Chapter 6 The Skeletal System: Bone Tissue PDF

CHAPTER 6 The Skeletal System: Bone Tissue Bone tissue The Skeletal System 6 Basic Functions: - continuously growing, remodeling, and repairing itself. 1. Support - contributes to homeostasis o...

CHAPTER 6 The Skeletal System: Bone Tissue Bone tissue The Skeletal System 6 Basic Functions: - continuously growing, remodeling, and repairing itself. 1. Support - contributes to homeostasis of the body by - The skeleton serves as the structural providing support and protection, producing framework for the body by supporting soft blood cells, and storing minerals and tissues and providing attachment points for the triglycerides. tendons of most skeletal muscles. - is a complex and dynamic living tissue. - it continually 2. Protection engages in a process called bone remodeling - The skeleton protects the most important internal organs from injury. Bone Remodeling - ie, cranial bones protect the brain, - the building of new bone tissue and breaking and the rib cage protects the heart and lungs. down of old bone tissue. 3. Assistance in Movement ➔ In the early days of space exploration, young, healthy - Most skeletal muscles attach to bones; men in prime physical shape returned from their When they contract, they pull on bones to space flights only to alarm their physicians. produce movement. ➔ Physical examinations of the astronauts revealed that 4. Mineral Homeostasis (Storage and they had lost up to 20% of their total bone density Release) during their extended stay in space. - Bone tissue makes up about 18% of the weight of the human body. ➔ The zero-gravity (weightless) environment of space, - It stores several minerals, especially coupled with the fact that the astronauts traveled in calcium and phosphorus, which contribute to small capsules that greatly limited their movement for the strength of bone. extended periods of time, placed minimal strain on - Bone tissue stores about 99% of the their bones. body’s calcium. - On demand, bone releases minerals ➔ In contrast, athletes subject their bones to great into the blood to maintain critical mineral forces, which place significant strain on the bone balances (homeostasis). tissue. Accomplished athletes show an increase in overall bone density. 5. Blood Cell Production - Within certain bones, a connective 6.1 Functions of Bone and the Skeletal System tissue called red bone marrow produces: red blood cells Bone white blood cells - is an organ made up of several different platelets tissues working together: ➔ a process called Hemopoiesis Bone (osseous) Tissue Cartilage ➔ Within a network of reticular fibers, Red Bone Marrow Dense Connective Tissue consists of: Epithelium Developing Blood Cells Adipose Tissue Adipocytes Nervous Tissue Fibroblasts Macrophages ➔ The entire framework of bones and their cartilages constitute the skeletal system. ➔ RBM is present in developing bones of the fetus and in some adult bones: ➔ The study of bone structure and the treatment of bone Hip (pelvic) bones disorders is referred to as Osteology. Ribs Sternum (breastbone) Vertebrae (backbones) Skull  ends of the bones of the Humerus the bone surface wherever it is not covered by (arm bone) & Femur (thigh bone) articular cartilage. - it is composed of: ➔ In a newborn, all bone marrow is red and is involved Outer Fibrous Layer of Dense Irregular in Hemopoiesis. With increasing age, much of the Connective Tissue bone marrow changes from red to yellow. Inner Osteogenic Layer That Consists of Cells 6. Triglyceride Storage - Yellow bone marrow consists mainly of ➔ Some of the cells enable bone to grow in thickness, adipose cells, which store triglycerides. but not in length. - The stored triglycerides are a potential chemical energy reserve. The periosteum: protects the bone 6.2 Structure of Bone assists in fracture repair ➔ Macroscopic bone structure may be analyzed by helps nourish bone tissue considering the parts of a long bone. A long bone is serves as an attachment point for ligaments one that has greater length than width. and tendons Long Bone consists of the following parts: The periosteum is attached to the underlying bone by perforating fibers or Sharpey’s fibers. 1. Diaphysis - is the bone’s shaft or body—the long, Perforating Fibers or Sharpey’s fibers cylindrical, main portion of the bone. ➔ thick bundles of collagen that extend from the periosteum into the bone extracellular matrix. 2. Epiphyses - are the proximal and distal ends of 6. Medullary Cavity the bone. - or marrow cavity - is a hollow, cylindrical space within 3. Metaphyses the diaphysis that contains (in adults); - are the regions between the fatty yellow bone marrow diaphysis and the epiphyses. numerous blood vessels - In a growing bone, each metaphysis contains an epiphyseal plate ➔ MC minimizes the weight of the bone by reducing the dense bony material where it is least needed. Epiphyseal (Growth) Plate ➔ a layer of hyaline cartilage that allows the ➔ The long bones’ tubular design provides maximum diaphysis of the bone to grow in length strength with minimum weight. ➔ When a bone ceases to grow in length at about ages 7. Endosteum 14–24, the cartilage in the epiphyseal plate is - a thin membrane that lines the medullary replaced by bone; the resulting bony structure is cavity known as the Epiphyseal Line. - it contains a single layer of bone-forming cells and a small amount of connective tissue. 4. Articular Cartilage - is a thin layer of hyaline cartilage covering ➔ The spongy bone tissue of the epiphyses and the part of the epiphysis where the bone forms metaphyses contains red bone marrow. an articulation (joint) with another bone. - reduces friction and absorbs shock at freely ➔ The medullary cavity of the diaphysis contains yellow movable joints. bone marrow (in adults). - bcz articular cartilage lacks a perichondrium and lacks blood vessels, repair of damage is ➔ A long bone is covered by articular cartilage at the limited. articular surfaces of its proximal and distal epiphyses and by periosteum around all other parts of the bone. 5. Periosteum - is a tough connective tissue sheath and its associated blood supply that surrounds 6.3 Histology of Bone Tissue This process, called calcification, is initiated by bone-building cells called osteoblasts. Like other connective tissues, bone, or osseous It was once thought that calcification simply occurred tissue, contains an abundant extracellular matrix that when enough mineral salts were present to form surrounds widely separated cells. crystals. But we now know that Calcification requires the presence of collagen fibers. The extracellular matrix is about: 15% water Mineral salts first begin to crystallize in the 30% collagen fibers microscopic spaces between collagen fibers. 55% crystallized mineral salts After the spaces are filled, mineral crystals The most abundant mineral salt is calcium phosphate accumulate around the collagen fibers. [Ca3(PO4)2]. It combines with another mineral salt, calcium hydroxide [Ca(OH)2], to form crystals of The combination of crystallized salts and collagen hydroxyapatite [Ca10(PO4)6(OH)2]. fibers are responsible for the characteristics of bone. As the crystals form, they combine with still other Although a bone’s hardness depends on the mineral salts: crystallized inorganic mineral salts, a bone’s flexibility Calcium carbonate (CaCO3) depends on its collagen fibers. And combine with other ions: Like reinforcing metal rods in concrete, collagen fibers Magnesium and other organic molecules provide tensile strength, Fluoride resistance to being stretched or torn apart. Potassium Sulfate Soaking a bone in an acidic solution, such as vinegar, dissolves its mineral salts, causing the bone to As these mineral salts are deposited in the framework become rubbery and flexible. formed by the collagen fibers of the extracellular matrix, they crystallize and the tissue hardens. Functions of Bone Tissue - like osteoblasts, osteocytes do not undergo cell division 1. Supports soft tissue and provides attachment for skeletal muscles. (The ending -cyte in the name of a bone cell or any other tissue cell means that the cell maintains and 2. Protects internal organs. monitors the tissue.) 3. Assists in movement, along with 4. Osteoclasts skeletal muscles. - are huge cells derived from the fusion of as many as 50 monocytes (a type of 4. Stores and releases minerals. white blood cell) - are concentrated in the endosteum. 