, or haversian systems (ha‐VER‐shan). • central or Haversian canal- Each osteon consists of concentric lamellae arranged around a central (haversian) canal. • concentric lamellae- Resembling the growth rings of a tree, the concentric lamellae (LA‐ mel‐ē) are circular plates of mineralized extracellular matrix of increasing diameter, surrounding a small network of blood vessels, lymphatics, and nerves located in the central canal (Figure 6.3a). These tube‐like units of bone generally form a series of parallel cylinders that, in long bones, tend to run parallel to the long axis of the bone. -The areas between neighboring osteons contain lamellae called interstitial lamellae (in′‐ter‐STISH‐ al), which also have lacunae with osteocytes and canaliculi. Interstitial lamellae are fragments of older osteons that have been partially destroyed during bone rebuilding or growth. -Arranged around the entire outer and inner circumference of the shaft of a long bone are lamellae called circumferential lamellae (ser′‐kum‐fer‐EN‐shē‐al). They develop during initial bone formation. The circumferential lamellae directly deep to the periosteum are called outer circumferential lamellae. They are connected to the periosteum by perforating (Sharpey's) fibers. The circumferential lamellae that line the medullary cavity are called inner circumferential lamellae (Figure 6.3a). • lacuna (plural is lacunae)- Between the concentric lamellae are small spaces called lacunae (la‐KOO‐neē = little lakes; singular is lacuna), which contain osteocytes. • canaliculus (plural is canaliculi)- Radiating in all directions from the lacunae are tiny canaliculi (kan‐a‐LIK‐ū‐lī = small channels), which are filled with extracellular fluid. Inside the canaliculi are slender fingerlike processes of osteocytes (see inset at right of Figure 6.3a). Neighboring osteocytes communicate via gap junctions (see Section 4.2). The canaliculi connect lacunae with one another and with the central canals, forming an intricate, miniature system of interconnected canals throughout the bone. This system provides many routes for nutrients and oxygen to reach the osteocytes and for the removal of wastes. - Osteons in compact bone tissue are aligned in the same direction and are parallel to the length of the diaphysis. This is why the shaft of the long bone resists bending or fracturing even when considerable force is applied from either end. - the organization of osteons is not static but changes over time in response to the physical demands placed on the skeleton. • spongy bone tissue also referred to as trabecular or cancellous bone tissue, does not contain osteons (Figure 6.3b, 6.3c). Spongy bone tissue is always located in the interior of a bone, protected by a covering of compact bone. • trabecula (plural is trabaculae) It consists of lamellae that are arranged in an irregular pattern of thin columns called trabeculae (tra‐ BEK‐ū‐lē = little beams; singular istrabecula). Between the trabeculae are spaces that are visible to the unaided eye. These macroscopic spaces are filled with red bone marrow in bones that produce blood cells, and yellow bone marrow (adipose tissue) in other bones. Both types of bone marrow contain numerous small blood vessels that provide nourishment to the osteocytes. Each trabecula consists of concentric lamellae, osteocytes that lie in lacunae, and canaliculi that radiate outward from the lacunae. • blood and nerve supply of bone -Blood vessels pass into bones from the periosteum. • nutrient foramen- Near the center of the diaphysis, a large nutrient artery passes through a hole in compact bone called the nutrient foramen. On entering the medullary cavity, the nutrient artery divides into proximal and distal branches that course toward each end of the bone. These branches supply both the inner part of compact bone tissue of the diaphysis and the spongy bone tissue and red bone marrow as far as the epiphyseal plates (or lines). Some bones, like the tibia, have only one nutrient artery; others, like the femur (thigh bone), have several. • nutrient artery • nutrient veins- Veins that carry blood away from long bones are evident in three places: (1) One or two nutrient veins accompany the nutrient artery and exit through the diaphysis; 4. Describe the steps in intramembranous and endochondral ossification. • bone formation • ossification or osteogenesis The process by which bone forms is called ossification (os′‐i‐ fi‐KĀ‐shun; ossi‐ = bone; ‐fication = making) or osteogenesis (os′‐tē‐ō‐JEN‐e‐sis). Bone formation occurs in four principal situations: (1) the initial formation of bones in an embryo and fetus, (2) the growth of bones during infancy, childhood, and adolescence until their adult sizes are reached, (3) the remodeling of bone (replacement of old bone by new bone tissue throughout life), and (4) the repair of fractures (breaks in bones) throughout life. • intramembranous ossification Intramembranous ossification is the simpler of the two methods of bone formation. The flat bones of the skull, most of the facial bones, mandible (lower jawbone), and the medial part of the clavicle (collar bone) are formed in this way. Also, the “soft spots” that help the fetal skull pass through the birth canal later harden as they undergo intramembranous ossification, which occurs as follows (Figure 6.5): 1. Development of the ossification center. At the site where the bone will develop, specific chemical messages cause the mesenchymal cells to cluster together and differentiate, first into osteogenic 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. 