Chapter6 Functions of Bone and the Skeletal System

Blood cell production.

Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis (hēm-ō-poy-ē-sis; hemo- = blood; -poiesis = making)

Besides gender, risk factors for developing osteoporosis include

a family history of the disease, European or Asian ancestry, thin or small body build, an inactive lifestyle, cigarette smoking, a diet low in calcium and vitamin D, more than two alcoholic drinks a day, and the use of certain medications.

Yellow bone marrow

fatty tissue found in the medullary cavity of most adult long bones

Rickets is a disease of

children in which the growing bones become "soft" or rubbery and are easily deformed. Because new bone formed at the epiphyseal (growth) plates fails to ossify, bowed legs and deformities of the skull, rib cage, and pelvis are common.

As the cartilage model continues to grow

chondrocytes in its midregion hypertrophy (increase in size) and the surrounding cartilage extracellular matrix begins to calcify. Other chondrocytes within the calcifying cartilage die because nutrients can no longer diffuse quickly enough through the extracellular matrix. As these chondrocytes die, the spaces left behind by dead chondrocytes merge into small cavities called lacunae.

Resembling the growth rings of a tree, the concentric lamellae (la-MEL-ē) are

circular plates of mineralized extracellular matrix of increasing diameter, surrounding a small network of blood vessels and nerves located in the central canal (Figure 6.3a). These tubelike units of bone generally form a series of parallel cylinders that, in long bones, tend to run parallel to the long axis of the bone.

Each trabecula consists of

concentric lamellae, osteocytes that lie in lacunae, and canaliculi that radiate outward from the lacunae.

Also, the "soft spots" that help the fetal skull pass through the birth canal later harden as they undergo intramembranous ossification, which occurs as follows

note

The process by which bone forms is called

ossification (os′-i-fi-KĀ-shun; ossi- = bone; -fication = making)ossification is formation of bone. Also called osteogenesis

difference between the function of osteoblasts and osteoclasts is as follows:

osteoBlasts Build bone, while osteoClasts Carve out bone.

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.

red bone marrow

produces red blood cells, a highly vascularized connective tissue located in microscopic spaces between trabeculae of spongy bone tissue.

The periosteum also

protects the bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons

The epiphyses

proximal and distal ends of the bone

3.The metaphyses (me-TAF-i-sēz; meta- = between; singular is metaphysis)

Region of a long bone between the diaphysis and epiphysis that contains the epiphyseal plate in a growing bone.

Older women suffer from osteoporosis more often than men for two reasons

(1) Women's bones are less massive than men's bones, and (2) production of estrogens in women declines dramatically at menopause, whereas production of the main androgen, testosterone, in older men wanes gradually and only slightly

Bone formation occurs in four principal situations:

(1) the initial formation of bones in an embryo and fetus, (2) the growth of bones during infancy, childhood, and adolescence until their adult sizes are reached, (3) the remodeling of bone (replacement of old bone by new bone tissue throughout life), and (4) the repair of fractures (breaks in bones) throughout life.

Functions of Bone Tissue

1.Supports soft tissue and provides attachment for skeletal muscles. 2.Protects internal organs. 3.Assists in movement, along with skeletal muscles. 4.Stores and releases minerals. 5.Contains red bone marrow, which produces blood cells. 6.Contains yellow bone marrow, which stores triglycerides (fats).

4.Osteoclasts (OS-tē-ō-klasts′; -clast = break)

A large, multinuclear cell that resorbs (destroys) bone matrix.are huge cells derived from the fusion of as many as 50 monocytes (a type of white blood cell) and are concentrated in the endosteum. On the side of the cell that faces the bone surface, the osteoclast's plasma membrane is deeply folded into a ruffled border. Here the cell releases powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying extracellular bone matrix.

Greenstick

A partial fracture in which one side of the bone is broken and the other side bends; similar to the way a green twig breaks on one side while the other side stays whole, but bends; occurs only in children, whose bones are not fully ossified and contain more organic material than inorganic material.

note

Although bone has a generous blood supply, healing sometimes takes months. The calcium and phosphorus needed to strengthen and harden new bone are deposited only gradually, and bone cells generally grow and reproduce slowly. The temporary disruption in their blood supply also helps explain the slowness of healing of severely fractured bones

3Formation of trabeculae.