5. Contains red bone marrow, which produces blood cells. On the side of the cell that faces the bone surface, the osteoclast’s plasma membrane is deeply folded into a 6. Contains yellow bone marrow, which ruffled border. stores triglycerides (fats). Here (RB) the cell releases powerful lysosomal Four types of cells are present in bone tissue: enzymes and acids that digest the protein and 1. Osteoprogenitor Cells mineral components of the underlying extracellular - are unspecialized bone stem cells bone matrix. derived from mesenchyme - they are the only bone cells to This breakdown of bone extracellular matrix, termed undergo cell division; the resulting cells bone resorption is part of the normal development, develop into osteoblasts. maintenance, and repair of bone. - found along the inner portion of the (The ending -clast means that the cell breaks down periosteum extracellular matrix.) in the endosteum in the canals within bone that ➔ In response to certain hormones, osteoclasts help contain blood vessels. regulate blood calcium level. They are also target cells for drug therapy used to treat osteoporosis. Mesenchyme ➔ the tissue from which almost all ➔ osteoBlasts Build bone, while osteoClasts Carve out connective tissues are formed. bone 2. Osteoblasts ➔ Bone is not completely solid but has many small - are bone-building cells. spaces between: - they synthesize and secrete collagen fibers ➔ its cells and other organic components needed to build the ➔ extracellular matrix components extracellular matrix of bone tissue, and they initiate calcification ➔ Some spaces serve as channels for blood vessels - as osteoblasts surround themselves with that supply bone cells with nutrients. extracellular matrix, they become trapped in their secretions and become osteocytes. ➔ Other spaces act as storage areas for red bone - do not undergo cell division. marrow. (The ending -blast in the name of a bone cell or any ➔ Depending on the size and distribution of the spaces, other connective tissue cell means that the cell the regions of a bone may be categorized as secretes extracellular matrix. compact or spongy. ➔ Overall, about 80% of the skeleton is compact bone 3. Osteocytes and 20% is spongy bone. - 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.  Compact Bone Tissue - contains few spaces and is the ➔ Neighboring osteocytes communicate via gap strongest form of bone tissue junctions. - it is found beneath the periosteum of all bones and makes up the bulk of the diaphyses of long ➔ The canaliculi connect lacunae with one another and bones with the central canals, forming an intricate, - provides protection and support and resists miniature system of interconnected canals the stresses produced by weight and movement throughout the bone. - compact bone tissue is composed of repeating structural units called osteons, or haversian ➔ System of Interconnected Canals provides many systems routes for nutrients and oxygen to reach the osteocytes and for the removal of wastes. ➔ Each osteon consists of concentric lamellae arranged around an osteonic (haversian or central) ➔ Osteons in compact bone tissue are aligned in the canal. same direction and are parallel to the length of the diaphysis. ➔ Resembling the growth rings of a tree, the concentric lamellae are circular plates of mineralized ➔ As a result, the shaft of a long bone resists bending or extracellular matrix of increasing diameter, fracturing even when considerable force is applied surrounding: from either end. a small network of blood vessels nerves located in the central canal ➔ Compact bone tissue tends to be thickest in those parts of a bone where stresses are applied in ➔ These (CL) tubelike units of bone generally form a relatively few directions. series of parallel cylinders that, in long bones, tend to run parallel to the long axis of the bone. ➔ The lines of stress in a bone are not static. They change as a person learns to walk and in response to ➔ Between the concentric lamellae are small spaces repeated strenuous physical activity, such as weight called lacunae, which contain osteocytes. training. ➔ Radiating in all directions from the lacunae are tiny ➔ The lines of stress in a bone also can change canaliculi, which are filled with extracellular fluid. because of fractures or physical deformity. Thus, the organization of osteons is not static but changes ➔ Inside the canaliculi are slender fingerlike processes over time in response to the physical demands of osteocytes. placed on the skeleton. ➔ The areas between neighboring osteons contain Spongy bone tissue makes up most of the interior lamellae called interstitial lamellae, which also have bone tissue of: lacunae with osteocytes and canaliculi. Short shaped bones Flat shaped bones ➔ Interstitial lamellae are fragments of older osteons Sesamoid shaped bones that have been partially destroyed during bone Irregularly shaped bones rebuilding or growth. In long bones, SB form the core of the epiphyses beneath ➔ Blood vessels and nerves from the periosteum the paper-thin layer of compact bone, and forms a penetrate the compact bone through transverse variable narrow rim bordering the medullary cavity of the interosteonic (Volkmann’s or perforating) canals. diaphysis. ➔ The vessels and nerves of the interosteonic canals Spongy bone is always covered by a layer of compact connect with: bone for protection. medullary cavity periosteum At first glance, the trabeculae of spongy bone tissue may central canals appear to be less organized than the osteons of compact bone tissue. However, they are precisely oriented along ➔ Arranged around the entire outer and inner lines of stress, a characteristic that helps bones resist circumference of the shaft of a long bone are lamellae stresses and transfer force without breaking. called circumferential lamellae. They develop during initial bone formation. Spongy bone tissue tends to be located where bones are not heavily stressed or where stresses are applied ➔ The circumferential lamellae directly deep to the from many directions. periosteum are called external circumferential lamellae. They are connected to the periosteum by The trabeculae do not achieve their final arrangement perforating (Sharpey’s) fibers. until locomotion is completely learned. In fact, the arrangement can even be altered as lines of stress ➔ The circumferential lamellae that line the medullary change due to a poorly healed fracture or a deformity. cavity are called internal circumferential lamellae. Spongy bone tissue is different from compact bone Spongy Bone Tissue tissue in two respects: - In contrast to compact bone tissue, spongy 1. Spongy bone tissue is light, which reduces the bone tissue, also referred to as trabecular or overall weight of a bone. This reduction in cancellous bone tissue, does not contain osteons weight allows the bone to move more readily - is always located in the interior of when pulled by a skeletal muscle. a bone, protected by a covering of compact bone. 2. The trabeculae of spongy bone tissue support - it consists of lamellae that are arranged in an and protect the red bone marrow. irregular pattern of thin columns called trabeculae. Spongy bone in the: Between the trabeculae are spaces that are visible to Hip bones the unaided eye. These macroscopic spaces are Ribs filled with: Sternum (breastbone) red bone marrow in bones that produce blood Vertebrae cells Proximal ends of the humerus and femur yellow bone marrow (adipose tissue) in other bones ➔ is the only site where red bone marrow is stored Both types of bone marrow contain numerous small ➔ and thus, the site where hemopoiesis (blood cell blood vessels that provide nourishment to the production) occurs in adults. osteocytes. ➔ Bone tissue is organized in concentric lamellae Each trabecula consists of: around an osteonic canal in compact bone and in concentric lamellae irregularly arranged lamellae in the trabeculae in osteocytes that lie in lacunae spongy bone. canaliculi that radiate outward from the lacunae 6.4 Blood and Nerve Supply of Bone The ends of long bones are supplied by the metaphyseal Bone is richly supplied with blood/blood vessels. and epiphyseal arteries, which arise from arteries that supply the associated joint. Blood vessels, which are especially abundant in portions of bone containing red bone marrow, pass into bones The metaphyseal arteries enter the metaphyses of a from the periosteum. long bone and, together with the nutrient artery, supply the red bone marrow and bone tissue of the metaphyses. The epiphyseal arteries enter the epiphyses of a long bone and supply the red bone marrow and bone tissue of the epiphyses. 