bio ch 6 c

Osteoblasts add to bone matrix and osteoclasts remove the matrix, thus maintaining a delicate balance. Osteoblast and osteoclast activity are affected by hormonal levels (discussed in section 6.5a), the body's need for calcium and/or phosphorus, and gravitational or mechanical stressors to bone. For example, when a person wears orthodontic braces, osteoblasts and osteoclasts work together to modify the tooth-jaw junction, in response to the mechanical stress applied by the braces to the teeth and jaw. If osteoclasts resorb the bone to remove calcium salts at a faster rate than osteoblasts produce matrix to stimulate deposition, bones lose mass and become weaker; in contrast, when osteoblast activity outpaces osteoclast activity, bones have a greater mass.

The matrix of bone connective tissue has both organic and inorganic components. About one-third of bone mass is composed of organic components called osteoid. It includes cells, collagen fibers, and ground substance. (The ground substance is the semisolid material that suspends and supports the collagen fibers.) The collagen fibers give a bone tensile strength by resisting stretching and twisting, and contribute to its overall flexibility. The inorganic components of the bone provide its compressional strength. Calcium phosphate, Ca3(PO4)2, accounts for most of the inorganic components of bone. Calcium phosphate and calcium hydroxide interact to form crystals of hydroxyapatite (hī-drok′sē-ap-ă-tīt), which is Ca10(PO4)6(OH)2. These crystals deposit around the collagen fibers, leading to hardening of the extracellular matrix. The crystals also incorporate other salts, such as calcium carbonate, and ions, such as sodium, magnesium, sulfate, and fluoride, in the process of calcification.

Finally, abnormal amounts of certain hormones can affect bone maintenance and growth. Recall that chronically low levels of growth hormone and/or thyroid hormone in a child inhibit bone growth and result in short stature. Another example are the glucocorticoids, a group of hormones produced by the adrenal cortex. Normal glucocorticoid levels tend not to have any major effects on bone growth or mass. However, if glucocorticoid levels are chronically too high, they stimulate bone resorption and can lead to significant loss of bone mass. This result is particularly concerning for children on corticosteroid treatment for chronic disorders such as Crohn disease.

A continual dietary source of vitamins is required for normal bone growth and maintenance. For example, vitamin A activates osteoblasts, whereas vitamin C is required for normal synthesis of collagen, the primary organic component in the bone matrix. Vitamin D stimulates the absorption and transport of calcium and phosphate ions into the blood. It also is necessary for the calcification of bone. As calcium and phosphate levels rise in the blood, calcitonin is secreted, which encourages the deposition of these minerals into bone.

A fracture hematoma forms. A bone fracture tears blood vessels inside the bone and within the periosteum, causing bleeding, and then a fracture hematoma forms from the clotted blood. A fibrocartilaginous (soft) callus forms. Regenerated blood capillaries infiltrate the fracture hematoma due to an increase in osteoblasts in both the periosteum and the endosteum near the fracture site. First, the fracture hematoma is reorganized into an actively growing connective tissue called a procallus. Fibroblasts within the procallus produce collagen fibers that help connect the broken ends of the bones. Chondroblasts in the newly growing connective tissue form a dense regular connective tissue associated with the cartilage. Eventually, the procallus becomes a fibrocartilaginous (soft) callus (kal′ŭs; hard skin). The fibrocartilaginous callus stage lasts at least 3 weeks.

A hard (bony) callus forms. Within a week, osteoprogenitor cells in areas adjacent to the fibrocartilaginous callus become osteoblasts and produce trabeculae of primary bone. The fibrocartilaginous callus is then replaced by this bone, which forms a hard (bony) callus. The trabeculae of the hard callus continue to grow and thicken for several months. The bone is remodeled. Remodeling is the final phase of fracture repair. The hard callus persists for at least 3 to 4 months as osteoclasts remove excess bony material from both exterior and interior surfaces. Compact bone replaces primary bone. The fracture usually leaves a slight thickening of the bone (as detected by x-ray); however, in many instances healing occurs with no obvious thickening

Bone Male Age at Epiphyseal Union (years) Humerus, lateral epicondyle 11-16 (female: 9-13) Humerus, medial epicondyle 11-16 (female: 10-15) Humerus, head 14.5-23.5 Proximal radius 14-19 Distal radius 17-22 Distal fibula and tibia 14.5-19.5 Proximal tibia 15-22 Femur, head 14.5-23.5 Distal femur 14.5-21.5 Clavicle 19-30

As with cartilage growth, a long bone's growth in length is called interstitial growth, and its growth in diameter or thickness is called appositional growth. Interstitial growth occurs within the epiphyseal plate as chondrocytes undergo mitotic cell division in zone 2 and chondrocytes hypertrophy in zone 3. These activities combine to push the zone of resting cartilage toward the epiphysis. It is the flexible matrix of hyaline cartilage that permits this growth. Once growth in length has occurred, new bone connective tissue is produced at the same rate in zone 5. Thus, growth in length is due to growth in hyaline cartilage connective tissue that is later replaced with bone (endochondral ossification).