As the bone extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone around the network of blood vessels in the tissue. Connective tissue associated with the blood vessels in the trabeculae differentiates into red bone marrow.

4Development of the medullary (marrow) cavity

As the primary ossification center grows toward the ends of the bone, osteoclasts break down some of the newly formed spongy bone trabeculae. This activity leaves a cavity, the medullary (marrow) cavity, in the diaphysis (shaft). Eventually, most of the wall of the diaphysis is replaced by compact bone.

1Development of the cartilage model.

At the site where the bone is going to form, specific chemical messages cause the cells in mesenchyme to crowd together in the general shape of the future bone, and then develop into chondroblasts. The chondroblasts secrete cartilage extracellular matrix, producing a cartilage model (future diaphysis) consisting of hyaline cartilage. A covering called the perichondrium (per′-i-KON-drē-um) develops around the cartilage model

1Development of the ossification center

At the site where the bone will develop, specific chemical messages cause the cells of the mesenchyme to cluster together and differentiate, first into osteoprogenitor cells and then into osteoblasts. The site of such a cluster is called an ossification center(An area in the cartilage model of a future bone where the cartilage cells hypertrophy, secrete enzymes that calcify their extracellular matrix, and die, and the area they occupied is invaded by osteoblasts that then lay down bone). Osteoblasts secrete the organic extracellular matrix of bone until they are surrounded by it.

hemopoiesis

Blood cell production, which occurs in red bone marrow after birth.

4.Mineral homeostasis (storage and release)

Bone tissue makes up about 18% of the weight of the human body. It stores several minerals, especially calcium and phosphorus, which contribute to the strength of bone. Bone tissue stores about 99% of the body's calcium. On demand, bone releases minerals into the blood to maintain critical mineral balances (homeostasis) and to distribute the minerals to other parts of the body.

Compact Bone Tissue

Bone tissue that contains few spaces between osteons (haversian systems); forms the external portion of all bones and the bulk of the diaphysis (shaft) of long bones; is found immediately deep to the periosteum and external to spongy bone. contains few spaces (Figure 6.3a) and is the strongest form of bone tissue. It is found beneath the periosteum of all bones and makes up the bulk of the diaphyses of long bones. Compact bone tissue provides protection and support and resists the stresses produced by weight and movement.

2.Osteoblasts (OS-tē-ō-blasts′; -blasts = buds or sprouts)

Cell formed from an osteogenic cell that participates in bone formation by secreting some organic components and inorganic salts. also are bone-building cells. They synthesize and secrete collagen fibers and other organic components needed to build the extracellular matrix of bone tissue, and they initiate calcification . As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes. (Note: The ending -blast in the name of a bone cell or any other connective tissue cell means that the cell secretes extracellular matrix.)

Spongy bone tissue is different from compact bone tissue in two respects.

First, spongy bone tissue is light, which reduces the overall weight of a bone. This reduction in weight allows the bone to move more readily when pulled by a skeletal muscle Second, the trabeculae of spongy bone tissue support and protect the red bone marrow. Spongy bone in the hip bones, ribs, sternum (breastbone), vertebrae, and the proximal ends of the humerus and femur is the only site where red bone marrow is stored and, thus, the site where hemopoiesis (blood cell production) occurs in adults.

Fracture and Repair of Bone

Fracture - Break in bone Repair - Stress fracture

Colles

Fracture of the distal end of the lateral forearm bone (radius) in which the distal fragment is displaced posteriorly.

Pott

Fracture of the distal end of the lateral leg bone (fibula), with serious injury of the distal tibial articulation.

note:

In a growing bone, each metaphysis contains an epiphyseal (growth) plate (ep′-i-FIZ-ē-al), a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length . When a bone ceases to grow in length at about ages 14-24, the cartilage in the epiphyseal plate is replaced by bone; the resulting bony structure is known as the epiphyseal line.

2bReparative phase: Bony callus formation

In areas closer to well-vascularized healthy bone tissue, osteoprogenitor cells develop into osteoblasts, which begin to produce spongy bone trabeculae. The trabeculae join living and dead portions of the original bone fragments. In time, the fibrocartilage is converted to spongy bone, and the callus is then referred to as a bony (hard) callus. The bony callus lasts about 3 to 4 months.