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 2. Numerous epiphyseal veins and metaphyseal veins accompany their respective arteries and exit through the epiphyses and metaphyses, respectively 3. Many small periosteal veins accompany their respective arteries and exit through the periosteum. Nerves accompany the blood vessels that supply bones. The periosteum is rich in sensory nerves, some of which carry pain sensations. These nerves are especially sensitive to tearing or tension, which explains the severe pain resulting from a fracture or a bone tumor. For the same reason, there is some pain associated with Periosteal Arteries a bone marrow needle biopsy. - small arteries accompanied by nerves - enter the diaphysis through many interosteonic In this procedure, a needle is inserted into the middle of (Volkmann’s or perforating) canals the bone to withdraw a sample of red bone marrow to - supply the periosteum and outer part of the compact examine it for conditions such as: bone. Leukemia Metastatic Neoplasm Near the center of the diaphysis, a large nutrient artery Lymphoma passes through a hole in a compact bone called the Hodgkin’s disease nutrient foramen (foramina is plural). Aplastic anemia On entering the medullary cavity, the nutrient artery As the needle penetrates the periosteum, pain is felt. divides into proximal and distal branches that course Once it passes through, there is little pain. toward each end of the bone. 6.5 Bone Formation These proximal and distal branches supply both the Ossification inner part of compact bone tissue of the diaphysis and the - or osteogenesis spongy bone tissue and red bone marrow as far as the - the process by which bone forms epiphyseal plates (or lines). Bone formation occurs in four principal situations: Some bones, like the tibia, have only one nutrient artery; others, like the femur (thigh bone), have several. A. The initial formation of bones in an embryo and fetus  B. The growth of bones during infancy, childhood, 2. Calcification. and adolescence until their adult sizes are - Next, the secretion of the extracellular matrix reached stops,and the cells, now called osteocytes, lie in C. The remodeling of bone (replacement of old lacunae and extend their narrow cytoplasmic bone by new bone tissue throughout life) processes into canaliculi that radiate in all directions. D. The repair of fractures (breaks in bones) - Within a few days, calcium and other mineral throughout life. salts are deposited and the extracellular matrix hardens or calcifies (calcification). A. Initial Formation of Bones in an Embryo and Fetus 3. Formation of trabeculae. - As the bone extracellular matrix forms, it The Embryonic “Skeleton” develops into trabeculae that fuse with one another to - initially composed of mesenchyme in the form spongy bone around the network of blood vessels general shape of bones in the tissue. - is the site where cartilage formation and - Connective tissue associated with the blood ossification occur during the sixth week of embryonic vessels in the trabeculae differentiates into red bone development marrow. - Bone formation follows one of two patterns. 4. Development of the periosteum. The two patterns of bone formation, which both involve - In conjunction with the formation of the replacement of a preexisting connective tissue with trabeculae, the mesenchyme condenses at the bone, do not lead to differences in the structure of periphery of the bone and develops into the mature bones, but are simply different methods of periosteum. bone development. - Eventually, a thin layer of compact bone replaces the surface layers of the spongy bone, but the Intramembranous Ossification spongy bone remains in the center. - the first type of ossification - Much of the newly formed bone is remodeled - bone forms directly within mesenchyme, (destroyed and reformed) as the bone is transformed which is arranged in sheetlike layers that resemble into its adult size and shape. membranes - is the simpler of the two methods of bone formation. Endochondral Ossification - the second type of ossification These are formed in this way (IO): - bone forms within hyaline cartilage The flat bones of the skull that develops from mesenchyme Most of the facial bones - the replacement of cartilage by bone Mandible (lower jawbone) - although most bones of the body are formed in The medial part of the clavicle (collar bone) this way, the process is best observed in a long bone. ➔ Also, the “soft spots” that help the fetal skull pass through the birth canal later harden as they undergo Endochondral Ossification occurs as follows: intramembranous ossification 1. Development of the cartilage model. Intramembranous Ossification occurs as follows: - At the site where the bone is going to form, specific chemical messages cause the cells in 1. Development of the Ossification Center. mesenchyme to crowd together in the general shape - At the site where the bone will develop, of the future bone, and then develop into specific chemical messages cause the cells of the chondroblasts. mesenchyme to cluster together and differentiate, first - The chondroblasts secrete cartilage into osteoprogenitor cells and then into osteoblasts. extracellular matrix, producing a cartilage model - The site of such a cluster is called an (future diaphysis) consisting of hyaline cartilage. ossification center. - A covering called the perichondrium - Osteoblasts secrete the organic extracellular develops around the cartilage model. matrix of bone until they are surrounded by it.  2. Growth of the cartilage model. - This activity leaves a cavity, the medullary - Once chondroblasts become deeply buried in (marrow) cavity, in the diaphysis (shaft). the cartilage extracellular matrix, they are called - Eventually, most of the wall of the diaphysis chondrocytes. is replaced by compact bone. - The cartilage model grows in length by continual cell division of chondrocytes, accompanied by further secretion of the cartilage extracellular matrix. 5. Development of the secondary ossification - This type of cartilaginous growth, called centers. interstitial (endogenous) growth (growth from within), - When branches of the epiphyseal artery results in an increase in length. enter the epiphyses, secondary ossification centers - In contrast, growth of the cartilage in develop, usually around the time of birth. thickness is due mainly to the deposition of - Bone formation is similar to what occurs in extracellular matrix material on the cartilage surface of primary ossification centers. the model by new chondroblasts that develop from the - However, in the secondary ossification perichondrium. centers spongy bone remains in the interior of the - This process is called appositional epiphyses (no medullary cavities are formed here). (exogenous) growth meaning growth at the outer - In contrast to primary ossification, secondary surface. ossification proceeds outward from the center of the - As the cartilage model continues to grow, epiphysis toward the outer surface of the bone. chondrocytes in its midregion hypertrophy (increase in size) and the surrounding cartilage extracellular matrix 6. Formation of articular cartilage and the begin to calcify. epiphyseal (growth) plate. - Other chondrocytes within the calcifying - The hyaline cartilage that covers the cartilage die because nutrients can no longer diffuse epiphyses becomes the articular cartilage. quickly enough through the extracellular matrix. - Prior to adulthood, hyaline cartilage remains - As these chondrocytes die, the spaces left between the diaphysis and epiphysis as the epiphyseal behind by dead chondrocytes merge into small cavities (growth) plate, the region responsible for the called lacunae. lengthwise growth of long bones that you will learn about next. 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 osteoprogenitor 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.  