Mechanical stress, in the form of exercise, is required for normal bone remodeling. In response to mechanical stress, bone has the ability to increase its strength over a period of time by increasing the amounts of mineral salts deposited and collagen fibers synthesized. Stress also increases the production of the hormone calcitonin, which helps inhibit bone resorption by osteoclasts and encourage bone deposition by osteoblasts. Mechanical stresses that significantly affect bone result from repeated skeletal muscle contraction and gravitational forces. Typically, the bones of athletes become noticeably thicker as a result of repetitive and stressful exercise. Weight-bearing activities, such as weight lifting or walking, help build and retain bone mass. In contrast, lack of mechanical stress weakens bone through both demineralization of the bone matrix and reduction of collagen formation. For example, if a person has a fractured bone in a cast or is bedridden, the mass of the unstressed bone decreases in the immobilized limbs. While in space, astronauts must exercise so that the lesser gravity won't weaken their bones.

Bone has great mineral strength, and yet it may break as a result of unusual stress or a sudden impact. Breaks in bones, called fractures, are classified in several ways. They can be named from the cause: stress, trauma, or pathology. A stress fracture is a thin break caused by recent increased physical activity in which the bone experiences repetitive loads (e.g., as seen in some runners). Stress fractures tend to occur in the weight-bearing bones (e.g., pelvis and lower limb). A traumatic fracture is a result of impact or excess stress to the bone, and a pathologic fracture usually occurs in bone that has been weakened by disease, such as when the vertebrae fracture in someone with osteoporosis (a bone condition discussed in Clinical View 6.6 "Osteoporosis" in section 6.7). osteoblast is reduded

Two types of bone connective tissue are present in most of the bones of the body: compact bone (also called dense or cortical bone) and spongy bone (also called cancellous or trabecular bone). As their names imply, compact bone is solid and relatively dense, whereas spongy bone appears more porous, like a sponge. The arrangement of compact bone and spongy bone components differs at the microscopic level. In a long bone, compact bone forms the solid external walls of the bone, and spongy bone is located internally, primarily within the epiphyses. Spongy bone forms an open lattice of narrow plates of bone, called trabeculae (tră-bek′yū-lē; sing., trabecula, tră-bek′yū-lă; trabs = a beam). In a flat bone of the skull, the spongy bone, also called diploë (dip′lō-ē; diplous = double), is sandwiched between two layers of compact bone

Compact bone has an organized structure when viewed under the microscope. A cylindrical osteon (os′tē-on), or Haversian system, is the basic functional and structural unit of mature compact bone. Osteons run parallel to the diaphysis of the long bone. When viewed in cross Page 154section, an osteon has the appearance of a bull's-eye target. An osteon is a three-dimensional structure that has several components

1.Ossification centers form within thickened regions of mesenchyme, beginning in the eighth week of development. Some cells in the thickened, condensed mesenchyme divide, and the committed cells that result differentiate into osteoprogenitor cells. Some osteoprogenitor cells become osteoblasts, which secrete the semisolid organic components of the bone matrix called osteoid. Multiple ossification centers develop within the thickened mesenchyme as the number of osteoblasts increases. 2. Osteoid undergoes calcification. Osteoid formation is quickly followed by initiation of the process of calcification, as calcium salts are deposited onto the osteoid and then crystallize (solidify). Both organic matrix formation and calcification occur simultaneously at several sites within the condensed mesenchyme. When calcification entraps osteoblasts within lacunae in the matrix, the entrapped cells become osteocytes. 3. Woven bone and its surrounding periosteum form. Initially, the newly formed bone connective tissue is immature and not well organized, a type called woven bone, or primary bone. Eventually, woven bone is replaced by lamellar bone, or secondary bone. The mesenchyme that still surrounds the woven bone begins to thicken and eventually organizes to form the periosteum. Mesenchymal cells continue to grow and develop to produce additional osteoblasts. Newly formed blood vessels also branch throughout this region. The calcified trabeculae and intertrabecular spaces are composed of spongy bone.

Endochondral (en′dō-kon′drăl) ossification begins with a hyaline cartilage model and produces most of the other bones of the skeleton, including those of the upper and lower limbs, the pelvis, the vertebrae, and the ends of the clavicle. Long bone development in the limb is a good example of this process, which takes place in the following steps (figure 6.11): The fetal hyaline cartilage model develops. During the eighth to twelfth week of development, chondroblasts secrete cartilage matrix, and a hyaline cartilage model forms. Within this cartilage model, are chondrocytes trapped within lacunae. A perichondrium surrounds the cartilage.