4Development of the periosteum

In conjunction with the formation of trabeculae, the mesenchyme condenses at the periphery of the bone and develops into the periosteum. Eventually, a thin layer of compact bone replaces the surface layers of the spongy bone, but spongy bone remains in the center. Much of the newly formed bone is remodeled (destroyed and reformed) as the bone is transformed into its adult size and shape.

Intramembranous Ossification

Intramembranous ossification is the simpler of the two methods of bone formation. The flat bones of the skull, most of the facial bones, mandible (lower jawbone), and the medial part of the clavicle (collar bone) are formed in this way. Also, the "soft spots" that help the fetal skull pass through the birth canal later harden as they undergo intramembranous ossification,

4.The articular cartilage

Is a thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation (joint) with another bone. Articular cartilage reduces friction and absorbs shock at freely movable joints. Because articular cartilage lacks a perichondrium and lacks blood vessels, repair of damage is limited.Hyaline cartilage attached to articular bone surfaces

3.Assistance in movement.

Most skeletal muscles attach to bones; when they contract, they pull on bones to produce movement.

note

Neighboring osteocytes communicate via gap junctions (see Section 4.2). The canaliculi connect lacunae with one another and with the central canals, forming an intricate, miniature system of interconnected canals throughout the bone. This system provides many routes for nutrients and oxygen to reach the osteocytes and for the removal of

2Calcification.

Next, the secretion of extracellular matrix stops, and the cells, now called osteocytes, lie in lacunae and extend their narrow cytoplasmic processes into canaliculi that radiate in all directions. Within a few days, calcium and other mineral salts are deposited and the extracellular matrix hardens or calcifies (calcification).

2Growth of the cartilage model.

Once chondroblasts become deeply buried in the cartilage extracellular matrix, they are called chondrocytes. The cartilage model grows in length by continual cell division of chondrocytes, accompanied by further secretion of the cartilage extracellular matrix.

Impacted

One end of the fractured bone is forcefully driven into the interior of the other.

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.

note

Some of the cells enable bone to grow in thickness, but not in length

Comminuted (KOM-i-noo-ted; com- = together; -minuted = crumbled)

The bone is splintered, crushed, or broken into pieces at the site of impact, and smaller bone fragments lie between the two main fragments.

Some Common Fractures Open (Compound)

The broken ends of the bone protrude through the skin. Conversely, a closed (simple) fracture does not break the skin.

3Bone remodeling phase

The final phase of fracture repair is bone remodeling of the callus. Dead portions of the original fragments of broken bone are gradually resorbed by osteoclasts. Compact bone replaces spongy bone around the periphery of the fracture. Sometimes, the repair process is so thorough that the fracture line is undetectable, even in a radiograph (x-ray). However, a thickened area on the surface of the bone remains as evidence of a healed fracture.

6Formation of articular cartilage and the epiphyseal (growth) plate.

The hyaline cartilage that covers the epiphyses becomes the articular cartilage. Prior to adulthood, hyaline cartilage remains between the diaphysis and epiphysis as the epiphyseal (growth) plate, the region responsible for the lengthwise growth of long bones that you will learn about next.

7.The endosteum (end-OS-tē-um; endo- = within)

The membrane that lines the medullary (marrow) cavity of bones, consisting of osteogenic cells and scattered osteoclasts. Also is a thin membrane that lines the medullary cavity. It contains a single layer of bone-forming cells and a small amount of connective tissue.

2aReparative phase: Fibrocartilaginous callus formation

The reparative phase is characterized by two events: the formation of a fibrocartilaginous callus, and a bony callus to bridge the gap between the broken ends of the bones. Blood vessels grow into the fracture hematoma and phagocytes begin to clean up dead bone cells. Fibroblasts from the periosteum invade the fracture site and produce collagen fibers In addition, cells from the periosteum develop into chondroblasts and begin to produce fibrocartilage in this region.

Endochondral Ossification

The replacement of cartilage by bone is called endochondral ossification. Although most bones of the body are formed in this way, the process is best observed in a long bone. It proceeds as follows (Figure 6.6):

A typical long bone consists of the following parts:1.The diaphysis(dī-AF-i-sis = growing between)

The shaft of a long bone or body—the long, cylindrical, main portion of the bone.

2.Protection

The skeleton protects the most important internal organs from injury. For example, cranial bones protect the brain, and the rib cage protects the heart and lungs.