B. Bone Growth During Infancy, Childhood, - The term “resting” is used because and Adolescence. the cells do not function in bone growth. Rather, they anchor the epiphyseal plate to the During infancy, childhood, and adolescence, epiphysis of the bone. bones throughout the body grow in thickness by appositional growth, and long bones 2. Zone of proliferating cartilage. lengthen by the addition of bone material on the - Slightly larger chondrocytes in this diaphyseal side of the epiphyseal plate by zone are arranged like stacks of coins. interstitial growth. - These chondrocytes undergo interstitial growth as they divide and secrete Growth in Length extracellular matrix. The growth in length of long bones involves - The chondrocytes in this zone divide the following two major events: to replace those that die at the diaphyseal side of the epiphyseal plate. interstitial growth of cartilage on the epiphyseal side of the epiphyseal plate 3. Zone of hypertrophic cartilage - This layer consists of large, maturing replacement of cartilage on the diaphyseal chondrocytes arranged in columns. side of the epiphyseal plate with bone by endochondral ossification 4. Zone of calcified cartilage. - The final zone of the epiphyseal plate To understand how a bone grows in length, you is only a few cells thick and consists mostly of need to know some of the details of the chondrocytes that are dead because the structure of the epiphyseal plate. extracellular matrix around them has calcified. - Osteoclasts dissolve the calcified The Epiphyseal (growth) Plate cartilage, and osteoblasts and capillaries from - is a layer of hyaline cartilage in the the diaphysis invade the area. metaphysis of a growing bone that consists of - The osteoblasts lay down bone four zones: extracellular matrix, replacing the calcified cartilage by the process of endochondral 1. Zone of resting cartilage. ossification. - This layer is nearest to the epiphysis - Recall that endochondral ossification and consists of small, scattered chondrocytes. is the replacement of cartilage with bone.  - As a result, the zone of calcified ➔ It should also be kept in mind that closure of cartilage becomes the “new diaphysis” that is the epiphyseal plate, on average, takes place firmly cemented to the rest of the diaphysis of 1–2 years earlier in females. the bone. Growth in Thickness ➔ The activity of the epiphyseal plate is the Like cartilage, bone can grow in thickness only way that the diaphysis can increase in (diameter) only by appositional growth: length. 1. At the bone surface, periosteal cells ➔ As a bone grows, chondrocytes proliferate on differentiate into osteoblasts, which secrete the the epiphyseal side of the plate. collagen fibers and other organic molecules that form bone extracellular matrix. ➔ New chondrocytes replace older ones, which are destroyed by calcification. Thus, the The osteoblasts become surrounded by cartilage is replaced by bone on the diaphyseal extracellular matrix and develop into side of the plate. osteocytes. ➔ In this way the thickness of the epiphyseal plate This process forms bone ridges on either side remains relatively constant, but the bone on the of a periosteal blood vessel. diaphyseal side increases in length. The ridges slowly enlarge and create a groove ➔ If a bone fracture damages the epiphyseal for the periosteal blood vessel. plate, the fractured bone may be shorter than normal once adult stature is reached. 2. Eventually, the ridges fold together and fuse, and the groove becomes a tunnel that encloses ➔ This is because damage to cartilage, which is the blood vessel. avascular, accelerates closure of the epiphyseal plate due to the cessation of The former periosteum now becomes the cartilage cell division, thus inhibiting lengthwise endosteum that lines the tunnel. growth of the bone. 3. Osteoblasts in the endosteum deposit bone ➔ When adolescence comes to an end (at about extracellular matrix, forming new concentric age 18 in females and age 21 in males), the lamellae. epiphyseal plates close; that is, the epiphyseal cartilage cells stop dividing and bone replaces The formation of additional concentric lamellae all remaining cartilage. proceeds inward toward the periosteal blood vessels. ➔ The epiphyseal plate fades, leaving a bony structure called the epiphyseal line. In this way, the tunnel fills in, and a new osteon is created. ➔ With the appearance of the epiphyseal line, bone growth in length stops completely. 4. As an osteon is forming, osteoblasts under the periosteum deposit new circumferential ➔ Closure of the epiphyseal plate is a gradual lamellae, further increasing the thickness of the process and the degree to which it occurs is bone. useful in determining bone age, predicting adult height, and establishing age at death from As additional periosteal blood vessels skeletal remains, especially in infants, children, become enclosed as in step 1 , the growth and adolescents. process continues. ➔ For example, an open epiphyseal plate ➔ Recall that as new bone tissue is being indicates a younger person, while a partially deposited on the outer surface of bone, the closed epiphyseal plate or a completely closed bone tissue lining the medullary cavity is one indicates an older person. destroyed by osteoclasts in the endosteum. In this way, the medullary cavity enlarges as the bone increases in thickness.  C. REMODELING OF BONE ➔ Since the strength of bone is related to the Like skin, bone forms before birth but degree to which it is stressed, if newly formed continually renews itself thereafter. bone is subjected to heavy loads, it will grow thicker and therefore be stronger than the old Bone Remodeling bone. - is the ongoing replacement of old bone tissue by new bone tissue. ➔ Also, the shape of a bone can be altered for - it involves bone resorption & bone proper support based on the stress patterns deposition experienced during the remodeling process. - also takes place at different rates in different regions of the body ➔ Finally, new bone is more resistant to fracture - also removes injured bone, replacing than old bones. it with new bone tissue - may be triggered by factors such as: ➔ During the process of bone resorption, an Exercise osteoclast attaches tightly to the bone surface Sedentary Lifestyle at the endosteum or periosteum and forms a Changes in Diet leak proof seal at the edges of its ruffled border. Bone Resorption - the removal of minerals and collagen ➔ Then it releases protein-digesting lysosomal fibers from bone by osteoclasts enzymes and several acids into the sealed - results in the destruction of bone pocket. extracellular matrix ➔ The enzymes digest collagen fibers and other Bone Deposition organic substances while the acids dissolve - the addition of minerals and collagen the bone minerals. fibers to bone by osteoblasts - results in the formation of bone ➔ Working together, several osteoclasts carve extracellular matrix out a small tunnel in the old bone. ➔ At any given time, about 5% of the total bone ➔ The degraded bone proteins and extracellular mass in the body is being remodeled. matrix minerals, mainly calcium and phosphorus, enter an osteoclast by ➔ The distal portion of the femur is replaced endocytosis, cross the cell in vesicles, and about every four months. undergo exocytosis on the side opposite the ruffled border. ➔ By contrast, bone in certain areas of the shaft of the femur will not be replaced completely ➔ Now in the interstitial fluid, the products of during an individual’s life. bone resorption diffuse into nearby blood capillaries. ➔ Even after bones have reached their adult shapes and sizes, old bone is continually ➔ Once a small area of bone has been resorbed, destroyed and new bone is formed in its place. osteoclasts move in to rebuild the bone in that area. ➔ Remodeling has several other benefits:  D. Factors Affecting Bone Growth Sex Hormones and Bone Remodeling - at puberty, the secretion of hormones known as sex hormones causes a dramatic Normal bone metabolism—growth in the young effect on bone growth. and bone remodeling in the adult—depends on - include: several factors. These include adequate dietary Estrogens (produced by the intake of minerals and vitamins, as well as ovaries) sufficient levels of several hormones: Androgens such as testosterone (produced by the 1. Minerals. testes) ➔ Large amounts of calcium and phosphorus are needed while bones are growing ➔ Although females have much higher levels of estrogens and males have higher levels of ➔ As are smaller amounts of magnesium, androgens, females also have low levels of fluoride, and manganese, these minerals are androgens, and males have low levels of also necessary during bone remodeling. estrogens. 