Periosteal blood vessels (periosteal arteries and periosteal veins) provide blood to the external circumferential lamellae and the superficial osteons within the compact bone at the external edge of the bone. These vessels and the accompanying periosteal nerves penetrate the diaphysis and enter the perforating canals at many locations. Nerves that supply bones accompany blood vessels through the nutrient foramen and innervate the bone as well as its periosteum, endosteum, and marrow cavity. These are mainly sensory nerves that signal injuries to the skeleton.

Growth hormone Stimulates liver to produce the hormone somatomedin, which causes cartilage proliferation at epiphyseal plate and resulting bone elongation; too little growth hormone results in short stature in the child Thyroid hormone Stimulates bone growth by stimulating metabolic rate of osteoblasts; too little thyroid hormone results in short stature Calcitonin Promotes calcium deposition in bone and inhibits osteoclast activity Parathyroid hormone Increases blood calcium levels by encouraging bone resorption by osteoclasts Sex hormones (estrogen and testosterone) Stimulate osteoblasts; promote epiphyseal plate growth and closure Glucocorticoids If levels are chronically too high, bone resorption occurs and significant bone mass is lost

Bone connective tissue and the bones that compose the skeletal system perform several basic functions: support and protection, movement, hemopoiesis, and storage of mineral and energy reserves. Support and Protection Bones provide structural support and serve as a framework for the entire body. Bones also protect many delicate tissues and organs from injury and trauma. The rib cage protects the heart and lungs, the cranial bones enclose and protect the brain, the vertebrae enclose the spinal cord, and the pelvis cradles some digestive, urinary, and reproductive organs. Movement Bones serve as attachment sites for skeletal muscles, other soft tissues, and some organs. Muscles attached to the bones of the skeleton contract and exert a pull on the skeleton that then functions as a system of levers. The bones of the skeleton can alter the direction and magnitude of the forces generated by the skeletal muscles. Potential movements range from powerful contractions needed for running and jumping to delicate, precise movements required to remove a splinter from a finger.

Hemopoiesis (hē′mō-poy-ē′sis; haima = blood, poiesi = making) is the process of blood cell production. It occurs in a connective tissue called red bone marrow, which is located in some spongy bone. Red bone marrow contains stem cells that form all blood cells and platelets (see section 21.4). The locations of red bone marrow differ between children and adults. In children, red bone marrow is located in the spongy bone and the medullary cavity of most of the bones of the body. As children mature into adults, much of the red bone marrow degenerates and turns into a fatty tissue called yellow bone marrow. As a result, adults have red bone marrow only in selected portions of the axial skeleton, such as the flat bones of the skull, the vertebrae, the ribs, the sternum (breastbone), and the ossa coxae (hip bones). Adults also have red bone marrow in the proximal epiphyses of each humerus and femur.

Bone replaces cartilage, except the articular cartilage and epiphyseal plates. By late stages of bone development, almost all of the hyaline cartilage is replaced by bone. Hyaline cartilage is found only as articular cartilage on the articular surface of each epiphysis, and as a region called the epiphyseal (ep′i-fiz′ē-ăl) plate, sandwiched between the diaphysis and the epiphysis. Lengthwise growth continues until the epiphyseal plates ossify and form epiphyseal lines. Bone growth continues through puberty until ossification of the epiphyseal plates, indicating the bone has reached its adult length. Page 159Eventually, the only remnant of each epiphyseal plate is an internal thin line of compact bone called an epiphyseal line. Depending upon the bone, most epiphyseal plates fuse between the ages of 10 and 25. (The last epiphyseal plates to ossify are those of the clavicle in the late 20s.)

Interstitial growth is dependent upon growth of cartilage within the epiphyseal plate. The epiphyseal plate exhibits five distinct microscopic zones, which are continuous from the first zone (zone 1) nearest the epiphysis to the last zone (zone 5) nearest the diaphysis

Ossification (os′i-fi-kā′shŭn; os = bone, facio = to make), or osteogenesis (os′tē-ō-jen′ĕ-sis; genesis = beginning), refers to the formation and development of bone connective tissue. Ossification begins in the embryo and continues as the skeleton grows during Page 157childhood and adolescence. Even after the adult bones have formed, ossification continues. By the eighth through twelfth weeks of development, the skeleton begins forming from either thickened condensations of mesenchyme or a hyaline cartilage model of bone. Thereafter, these models are replaced by hard bone.