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 all directions. Within a few days, calcium and other mineral salts are deposited and the extracellular matrix hardens or calcifies (calcification). 3. Formation of trabeculae. As the bone extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone around the network of blood vessels in the tissue. Connective tissue that is associated with the blood vessels in the trabeculae differentiates into red bone marrow. 4. Development of the periosteum. In conjunction with the formation of trabeculae, the mesenchyme condenses at the periphery of the bone and develops into the periosteum. Eventually, a thin layer of compact bone replaces the surface layers of the spongy bone, but spongy bone remains in the center. Much of the newly formed bone is remodeled (destroyed and reformed) as the bone is transformed into its adult size and shape. 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. • endochondral ossification The replacement of cartilage 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 bone. It proceeds as follows (Figure 6.6): 1. Development of the cartilage model. At the site where the bone is going to form, specific chemical messages cause the mesenchymal cells to crowd together in the general shape of the future bone, and then develop into chondroblasts. The chondroblasts secrete cartilage extracellular matrix, producing a cartilage model consisting of hyaline cartilage. A covering called the perichondrium (per′‐i‐KON‐drē‐um) develops around the cartilage model. 2. Growth of the cartilage model. Once chondroblasts become deeply buried in the cartilage extracellular matrix, they are called chondrocytes. The cartilage model grows in length by continual cell division of chondrocytes, accompanied by further secretion of the cartilage extracellular matrix. This type of cartilaginous growth, called interstitial (endogenous) growth (growth from within), results in an increase in length. In contrast, growth of the cartilage in thickness is due mainly to the deposition of extracellular matrix material on the cartilage surface of the model by new chondroblasts that develop from the perichondrium. This process is called appositional (exogenous) growth (a‐pō‐ZISH‐o‐nal), meaning growth at the outer surface. Interstitial growth and appositional growth of cartilage are described in more detail in Section 4.5. As the cartilage model continues to grow, chondrocytes in its mid‐region hypertrophy (increase in size) and the surrounding cartilage extracellular matrix begins to calcify. Other chondrocytes within the calcifying cartilage die because nutrients can no longer diffuse quickly enough through the extracellular matrix. As these chondrocytes die, the spaces left behind by dead chondrocytes merge into small cavities called lacunae. 3. Development of the primary ossification center. Primary ossification proceeds inward from the external surface of the bone. A nutrient artery penetrates the perichondrium and the calcifying cartilage model through a nutrient foramen in the midregion of the cartilage model, stimulating osteogenic cells in the perichondrium to differentiate into osteoblasts. Once the perichondrium starts to form bone, it is known as the periosteum. Near the middle of the model, periosteal capillaries grow into the disintegrating calcified cartilage, inducing growth of a primary ossification center, a region where bone tissue will replace most of the cartilage. Osteoblasts then begin to deposit bone extracellular matrix over the remnants of calcified cartilage, forming spongy bone trabeculae. Primary ossification spreads from this central location toward both ends of the cartilage model. 4. Development of the medullary (marrow) cavity. As the primary ossification center grows toward the ends of the bone, osteoclasts break down some of the newly formed spongy bone trabeculae. This activity leaves a cavity, the medullary (marrow) cavity, in the diaphysis (shaft). Eventually, most of the wall of the diaphysis is replaced by compact bone. 5. Development of the secondary ossification centers. When branches of the epiphyseal artery enter the epiphyses, secondary ossification centers develop, usually around the time of birth. Bone formation is similar to what occurs in primary ossification centers. However, in the secondary ossification centers spongy bone remains in the interior of the epiphyses (no medullary cavities are formed here). In contrast to primary ossification, secondary ossification proceeds outward from the center of the epiphysis toward the outer surface of the bone. 6. Formation of articular cartilage and the epiphyseal (growth) plate. The hyaline cartilage that covers the epiphyses becomes the articular cartilage. Prior to adulthood, hyaline cartilage remains between the diaphysis and epiphysis as the epiphyseal (growth) plate, the region responsible for the lengthwise growth of long bones that you will learn about next.