2. Vitamins. ➔ The adrenal glands of both sexes produce ➔ Vitamin A stimulates activity of osteoblasts. androgens, and other tissues, such as adipose tissue, can convert androgens to estrogens. ➔ Vitamin C is needed for synthesis of collagen, the main bone protein. ➔ These hormones (A&E) are responsible for: increased osteoblast activity ➔ Vitamin D helps build bone by increasing the synthesis of bone extracellular matrix absorption of calcium from foods in the sudden “growth spurt” that occurs gastrointestinal tract into the blood. during the teenage years. ➔ Vitamins K and B12 are also needed for ➔ Estrogens also promote changes in the synthesis of bone proteins. skeleton that are typical of females, such as widening of the pelvis. 3. Hormones. ➔ During childhood, the hormones most ➔ Ultimately sex hormones, especially estrogens important to bone growth are the insulin-like in both sexes, shut down growth at epiphyseal growth factors (IGFs), which are produced by (growth) plates, causing elongation of the the liver and bone tissue. bones to cease. IGFs (insulin-like growth factors) ➔ Lengthwise growth of bones typically ends - are produced in response to the earlier in females than in males due to their secretion of growth hormone (GH) from the higher levels of estrogens. anterior lobe of the pituitary gland - they: ➔ During adulthood, sex hormones contribute to stimulate osteoblasts bone remodeling by slowing resorption of old promote cell division at the bone and promoting deposition of new bone. epiphyseal plate and in the periosteum ➔ One way that estrogens slow resorption is by enhance synthesis of the promoting apoptosis (programmed death) of proteins needed to build a new osteoclasts. bone ➔ Parathyroid hormone, calcitriol (the active form ➔ Thyroid hormones (T3 and T4) from the of vitamin D), and calcitonin are other thyroid glands also promote bone growth by hormones that can affect bone remodeling. stimulating osteoblasts. ➔ Moderate weight-bearing exercises maintain ➔ In addition, the hormone insulin from the sufficient strain on bones to increase and pancreas promotes bone growth by increasing maintain their density. the synthesis of bone proteins.  Orthodontics ➔ Dwarfism is a condition of small stature in - is the branch of dentistry concerned which the height of an individual is typically with the prevention and correction of poorly under 4 feet 10 inches, usually averaging 4 aligned teeth feet. - the movement of teeth by braces places a stress on the bone that forms the ➔ Generally, there are two types of dwarfism: sockets that anchor the teeth. proportionate - in response to this artificial stress, disproportionate osteoclasts and osteoblasts remodel the sockets so that the teeth align properly Proportionate Dwarfism - all parts of the body are small but they Paget’s Disease are proportionate to each other ➔ A delicate balance exists between the actions - one cause of proportionate dwarfism of osteoclasts and osteoblasts. is called pituitary dwarfism. ➔ Should too much new tissue be formed, the ➔ Pituitary Dwarfism bones become abnormally thick and heavy. - a hyposecretion of GH during childhood and the condition is ➔ If too much mineral material is deposited in the appropriately bone, the surplus may form thick bumps, - the condition can be treated called spurs, on the bone that interfere with medically with administration of GH movement at joints. until epiphyseal plate closure. ➔ Excessive loss of calcium or tissue weakens Disproportionate Dwarfism the bones and they may break, as occurs in - some parts of the body are normal osteoporosis, or they may become too flexible, size or larger than normal while others are as in rickets and osteomalacia. smaller than normal. - for example, the trunk can be average ➔ In Paget’s disease, there is an excessive size while the limbs are short and the head may proliferation of osteoclasts so that bone be large in relation to the rest of the body, with resorption occurs faster than bone deposition. a prominent forehead and flattened nose at the bridge. ➔ In response, osteoblasts attempt to - most common cause of this type of compensate, but the new bone is weaker dwarfism is a condition called achondroplasia because it has a higher proportion of spongy to compact bone, mineralization is decreased, ➔ Achondroplasia and the newly synthesized extracellular matrix - an inherited condition in contains abnormal proteins. which the conversion of hyaline cartilage to bone is abnormal and the ➔ The newly formed bone, especially that of the long bones of the limbs stop growing in pelvis, limbs, lower vertebrae, and skull, childhood. becomes enlarged, hard, and brittle and - other bones are unaffected, fractures easily. and thus the person has short stature but a normal size head and trunk. Hormonal Abnormalities That Affect Height - this type of dwarfism is called achondroplastic dwarfism, the condition ➔ Excessive or deficient secretion of hormones is essentially untreatable, although that normally control bone growth can cause a some individuals opt for person to be abnormally tall or short. limb-lengthening surgery. ➔ Oversecretion of growth hormone (GH) during childhood produces giantism, in which a person becomes much taller and heavier than normal.  6.6 Fracture and Repair of Bone The reparative phase is characterized by Fracture two events: - is any break in a bone the formation of a fibrocartilaginous - named according to their: callus severity formation of a bony callus to bridge the the shape or position of the gap between the broken ends of the fracture line bones or even the physician who first described them 2a. Reparative Phase: Fibrocartilaginous - in some cases, a bone may fracture Callus Formation. without visibly breaking ➔ Blood vessels grow into the fracture hematoma and phagocytes begin to clean up Stress Fracture dead bone cells. - is a series of microscopic fissures in bone that forms without any evidence of injury ➔ Fibroblasts from the periosteum invade the to other tissues. I fracture site and produce collagen fibers. - in healthy adults, stress fractures result from repeated, strenuous activities such ➔ In addition, cells from the periosteum develop as running, jumping, or aerobic dancing into chondroblasts and begin to produce - are quite painful and also result from fibrocartilage in this region. disease processes that disrupt normal bone calcification, such as osteoporosis ➔ These events lead to the development of a - about 25% of stress fractures fibrocartilaginous (soft) callus, a mass of repair involve the tibia. tissue consisting of collagen fibers and - although standard x-ray images often cartilage that bridges the broken ends of the fail to reveal the presence of stress fractures, bone. they show up clearly in a bone scan. - the repair of a bone fracture ➔ Formation of the fibrocartilaginous callus takes involves the following phases: about 3 weeks. 1. Reactive Phase. 