Intramembranous (in′tră-mem′brā-nŭs) ossification literally means "bone growth within a membrane," and is so named because the thin layer of mesenchyme in these areas is sometimes referred to as a membrane. Intramembranous ossification is also sometimes called dermal ossification, because the mesenchyme that is the source of these bones is in the area of the future dermis. Recall from section 4.2c that mesenchyme is an embryonic connective tissue that has mesenchymal cells and abundant ground substance. Intramembranous ossification produces the flat bones of the skull, some of the facial bones (zygomatic bone, maxilla), the mandible (lower jaw), and the central part of the clavicle (collarbone). It begins when mesenchyme becomes thickened and condensed with a dense supply of blood capillaries, and continues in the following steps

Distinctive bone markings, or surface features, characterize each bone in the body. Projections from the bone surface mark the point where tendons and ligaments attach. Sites of articulation between adjacent bones are smooth areas. Depressions, grooves, and tunnels through bones indicate sites where blood vessels and nerves travel. Anatomists use specific terms to describe these elevations and depressions

Knowing the names of bone markings will help you learn about specific bones in chapters 7 and 8. For example, knowing that foramen means "hole" or "passageway" will tell you to look for a hole when trying to find the foramen magnum on the skull. Likewise, you can usually correctly assume that any smooth, oval prominence on a bone is called a condyle. Refer back to figure 6.17 frequently for assistance in learning the bones and their features. For professional criminologists, pathologists, and anthropologists, bones can tell an intricate anatomic story. Bone markings on skeletal remains indicate where the soft tissue components once were, often allowing an individual's height, age, sex, and general appearance to be determined.

A tough sheath called periosteum (per′ē-os′tē-ŭm; peri = around) covers the outer surface of the bone, except for the areas covered by articular cartilage. Periosteum is formed by dense irregular connective tissue and consists of an outer fibrous layer and an inner cellular layer (figure 6.5). Page 152The periosteum is anchored to the bone by numerous strong collagen fibers called perforating fibers, (also known as Sharpey fibers) which run perpendicular to the diaphysis. The periosteum protects the bone from surrounding structures, anchors blood vessels and nerves to the surface of the bone, and provides stem cells (osteoprogenitor cells and osteoblasts) for bone growth in width and fracture repair.

Long bones support soft tissues in the limbs. The femur, the bone of the thigh, is shown in both (a) anterior and (b) sectional views. (c) A typical long bone, such as the femur, contains both compact and spongy bone. Osteoprogenitor (os′tē-ō-prō-jen′i-tŏr; osteo = bone) cells are stem cells derived from mesenchyme. When they divide, they produce another stem cell and a "committed cell" that matures to become an osteoblast. These stem cells are located in both the periosteum and the endosteum.

Bone continues to grow and renew itself throughout life. The continual deposition of new bone connective tissue and the removal (resorption) of old bone connective tissue is called bone remodeling. Bone remodeling helps maintain calcium and phosphate levels in body fluids, and can be stimulated by stress on a bone (e.g., bone fracture, or exercise that builds up muscles that attach to bone). This ongoing process occurs at both the periosteal and endosteal surfaces of a bone. It either modifies the architecture of the bone or changes the total amount of minerals deposited in the skeleton. Prior to and throughout puberty, the formation of bone typically exceeds its resorption. In young adults, the processes of formation and resorption tend to occur at about the same rate. However, they become disproportionate in older adults when resorption of bone exceeds its formation.

Nutrient blood vessels, called the nutrient artery and the nutrient vein, supply the diaphysis of a long bone. Typically, only one nutrient artery enters and one nutrient vein leaves the bone via a nutrient foramen in the bone. These vessels branch and extend along the length of the shaft toward the epiphyses and into the central canal of osteons within compact bone and the marrow cavity. Metaphyseal blood vessels (metaphyseal arteries and metaphyseal veins) provide the blood supply to the diaphyseal side of the epiphyseal plate, which is the region where new bone ossification forms bone connective tissue to replace epiphyseal plate cartilage. Epiphyseal arteries and epiphyseal veins provide the blood supply to the epiphyses of the bone. In early childhood, the cartilaginous epiphyseal plate separates the epiphyseal and metaphyseal vessels. However, once an epiphyseal plate ossifies and becomes an epiphyseal line, the epiphyseal vessels and metaphyseal vessels anastomose (interconnect) through channels formed in the epiphyseal line (see figure 6.14 for examples).

Osteoblasts are formed from osteoprogenitor stem cells. Often, osteoblasts exhibit a somewhat cuboidal structure. They secrete the initial semisolid, organic form of bone matrix called osteoid (os′tē-oyd; eidos = resemblance). Osteoid later calcifies and hardens as a result of calcium salt deposition. Osteoblasts produce new bone, and once osteoblasts become entrapped in the matrix they produce and secrete, they differentiate into osteocytes. Osteocytes are mature bone cells derived from osteoblasts that have become entrapped in the matrix they secreted. They reside in small spaces within the matrix called lacunae. Osteocytes maintain the bone matrix and detect mechanical stress on a bone. This information is communicated to osteoblasts, and may result in the deposition of new bone matrix at the surface.