The skeletal system performs several basic functions: 1 Support

The skeleton serves as the structural framework for the body by supporting soft tissues and providing attachment points for the tendons of most skeletal muscles

6.The medullary cavity (MED-ul-er-ē; medulla- = marrow, pith)

The space within the diaphysis of a bone that contains yellow bone marrow. or marrow cavity, is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels in adults. This cavity minimizes the weight of the bone by reducing the dense bony material where it is least needed. The long bones' tubular design provides maximum strength with minimum weight.

Parts of a long bone

The spongy bone tissue of the epiphyses and metaphyses contains red bone marrow, and the medullary cavity of the diaphysis contains yellow bone marrow (in adults).A long bone is covered by articular cartilage at the articular surfaces of its proximal and distal epiphyses and by periosteum around all other parts of the bone.

Blood vessels and nerves from the periosteum penetrate the compact bone through transverse interosteonicA minute passageway by means of which blood vessels and nerves from the periosteum penetrate into compact bone. Also called Volkmann's (FŌLK-mans) or perforating canal. (Volkmann's or perforating) canals

The vessels and nerves of the interosteonic canals connect with those of the medullary cavity, periosteum, and central canals.

Arranged around the entire outer and inner circumference of the shaft of a long bone are lamellae called circumferential lamellae

They develop during initial bone formation,The circumferential lamellae directly deep to the periosteum are called external circumferential lamellae.

bone resorption

This breakdown of bone extracellular matrix, termed bone resorption (rē-SORP-shun), is part of the normal development, maintenance, and repair of bone. (Note: The ending -clast means that the cell breaks down extracellular matrix.) in response to certain hormones, osteoclasts help regulate blood calcium level They are also target cells for drug therapy used to treat osteoporosis

The repair of a bone fracture involves the following phases 1Reactive phase

This phase is an early inflammatory phase. Blood vessels crossing the fracture line are broken. As blood leaks from the torn ends of the vessels, a mass of blood (usually clotted) forms around the site of the fracture

Initial Bone Formation in an Embryo and Fetus

We will first consider the initial formation of bone in an embryo and fetus. The embryonic "skeleton," initially composed of mesenchyme in the general shape of bones, is the site where cartilage formation and ossification occur during the sixth week of embryonic development. Bone formation follows one of two patterns

5Development of the secondary ossification centers.

When branches of the epiphyseal artery enter the epiphyses

This mass of blood, called

a fracture hematoma (hē′-ma-TŌ-ma; hemat- = blood; -oma = tumor), usually forms 6 to 8 hours after the injury. Because the circulation of blood stops at the site where the fracture hematoma forms, nearby bone cells die. Swelling and inflammation occur in response to dead bone cells, producing additional cellular debris. Phagocytes (neutrophils and macrophages) and osteoclasts begin to remove the dead or damaged tissue in and around the fracture hematoma. This stage may last up to several weeks.

These events lead to the development of a fibrocartilaginous (soft) callus (fi-brō-kar-ti-LAJ-i-nus)

a mass of repair tissue consisting of collagen fibers and cartilage that bridges the broken ends of the bone. Formation of the fibrocartilaginous callus takes about 3 weeks

The trabeculae do not

achieve their final arrangement until locomotion is completely learned. In fact, the arrangement can even be altered as lines of stress change due to a poorly healed fracture or a deformity.

Prevention and treatment for rickets and osteomalacia consists of the

administration of adequate vitamin D and exposure to moderate amounts of sunlight.

Osteomalacia is the

adult counterpart of rickets, sometimes called adult rickets. New bone formed during remodeling fails to calcify, and the person experiences varying degrees of pain and tenderness in bones, especially the hip and legs.

2.The epiphyses (e-PIF-i-sēz = growing over; singular is epiphysis

are the proximal and distal the end of a long bone, usually larger in diameter than the shaft (diaphysis).

Rickets and Osteomalacia(os′-tē-ō-ma-LĀ-shē-a; malacia = softness)

are two forms of the same disease that result from inadequate calcification of the extracellular bone matrix, usually caused by a vitamin D deficiency.

1.Osteoprogenitor cells (os′-tē-ō-prō-JEN-i-tor; -genic = producing)

are unspecialized bone stem cell derived from mesenchyme that has mitotic potential and the ability to differentiate into an osteoblast. Also the tissue from which almost all connective tissues are formed. They are the only bone cells to undergo cell division; the resulting cells develop into osteoblasts. Osteoprogenitor cells are found along the inner portion of the periosteum, in the endosteum, and in the canals within bone that contain blood vessels.