2b. Reparative Phase: Bony Callus ➔ This phase is an early inflammatory phase. Formation. ➔ In areas closer to well-vascularized healthy ➔ Blood vessels crossing the fracture line are bone tissue, osteoprogenitor cells develop into broken. osteoblasts, which begin to produce spongy bone trabeculae. ➔ As blood leaks from the torn ends of the vessels, a mass of blood (usually clotted) ➔ The trabeculae join living and dead portions of forms around the site of the fracture. the original bone fragments. ➔ This mass of blood, called a fracture ➔ In time, the fibrocartilage is converted to hematoma, usually forms 6 to 8 hours after the spongy bone, and the callus is then referred to injury. as a bony (hard) callus. ➔ Because the circulation of blood stops at the ➔ The bony callus lasts about 3 to 4 months. site where the fracture hematoma forms, nearby bone cells die. 3. Bone Remodeling Phase. ➔ Swelling and inflammation occur in response ➔ The final phase of fracture repair is bone to dead bone cells, producing additional remodeling of the callus. cellular debris. ➔ Dead portions of the original fragments of ➔ Phagocytes (neutrophils and macrophages) broken bones are gradually reabsorbed by and osteoclasts begin to remove the dead or osteoclasts. damaged tissue in and around the fracture hematoma. ➔ Compact bone replaces spongy bone around the periphery of the fracture. ➔ This stage may last up to several weeks. ➔ Sometimes, the repair process is so thorough ➔ The ultimate goals of fracture treatment are that the fracture line is undetectable, even in a realignment of the bone fragments, radiograph (x-ray). immobilization to maintain realignment, and restoration of function. ➔ However, a thickened area on the surface of the bone remains as evidence of a healed ➔ For bones to unite properly, the fractured ends fracture. must be brought into alignment. This process, called reduction, is commonly referred to as ➔ Although bone has a generous blood supply, setting a fracture. healing sometimes takes months. ➔ In closed reduction, the fractured ends of a ➔ The calcium and phosphorus needed to bone are brought into alignment by manual strengthen and harden new bone are manipulation, and the skin remains intact. deposited only gradually, and bone cells generally grow and reproduce slowly. ➔ In open reduction, the fractured ends of a bone are brought into alignment by a surgical ➔ The temporary disruption in their blood supply procedure using internal fixation devices such also helps explain the slowness of healing of as screws, plates, pins, rods, and wires. severely fractured bones. ➔ Following reduction, a fractured bone may be Treatment for Fractures kept immobilized by a cast, sling, splint, elastic ➔ Treatments for fractures vary according to bandage, external fixation device, or a age, type of fracture, and the bone involved. combination of these devices. 6.7 Bone’s Role in Calcium detects the intracellular increase in cyclic AMP Homeostasis (the input). Bone is the body’s major calcium reservoir, As a result, PTH synthesis speeds up, and storing 99% of total body calcium. more PTH (the output) is released into the blood. One way to maintain the level of calcium in the blood is to control the rates of calcium The presence of higher levels of PTH increases resorption from bone into blood and of calcium the number and activity of osteoclasts deposition from blood into bone. (effectors), which step up the pace of bone resorption. Both nerve and muscle cells depend on a stable level of calcium ions (Ca2+) in The resulting release of Ca2+ from bone into extracellular fluid to function properly. blood returns the blood Ca2+ level to normal. Blood clotting also requires Ca2+. Also, many PTH also acts on the kidneys (effectors) to enzymes require Ca2+ as a cofactor (an decrease loss of Ca2+ in the urine, so more is additional substance needed for an enzymatic retained in the blood. reaction to occur). And PTH stimulates formation of calcitriol (the For this reason, the blood plasma level of Ca2+ active form of vitamin D), a hormone that is very closely regulated between 9 and 11 promotes absorption of calcium from foods in mg/100 mL. the gastrointestinal tract into the blood. Both of these actions also help elevate blood Even small changes in Ca2+ concentration Ca2+ level. outside this range may prove fatal: the heart may stop (cardiac arrest) if Another hormone works to decrease blood the concentration goes too high Ca2+ level. When blood Ca2+ rises above normal, parafollicular cells in the thyroid gland or breathing may cease (respiratory secrete calcitonin (CT): arrest) if the level falls too low inhibits activity of osteoclasts speeds blood Ca2+ uptake by bone The role of bone in calcium homeostasis is to accelerates Ca2+ deposition into bones help “buffer” the blood Ca2+ level, releasing Ca2+ into blood plasma (using osteoclasts) The net result is that CT promotes bone when the level decreases, and absorbing Ca2+ formation and decreases blood Ca2+ level. (using osteoblasts) when the level rises. Despite these effects, the role of CT in normal Ca2+ exchange is regulated by hormones, the calcium homeostasis is uncertain because it most important of which is parathyroid hormone can be completely absent without causing (PTH) secreted by the parathyroid glands.This symptoms. hormone increases blood Ca2+ level. Nevertheless, calcitonin harvested from salmon PTH secretion operates via a negative (Miacalcin®) is an effective drug for treating feedback system. osteoporosis because it slows bone resorption. If some stimulus causes the blood Ca2+ level to decrease, parathyroid gland cells (receptors) detect this change and increase their production of a molecule known as cyclic adenosine monophosphate (cyclic AMP). The gene for PTH within the nucleus of a parathyroid gland cell (the control center)  6.8 Exercise and Bone Tissue ➔ As the level of sex hormones diminishes during middle ➔ Within limits, bone tissue has the ability to alter its age, especially in women after menopause, a decrease in strength in response to changes in mechanical stress. bone mass occurs because bone resorption by osteoclasts outpaces bone deposition by osteoblasts. ➔ When placed under stress, bone tissue becomes stronger through increased deposition of mineral salts ➔ In old age, loss of bone through resorption occurs more and production of collagen fibers by osteoblasts. rapidly than bone gain. Because women’s bones generally are smaller and less massive than men’s bones ➔ Without mechanical stress, bone does not remodel to begin with, loss of bone mass in old age typically has a normally because bone resorption occurs more greater adverse effect in females. These factors quickly than bone formation. contribute to the higher incidence of osteoporosis in females. ➔ Research has shown that high-impact intermittent strains more strongly influence bone deposition as ➔ There are two principal effects of aging on bone tissue: compared with lower-impact constant strains. loss of bone mass and brittleness. ➔ Therefore, running and jumping stimulate bone ➔ Loss of bone mass results from demineralization, the loss remodeling more dramatically than walking. of calcium and other minerals from bone extracellular matrix. ➔ The main mechanical stresses on bone are those that result from the pull of skeletal muscles and the pull of ➔ This loss usually begins after age 30 in females, gravity. accelerates greatly around age 45 as levels of estrogens decrease, and continues until as much as 30% of the ➔ If a person is bedridden or has a fractured bone in a calcium in bones is lost by age 70. cast, the strength of the unstressed bones diminishes because of the loss of bone minerals and decreased ➔ Once bone loss begins in females, about 8% of bone numbers of collagen fibers. mass is lost every 10 years. ➔ Astronauts subjected to the microgravity of space ➔ In males, calcium loss typically does not begin until after also lose bone mass. In any of these cases, bone age 60, and about 3% of bone mass is lost every 10 loss can be dramatic—as much as 1% per week. years. ➔ In contrast, the bones of athletes, which are ➔ The loss of calcium from bones is one of the problems in repetitively and highly stressed, become notably osteoporosis. thicker and stronger than those of astronauts or nonathletes. ➔ The second principal effect of aging on the skeletal system, brittleness, results from a decreased rate of ➔ Weight-bearing activities, such as walking or protein synthesis. moderate weight lifting, help build and retain bone mass. ➔ Recall that the organic part of bone extracellular matrix, mainly collagen fibers, gives bone its tensile strength. ➔ Adolescents and young adults should engage in regular weight-bearing exercise prior to the closure of the epiphyseal plates to help build total mass prior to ➔ The loss of tensile strength causes the bones to become its inevitable reduction with aging. very brittle and susceptible to fracture. ➔ However, people of all ages can and should ➔ In some elderly people, collagen fiber synthesis slows, in strengthen their bones by engaging in weight-bearing part due to diminished production of growth hormone. exercise. ➔ In addition to increasing the susceptibility to fractures, 6.9 Aging and Bone Tissue loss of bone mass also leads to: ➔ From