Osteoclasts (os′tē-ō-klast′; klastos = broken) are large, multinuclear, phagocytic cells. They are derived from fused bone marrow cells similar to those that produce monocytes (described in section 21.3b). These cells exhibit a ruffled border where they contact the bone, which increases their surface area exposure to the bone. An osteoclast is often located within or adjacent to a depression or pit on the bone surface called a resorption lacuna (Howship lacuna). Osteoclasts are involved in an important process called bone resorption that takes place as follows: Osteoclasts secrete hydrochloric acid, which dissolves the mineral parts (calcium and phosphate) of the bone matrix, while lysosomes within the osteoclasts secrete enzymes that dissolve the organic part of the matrix. The release of the stored calcium and phosphate from the bone matrix is called osteolysis (os′tē-ol′i-sis; lysis = dissolution, loosening). The liberated calcium and phosphate ions enter the tissue fluid and then the blood.

Two hormones regulate calcium levels in the blood. Calcitonin (kal′si-tō′nin; calx = lime, tonos = stretching) is secreted by cells in the thyroid gland in response to elevated levels of calcium in the blood. Calcitonin promotes calcium deposition from blood into bone and inhibits osteoclast activity that resorbs bone. Parathyroid hormone is produced and released by the parathyroid glands in response to reduced calcium levels in the blood. It stimulates osteoclasts to resorb bone and thereby increase calcium levels in the blood. Parathyroid hormone presence also influences the production of calcitriol in the kidneys. Together they increase the release of calcium from bone.

Sex hormones (estrogen and testosterone), which begin to be secreted in great amounts at puberty, dramatically accelerate bone growth. Sex hormones increase the rate of bone formation by osteoblasts in ossification centers within the epiphyseal plate, resulting in increased length of long bones and increased height. The appearance of high levels of sex hormones at puberty also signals the beginning of the end for growth at the epiphyseal plate. Eventually, more bone is produced at the epiphyseal plate than the cartilage within the plate can support. As a result, the thickness of the epiphyseal plate cartilage begins to diminish, and eventually it disappears altogether, leaving behind the epiphyseal line. Older individuals (who have a normal reduction in sex hormones) may experience a decrease in bone mass as they age.

Perforating canals (Volkmann canals) resemble central canals in that they also contain blood vessels and nerves. However, perforating canals run perpendicular to the central canals and help connect multiple central canals, thus creating a vascular and innervation connection among the multiple osteons. Circumferential lamellae are rings of bone immediately internal to the periosteum of the bone (external circumferential lamellae) or internal to the endosteum (internal circumferential lamellae). These two distinct regions appear during the original formation of the bone. Both external and internal circumferential lamellae run the entire circumference of the bone itself (hence, their name). Page 156 Interstitial lamellae are the leftover parts of osteons that have been partially resorbed. They often look like a "bite" has been taken out of them. The interstitial lamellae are incomplete and typically have no central canal

Spongy bone contains no osteons (figure 6.9c). Instead, the trabeculae of spongy bone are composed of parallel lamellae (see figure 6.8c). Between adjacent lamellae are osteocytes resting in lacunae, with numerous canaliculi radiating from the lacunae. Nutrients reach the osteocytes by diffusion through canaliculi that open onto the surfaces of the trabeculae. Note that the trabeculae often form a meshwork of crisscrossing bars and plates of bone pieces. This structure provides great resistance to stresses applied in many directions by distributing the stress throughout the entire framework. As an analogy, visualize the jungle gym climbing apparatus on a children's playground. It is capable of supporting the weight of numerous children whether they are distributed throughout its structure or all localized in one area. This is accomplished because stresses and forces are distributed throughout the structure.

Cartilage calcifies, and a periosteal bone collar forms. Within the center of the cartilage model (future diaphysis), chondrocytes start to hypertrophy (enlarge) and resorb (eat away) some of the surrounding cartilage matrix, producing larger holes in the matrix. As these chondrocytes enlarge, the cartilage matrix begins to calcify. Chondrocytes in this region die and disintegrate because nutrients cannot diffuse to them through this calcified matrix. The result is a calcified cartilage shaft with large holes in the place where chondrocytes had been. As the cartilage in the shaft is calcifying, blood vessels grow toward the cartilage and start to penetrate the perichondrium around the shaft. Stem cells within the perichondrium divide to form osteoblasts. As the osteoblasts develop, this supporting connective tissue becomes highly vascularized, and the perichondrium becomes a periosteum. The osteoblasts within the internal layer of the periosteum start secreting a layer of osteoid around the calcified cartilage shaft. The osteoid hardens and forms a periosteal bone collar around this shaft.

The primary ossification center forms in the diaphysis. A growth of capillaries and osteoblasts, called a periosteal bud, extends from the periosteum into the core of the cartilage shaft, invading the spaces left by the chondrocytes. The remains of the calcified cartilage serve as a template on which osteoblasts begin to produce osteoid. This region is called the primary ossification center because it is the first major center of bone formation. Bone development extends in both directions toward the epiphyses from the primary ossification center. Healthy bone tissue quickly replaces the calcified, degenerating cartilage in the shaft. Most, but not all, primary ossification centers have formed by the twelfth week of development. Secondary ossification centers form in the epiphyses. The same basic process that formed the primary ossification center occurs later in the epiphyses. Beginning around the time of birth, the hyaline cartilage in the center of each epiphysis calcifies and begins to degenerate. Epiphyseal blood vessels and osteoprogenitor cells enter each epiphysis. Secondary ossification centers form as bone replaces calcified cartilage. Not all secondary ossification centers form at birth; some form later in childhood. As the secondary ossification centers form, osteoclasts resorb some bone matrix within the diaphysis, creating a hollow medullary cavity.

Epiphyseal Plate. (a) In a growing long bone, the epiphyseal plate, located at the boundary between the diaphysis and the epiphysis, exhibits five distinct but continuous zones. Zones 1-4 are cartilage, and zone 5 is bone. (b) An x-ray of a child's hand shows the cartilaginous epiphyseal plates as dark lines between the epiphysis and the diaphysis of long bones. Fusion of an epiphyseal plate is progressive, and is usually scored as follows: Open (no bony fusion or union between the epiphysis and the other bone end) Partial union (some fusion between the epiphysis and the rest of the bone, but a distinct line of separation may be seen) Complete union (all visible aspects of the epiphysis are united to the rest of the bone)

Zone of resting cartilage. This zone is farthest from the medullary cavity of the diaphysis and nearest the epiphysis. It is composed of small chondrocytes distributed throughout the cartilage matrix, and resembles mature, healthy hyaline cartilage. This region secures the epiphysis to the epiphyseal plate. Zone of proliferating cartilage. Chondrocytes in this zone undergo rapid mitotic cell division, enlarge slightly, and become aligned like a stack of coins into longitudinal columns of flattened lacunae. These columns are parallel to the diaphysis. Page 161 Zone of hypertrophic cartilage. Chondrocytes cease dividing and begin to hypertrophy (enlarge) greatly. The walls of the lacunae become thin because the chondrocytes resorb matrix as they hypertrophy. Zone of calcified cartilage. This zone usually is composed of two to three layers of cartilage. Minerals are deposited in the matrix between the columns of lacunae; this calcification kills the chondrocytes and makes the matrix appear opaque. Zone of ossification. The walls break down between lacunae in the columns, forming longitudinal channels. These spaces are invaded by capillaries and osteoprogenitor cells from the medullary cavity. New matrix of bone is deposited on the remaining calcified cartilage matrix.

More than 90% of the body's reserves of the minerals calcium and phosphate are stored within and released by bone. Calcium is an essential mineral for body functions such as muscle contraction, blood clotting, and nerve impulse transmission. Phosphate is a structural component of ATP, nucleotides, and phospholipids. When calcium or phosphate is needed by the body, some bone connective tissue is broken down, and the minerals are released into the blood. In addition, potential energy in the form of lipids is stored in yellow bone marrow, which is located in the shafts of adult long bones. Irregular bones have elaborate, complex shapes and do not fit into any of the preceding categories. These bones have compact bone covering internal spongy bone. The vertebrae, ossa coxae (hip bones), and several bones in the skull, such as the ethmoid and sphenoid bones, are examples of irregular bones.

Bones of the human skeleton occur in various shapes and sizes, depending upon their function. The four classes of bone as determined by shape are long bones, short bones, flat bones, and irregular bones (figure 6.3). Long bones have a greater length than width. These bones have an elongated, cylindrical shaft (diaphysis). This is the most common bone shape. Long bones occur in the upper limb (namely, the arm, forearm, palm, and fingers) and lower limb (thigh, leg, sole of the foot, and toes). Long bones vary in size; the small bones in the fingers and toes are long bones, as are the larger tibia and fibula of the lower limb. Short bones have a length nearly equal to their width. The external surfaces of short bones are covered by compact bone, and their interior is composed of spongy bone. Examples of short bones include the carpals (wrist bones) and tarsals (bones in the foot). Sesamoid bones, which are tiny, seed-shaped bones along the tendons of some muscles, are also classified as short bones. The patella (kneecap) is the largest sesamoid bone. Flat bones are so named because they have flat, thin surfaces. These bones are composed of roughly parallel surfaces of compact bone with a layer of internally placed spongy bone. They provide extensive surfaces for muscle attachment and protect underlying soft tissues. Flat bones form the roof of the skull, the scapulae (shoulder blades), the sternum (breastbone), and the ribs.