birth through adolescence, more bone tissue is Deformity produced than is lost during bone remodeling. Pain Loss of Height ➔ In young adults, the rates of bone deposition and Loss of Teeth resorption are about the same. Summary of Factors That Affect Bone Growth Minerals ○ Calcium and Phosphorus - Make bone extracellular matrix hard. ○ Magnesium - Helps form bone extracellular matrix. ○ Fluoride - Helps strengthen bone extracellular matrix. ○ Manganese - Activates enzymes involved in synthesis of bone extracellular matrix. Vitamins ○ Vitamin A - Needed for the activity of osteoblasts during remodeling of bone - Efficiency stunts bone growth - Toxic in high doses ○ Vitamin C - Needed for synthesis of collagen - The main bone protein - Deficiency leads to decreased collagen production, which slows down bone growth and delays repair of broken bones. ○ Vitamin D - Active form (calcitriol) is produced by the kidneys - Helps build bone by increasing absorption of calcium from gastrointestinal tract into blood - Deficiency causes faulty calcification and slows down bone growth - May reduce the risk of osteoporosis but is toxic if taken in high doses - People who have minimal exposure to ultraviolet rays or do not take vitamin D supplements may not have sufficient vitamin D to absorb calcium - This interferes with calcium metabolism. ○ Vitamin K & B12 - Needed for synthesis of bone proteins - Deficiency leads to abnormal protein production in the bone extracellular matrix and decreased bone density. Hormones ○ Growth Hormone (GH) - Secreted by the anterior lobe of the pituitary gland - Promotes general growth of all body tissues, including bone, mainly by stimulating production of insulin-like growth factors. ○ Insulin-like Growth Factors (IGFs) - Secreted by the liver, bones, and other tissues on stimulation by growth hormone - Promotes normal bone growth by stimulating osteoblasts and by increasing the synthesis of proteins needed to build new bone. ○ Thyroid Hormones (T2 &T4) - Secreted by thyroid gland - Promote normal bone growth by stimulating osteoblasts. ○ Insulin - Secreted by the pancreas - Promotes normal bone growth by increasing the synthesis of bone proteins.  ○ Sex Hormones (estrogens & testosterone) - Secreted by the ovaries in women (estrogens) and by the testes in men (testosterone) - Stimulate osteoblasts and promote the sudden “growth spurt” that occurs during the teenage years - Shut down growth at the epiphyseal plates around age 18–21, causing lengthwise growth of bone to end - Contribute to bone remodeling during adulthood by slowing bone resorption by osteoclasts and promoting bone deposition by osteoblasts. ○ Parathyroid Hormone (PTH) - Secreted by the parathyroid glands - Promotes bone resorption by osteoclasts - Enhances recovery of calcium ions from urine - Promotes formation of the active form of vitamin D (calcitriol) ○ Calcitonin - Secreted by the thyroid gland; inhibits bone resorption by osteoclasts. Exercise - Weight-bearing activities stimulate osteoblasts and, consequently, help build thicker, stronger bones and retard loss of bone mass that occurs as people age. Aging - As the level of sex hormones diminishes during middle age to older adulthood, especially in women after menopause, bone resorption by osteoclasts outpaces bone deposition by osteoblasts, which leads to a decrease in bone mass and an increased risk of osteoporosis. Disorders: Homeostatic Imbalances of increased metabolism that absorb more of the radioactive tracer due to increased blood flow. Bone Scan - is a diagnostic procedure that takes ➔ Hot spots may indicate bone cancer, abnormal advantage of the fact that bone is living tissue. healing of fractures, or abnormal bone growth. - a small amount of a radioactive tracer compound that is readily absorbed by bone is injected ➔ Lighter areas or “cold spots” are areas of decreased intravenously. metabolism that absorb less of the radioactive tracer - the degree of uptake of the tracer is related due to decreased blood flow. to the amount of blood flow to the bone. ➔ Cold spots may indicate problems such as ➔ A scanning device (gamma camera) measures the degenerative bone disease, decalcified bone, radiation emitted from the bones, and the information fractures, bone infections, Paget’s disease, or is translated into a photograph that can be read like rheumatoid arthritis. an x-ray on a monitor. ➔ A bone scan detects abnormalities 3 to 6 months ➔ Normal bone tissue is identified by a consistent gray sooner than standard x-ray procedures and exposes color throughout because of its uniform uptake of the the patient to less radiation. radioactive tracer. ➔ A bone scan is the standard test for bone density ➔ Darker or lighter areas may indicate bone screening, particularly important in screening for abnormalities. Darker areas or “hot spots” are areas osteoporosis in females  Osteoporosis - a condition of porous bones ➔ There is also a relatively new tool called FRAX® that - affects 10 million people a year in the USA incorporates risk factors besides bone mineral density - In addition, 18 million people have low bone to accurately estimate fracture risk. mass (osteopenia), which puts them at risk for osteoporosis. ➔ Patients fill out an online survey of risk factors such - The basic problem is that bone resorption as age, gender, height, weight, ethnicity, prior fracture (breakdown) outpaces bone deposition (formation). history, parental history of hip fracture, use of - In large part this is due to depletion of glucocorticoids (for example, cortisone), smoking, calcium from the body—more calcium is lost in urine, alcohol intake, and rheumatoid arthritis. feces, and sweat than is absorbed from the diet. - Bone mass becomes so depleted that bones ➔ Using the data, FRAX® provides an estimate of the fracture, often spontaneously, under the mechanical probability that a person will suffer a fracture of the stresses of everyday living, for example, a hip fracture hip or other major bone in the spine, shoulder, or might result from simply sitting down too quickly. forearm due to osteoporosis within 10 years. - In the United States, osteoporosis causes more than 1.5 million fractures a year, mainly in the ➔ Treatment options for osteoporosis are varied. With hips, wrists, and vertebrae. regard to nutrition, a diet high in calcium is important - Osteoporosis affects the entire skeletal to reduce the risk of fractures. system. In addition to fractures, osteoporosis causes shrinkage of vertebrae, height loss, hunched backs, ➔ Vitamin D is necessary for the body to utilize calcium. and bone pain. In terms of exercise, regular performance of - Osteoporosis primarily affects middle-aged weight-bearing exercises has been shown to maintain and elderly people, 80% of them women and build bone mass. These exercises include - Older women suffer from osteoporosis more walking, jogging, hiking, climbing stairs, playing often than men for two reasons: tennis, and dancing. Resistance exercises, such as weight lifting, also build bone strength and muscle 1) Women’s bones are less massive than men’s mass. bones 2) Production of estrogens in women declines ➔ Medications used to treat osteoporosis are generally dramatically at menopause, whereas of two types: production of the main androgen, testosterone, in older men wanes gradually 1. Antireabsorptive Drugs slow down the and only slightly. progression of bone loss 2. Bone-building Drugs promote increasing ➔ Estrogens and testosterone stimulate osteoblast bone mass. activity and synthesis of bone matrix. ➔ Among the antireabsorptive drugs are: ➔ Besides gender, risk factors for developing osteoporosis include a family history of the disease, Bisphosphonates - which inhibit osteoclasts European or Asian ancestry, thin or small body build, ○ Fosamax® an inactive lifestyle, cigarette smoking, a diet low in ○ Actonel® calcium and vitamin D, more than two alcoholic drinks ○ Boniva® a day, and the use of certain medications. ○ Calcitonin) ➔ Osteoporosis is diagnosed by taking a family history Selective Estrogen Receptor Modulators - and undergoing a bone mineral density (BMD) test. mimic the effects of estrogens without unwanted side effects Bone Mineral Density (BMD) Test ○ Raloxifene® - Performed like x-rays ○ Evista® - measure bone density. - They can also be used