The central canal (Haversian canal) is a cylindrical channel that lies in the center of the osteon. Traveling within the central canal are the blood vessels and nerves that supply the bone. Concentric lamellae (lă-mel′-ē; sing., lamella, lă-mel′ă; lamina = plate, leaf) are rings of bone connective tissue that surround the central canal and form the bulk of the osteon. The numbers of concentric lamellae vary among osteons. Each lamella contains collagen fibers oriented in one direction; adjacent lamellae contain collagen fibers oriented in perpendicular directions. In other words, if one lamella has collagen fibers directed superiorly and to the right, the next lamella will have collagen fibers directed superiorly and to the left. This alternating collagen fiber direction gives bone part of its strength and resilience. Osteocytes are housed in lacunae and occur between adjacent concentric lamellae.

Canaliculi (kan′ă-lik′yū-lī; sing., canaliculus, kan′ă-lik′yū-lŭs; canalis = canal) are tiny, interconnecting channels within the bone connective tissue that extend from each lacuna, travel through the lamellae, and connect to other lacunae and the central canal. Canaliculi house osteocyte cytoplasmic projections that permit intercellular contact and communication. Thus, nutrients, minerals, gases, and wastes can travel through these passageways between the central canal and the osteocytes.

Vitamin A Activates osteoblasts Vitamin C (ascorbic acid) Promotes collagen production Vitamin D Promotes absorption of calcium and phosphate into blood; helps with calcification of bone

Hormones are molecules released from one cell into the blood and they travel throughout the body to affect other cells (see section 20.1a). Hormones control and regulate bone growth and maintenance by altering the rates of osteoblast and osteoclast activity. Growth hormone, also called somatotropin (sō′mă-tō-trō′pin), is produced by the anterior pituitary gland. It affects bone growth by stimulating the liver to form another hormone called insulin-like growth factor (IGF), also called somatomedin (sō′mă-tō-mē′din). Growth hormone and IGF directly stimulate growth of cartilage in the epiphyseal plate. Thyroid hormone, secreted by the thyroid gland, stimulates bone growth by influencing the basal metabolic rate of bone cells. Together, growth hormone and thyroid hormone, if maintained in proper balance, regulate and maintain normal activity at the epiphyseal plates until puberty. If a child's growth hormone and/or thyroid hormone levels are chronically too low, then bone growth is adversely affected, and the child will be short in stature.

Appositional growth is an increase in width along the outside edge, or periphery, of the cartilage. Three steps occur in this process (figure 6.2b): Undifferentiated stem cells at the internal edge of the perichondrium begin to divide. (Note: The perichondrium contains mesenchymal cells as well as these stem cells.) New undifferentiated stem cells and committed cells that differentiate into chondroblasts are formed. These chondroblasts are located at the periphery of the old cartilage, where they begin to produce and secrete new cartilage matrix. The chondroblasts push apart as a result of matrix formation, and become chondrocytes, with each occupying its own lacuna. The cartilage continues to grow at the periphery as chondrocytes continue to produce more matrix.

The bones of the skeleton are complex, dynamic organs containing all tissue types. Their primary component is bone connective tissue, also called osseous (os′ē-ŭs) connective tissue (see section 4.2d). In addition, they contain connective tissue proper (periosteum), cartilage connective tissue (articular cartilage), smooth muscle tissue (forming the walls of blood vessels that supply bone), fluid connective tissue (blood), epithelial tissue (lining the inside opening of blood vessels), and nervous tissue (nerves that supply bone). The matrix of bone connective tissue is sturdy and rigid due to deposition of minerals in the matrix, a process called calcification (kal′si-fi-kā′shŭn), or mineralization.

Fractures are categorized by the amount of soft tissue damage associated with the fracture. In a simple fracture, the broken bone does not penetrate the skin, whereas in a compound fracture, one or both ends of the broken bone pierce the overlying skin and body tissues. Fractures are also known through eponyms (such as Colles or Pott), which have specific characteristics describing these fractures. Most often fractures are classified by the description of the fracture. Figure 6.15 shows some of the different classifications of fractures. Often many fracture classifications are used to describe a single fracture. For instance, a Colles fracture is a complete, transverse fracture of the distal radius with posterior displacement.