to confirm a diagnosis Estrogen Replacement Therapy (ERT)- which of osteoporosis, determine the rate of bone loss, and replaces estrogens lost during and after monitor the effects of treatment. menopause ○ Premarin®  Hormone Replacement Therapy (HRT) - which Osteomyelitis replaces estrogens and progesterone lost - An infection of bone characterized by: during and after menopause High fever ○ Prempro® Sweating Chills ➔ ERT helps maintain and increase bone mass after Pain menopause. Women on ERT have a slightly Nausea increased risk of stroke and blood clots. Pus formation Edema ➔ HRT also helps maintain and increase bone mass. Warmth over the affected bone and Women on HRT have increased risks of heart rigid overlying muscles. disease, breast cancer, stroke, blood clots, and - it is often caused by bacteria, usually dementia. Staphylococcus aureus. ➔ Among the bone-building drugs is: The bacteria may reach the bone from: outside the body (through open fractures, Parathyroid Hormone (PTH) - which stimulates penetrating wounds, or orthopedic surgical osteoblasts to produce new bone procedures) ○ (Forteo®). from other sites of infection in the body ○ Others are under development (abscessed teeth, burn infections, urinary tract infections, or upper respiratory infections) Rickets and Osteomalacia via the blood - are two forms of the same disease that result from adjacent soft tissue infections (as occurs from inadequate calcification of the extracellular bone in diabetes mellitus). matrix - usually caused by a vitamin D deficiency Osteopenia - Reduced bone mass due to a decrease in Rickets the rate of bone synthesis to a level too low to - is a disease of children in which the growing compensate for normal bone resorption bones become “soft ” or rubbery and are easily - any decrease in bone mass below normal deformed. - an example is osteoporosis. - Because new bone formed at the epiphyseal (growth) plates fails to ossify, bowed legs and Osteosarcoma deformities of the skull, rib cage, and pelvis are - Bone cancer that primarily affects common. osteoblasts and occurs most often in teenagers during their growth spurt Osteomalacia - the most common sites are the metaphyses - is the adult counterpart of rickets, sometimes of the thigh bone (femur), shin bone (tibia), and arm called adult rickets. bone (humerus). - New bone formed during remodeling fails to - Metastases occur most often in lungs calcify, and the person experiences varying degrees of - treatment consists of multidrug pain and tenderness in bones, especially the hip and chemotherapy and removal of the malignant growth, or legs. Bone fractures also result from minor trauma. amputation of the limb. Prevention and treatment for rickets and osteomalacia consists of the administration of adequate vitamin D and exposure to moderate amounts of sunlight. Osteoarthritis - The degeneration of articular cartilage such that the bony ends touch - the resulting friction of bone against bone worsens the condition. - usually associated with the elderly ➔ A bone is made up of several different tissues: bone ➔ Nerves accompany blood vessels in bone; the or osseous tissue, cartilage, dense connective tissue, periosteum is rich in sensory neurons. epithelium, adipose tissue, and nervous tissue. ➔ The process by which bone forms, called ossification, ➔ The entire framework of bones and their cartilages occurs in four principal situations: (1) the initial constitutes the skeletal system. formation of bones in an embryo and fetus; (2) the growth of bones during infancy, childhood, and ➔ The skeletal system functions in support, protection, adolescence until their adult sizes are reached; (3) movement, mineral homeostasis, blood cell the remodeling of bone (replacement of old bone by production, and triglyceride storage. new bone tissue throughout life); and (4) the repair of fractures (breaks in bones) throughout life. ➔ Parts of a typical long bone are the diaphysis (shaft), proximal and distal epiphyses (ends), metaphyses, ➔ Bone development begins during the sixth or seventh articular cartilage, periosteum, medullary (marrow) week of embryonic development. cavity, and endosteum. ➔ The two types of ossification, intramembranous and ➔ Bone tissue consists of widely separated cells endochondral, involve the replacement of a surrounded by large amounts of extracellular matrix. preexisting connective tissue with bone. ➔ The four principal types of cells in bone tissue are ➔ Intramembranous ossification refers to bone osteoprogenitor cells, osteoblasts (bone-building formation directly within mesenchyme arranged in cells), osteocytes (maintain daily activity of bone), sheetlike layers that resemble membranes. and osteoclasts (bone-destroying cells). ➔ Endochondral ossification refers to bone formation ➔ The extracellular matrix of bone contains abundant within hyaline cartilage that develops from mineral salts (mostly hydroxyapatite) and collagen mesenchyme. fibers. ➔ The primary ossification center of a long bone is in ➔ Compact bone tissue consists of osteons (haversian the diaphysis. Cartilage degenerates, leaving cavities systems) with little space between them. that merge to form the medullary cavity. Osteoblasts lay down bone. Next, ossification occurs in the ➔ Compact bone tissue lies over spongy bone tissue in epiphyses, where bone replaces cartilage, except for the epiphyses and makes up most of the bone tissue the epiphyseal (growth) plate. of the diaphysis. ➔ The epiphyseal plate consists of four zones: zone of ➔ Functionally, compact bone tissue is the strongest resting cartilage, zone of proliferating cartilage, zone form of bone and protects, supports, and resists of hypertrophic cartilage, and zone of calcified stress. cartilage. ➔ Spongy bone tissue does not contain osteons. It ➔ Because of the cell division in the epiphyseal (growth) consists of trabeculae surrounding many red bone plate, the diaphysis of a bone increases in length. marrow–filled spaces. ➔ Bone grows in thickness or diameter due to the ➔ Spongy bone tissue forms most of the structure of addition of new bone tissue by periosteal osteoblasts short, flat, and irregular bones, and the interior of the around the outer surface of the bone (appositional epiphyses in long bones. growth). ➔ Functionally, spongy bone tissue trabeculae offer ➔ Bone remodeling is an ongoing process in which resistance along lines of stress, support and protect osteoclasts carve out small tunnels in old bone tissue red bone marrow, and make bones lighter for easier and then osteoblasts rebuild it. movement. ➔ In bone resorption, osteoclasts release enzymes and ➔ Long bones are supplied by periosteal, nutrient, acids that degrade collagen fibers and dissolve metaphyseal, and epiphyseal arteries; veins mineral salts. accompany the arteries. ➔ Dietary minerals (especially calcium and phosphorus) and vitamins (A, C, D, K, and B12) are needed for ➔ Calcitonin (CT) from the thyroid gland has the bone growth and maintenance. potential to decrease blood Ca2+ level. ➔ Insulin-like growth factors (IGFs), growth hormone, ➔ Vitamin D enhances absorption of calcium and thyroid hormones, and insulin stimulate bone growth. phosphate and thus raises the blood levels of these Sex hormones slow resorption of old bone and substances. promote new bone deposition. ➔ Mechanical stress increases bone strength by ➔ A fracture is any break in a bone. Types of fractures increasing deposition of mineral salts and production include closed (simple), open (compound), of collagen fibers. comminuted, greenstick, impacted, stress, Pott, and Colles. ➔ Removal of mechanical stress weakens bone through demineralization and collagen fiber reduction. ➔ Fracture repair involves formation of a fracture hematoma during the reactive phase, ➔ The principal effect of aging is demineralization, a fibrocartilaginous callus and bony callus formation loss of calcium from bones, which is due to reduced during the reparative phase, and a bone remodeling osteoblast activity. phase. ➔ Another effect is decreased production of ➔ Bone is the major reservoir for calcium in the body. extracellular matrix proteins (mostly collagen fibers), which makes bones more brittle and thus more ➔ Parathyroid hormone (PTH) secreted by the susceptible to fracture. parathyroid glands increases blood Ca2+ level.