The healing of a simple fracture takes about 2 to 3 months, whereas a compound fracture takes longer to heal. Fractures heal much more quickly in young children (average healing time, 3 weeks) and become slower to heal as we age. In the elderly, the normal thinning and weakening of bone increases the incidence of fractures, and some severe fractures never heal without surgical intervention. Bone fracture repair can be described as a series of steps

Long bones, the most common bone shape in the body, serve as a useful model of bone structure. One example of a long bone is the femur (thigh bone) (figure 6.4). A typical long bone contains the following parts: One of the principal gross features of a long bone is its shaft, which is called the diaphysis (dī-af′i-sis; pl., diaphyses, dī-af′i-sēz; growing between). The elongated, usually cylindrical diaphysis provides for the leverage and major weight support of a long bone. At each end of a long bone is an expanded, knobby region called the epiphysis (e-pif′i-sis; pl., epiphyses, e-pif′i-sēz; epi = upon, physis = growth). The epiphysis is enlarged to strengthen the joint and provide added surface area for bone-to-bone articulation as well as tendon and ligament Page 151attachment. It is composed of an outer layer of compact bone and an inner layer of spongy bone. A proximal epiphysis is the end of the bone closest to the body trunk, and a distal epiphysis is the end farthest from the trunk.

The metaphysis (mĕ-taf′i-sis) is the region in a mature bone sandwiched between the diaphysis and the epiphysis. In a growing bone, this region contains the epiphyseal (growth) plate, thin layers of hyaline cartilage that provide for the continued lengthwise growth of the diaphysis. In adults, the remnant of the epiphyseal plate is a thin layer of compact bone called the epiphyseal line. The thin layer of hyaline cartilage covering the epiphysis at a joint surface is called articular cartilage. This cartilage helps reduce friction and absorb shock in movable joints. The hollow, cylindrical space within the diaphysis is called the medullary cavity (marrow cavity). In adults, it contains yellow bone marrow. The endosteum (en-dos′tē-ŭm; endo = within, osteon = bone) is an incomplete layer of cells that covers all internal surfaces of the bone, such as the medullary cavity. The endosteum contains osteoprogenitor cells, osteoblasts, and osteoclasts (figure 6.5), and is active during bone growth, repair, and remodeling.

Cartilage has three major functions in the body: Supporting tissues. For example, C-shaped hyaline cartilage rings in the trachea support the connective tissue and musculature of the tracheal wall; fibrocartilage provides both toughness and flexibility to the pubic symphysis and Page 148intervertebral discs; and flexible elastic cartilage supports the fleshy, external part of the ear called the auricle (aw′ri-kl; auris = ear). Providing a gliding surface at articulations (joints), where two bones meet. Providing a model for the formation of most of the bones in the body. Beginning in the embryonic period, cartilage serves as a "rough draft" form that is later replaced by bone tissue.

Cartilage grows in two ways. Growth from within the cartilage itself is termed interstitial (in′tĕr-stish′ăl) growth. Growth along the cartilage's outside edge, or periphery, is called appositional (ap′ō-zish′ŭn-ăl) growth Interstitial growth occurs within the internal regions of cartilage through a series of four steps (figure 6.2a): Chondrocytes housed within lacunae are stimulated to undergo mitotic cell division. Following cell division, two cells occupy a single lacuna; they are now called chondroblasts. As chondroblasts begin to synthesize and secrete new cartilage matrix, they are pushed apart. These cells now reside in their own lacuna and are called chondrocytes. The cartilage continues to grow in the internal region as chondrocytes continue to produce more matrix.

mention of the skeletal system conjures up images of dry, lifeless bones in various sizes and shapes. But the skeleton (skel′ĕ-tŏn; skeletos = dried) is much more than a supporting framework for the soft tissues of the body. The skeletal system is composed of dynamic living tissues; it interacts with all of the other organ systems and continually rebuilds and remodels itself. Our skeletal system includes the bones of the skeleton as well as cartilage, ligaments, and other connective tissues that stabilize or connect the bones. Bones support our weight and interact with muscles to produce precisely controlled movements. This interaction permits us to sit, stand, walk, and run. Further, our bones serve as vital reservoirs for calcium and phosphorus. We first examine the cartilage components of the skeleton before concentrating on the bone connective tissue.

Cartilage is found throughout the human body (figure 6.1). Cartilage is a semirigid connective tissue that is weaker than bone, but more flexible and resilient (see section 4.2d). As with all connective tissue types, cartilage contains a population of cells scattered throughout a matrix of protein fibers embedded within a gel-like ground substance. Chondroblasts (kon′drō-blast; chondros = grit or gristle, blastos = germ) are the cells that produce the matrix of cartilage. Once they become encased within the matrix they have produced and secreted, the cells are called chondrocytes (kon′drō-sīt; cyte = cell) and occupy small spaces named lacunae. These mature cartilage cells maintain the matrix and ensure that it remains healthy and viable. Mature cartilage is avascular (not penetrated by blood vessels) so nutrients must diffuse through the matrix. The three different types of cartilage—hyaline, elastic, and fibrocartilage—are described in detail in section 4.2d, so only cartilage functions, locations, and growth will be discussed in this chapter.