The periosteum also is

attached to the underlying bone by perforating fibers or Sharpey's fibers, thick bundles of collagen that extend from the periosteum into the bone extracellular matrix.

6.Triglyceride storage. Yellow bone marrow

consists mainly of adipose cells, which store triglycerides. The stored triglycerides are a potential chemical energy reserve.

Bone mass becomes so

depleted that bones fracture, often spontaneously, under the mechanical stresses of everyday living. For example, a hip fracture might result from simply sitting down too quickly

secondary ossification centers

develop, usually around the time of birth. Bone formation is similar to what occurs in primary ossification centers. However, in the secondary ossification centers spongy bone remains in the interior of the epiphyses (no medullary cavities are formed here). In contrast to primary ossification, secondary ossification proceeds outward from the center of the epiphysis toward the outer surface of the bone.

Red bone marrow consists of

developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers. It is present in developing bones of the fetus and in some adult bones, such as the hip (pelvic) bones, ribs, sternum (breastbone), vertebrae (backbones), skull, and ends of the bones of the humerus (arm bone) and femur (thigh bone). In a newborn, all bone marrow is red and is involved in hemopoiesis. With increasing age, much of the bone marrow changes from red to yellow.

Spongy Bone Tissue In contrast to compact bone tissue, spongy bone tissue, also referred to as trabecular or cancellous bone tissue

does not contain osteons (Figure 6.3b, c)

In the second type, endochondral ossification (en′-dō-KON-dral; endo- = within; -chondral = cartilage)

endochondral ossification is the replacement of cartilage by bone. endochondral ossification is bone forms within hyaline cartilage that develops from mesenchyme.

Bone fractures also result

from minor trauma

Spongy bone tissue tends to be located where bones are not

heavily stressed or where stresses are applied from many directions

white blood cells, and platelets, a process called

hemopoiesis (hēm-ō-poy-ē-sis; hemo- = blood; -poiesis = making).

Osteons in compact bone tissue are aligned

in the same direction and are parallel to the length of the diaphysis. As a result, the shaft of a long bone resists bending or fracturing even when considerable force is applied from either end. Compact bone tissue tends to be thickest in those parts of a bone where stresses are applied in relatively few directions. The lines of stress in a bone are not static. They change as a person learns to walk and in response to repeated strenuous physical activity, such as weight training. The lines of stress in a bone also can change because of fractures or physical deformity. Thus, the organization of osteons is not static but changes over time in response to the physical demands placed on the skeleton

Spongy bone tissue makes up most of the

interior bone tissue of short, flat, sesamoid, and irregularly shaped bones. In long bones it forms the core of the epiphyses beneath the paper-thin layer of compact bone, and forms a variable narrow rim bordering the medullary cavity of the diaphysis. Spongy bone is always covered by a layer of compact bone for protection.

Spongy bone tissue is always located in the

interior of a bone, protected by a covering of compact bone. It consists of lamellae that are arranged in an irregular pattern of thin columns called trabeculae little beams; singular is trabecula)is also(Irregular latticework of thin plates of spongy bone tissue. Fibrous cord of connective tissue serving as supporting fiber by forming a septum extending into an organ from its wall or capsule.)

The circumferential lamellae that line the medullary cavity are called

internal circumferential lamellae (Figure 6.3a)

This type of cartilaginous growth, called

interstitial (endogenous) growth (growth from within), results in an increase in length. In contrast, growth of the cartilage in thickness is due mainly to the deposition of extracellular matrix material on the cartilage surface of the model by new chondroblasts that develop from the perichondrium

In the first type of ossification, called

intramembranous ossification (in′-tra-MEM-bra-nus; intra- = within; -membran- = membrane),intramembranous ossification is the method of bone formation in which the bone is formed directly in mesenchyme arranged sheet like layers that resemble membranes. intramembranous ossification bone forms directly within mesenchyme, which is arranged in sheetlike layers that resemble membranes

A stress fracture

is a series of microscopic fissures in bone that forms without any evidence of injury to other tissues. In healthy adults, stress fractures result from repeated, strenuous activities such as running, jumping, or aerobic dancing.

A bone

is an organ made up of several different tissues working together: bone (osseous) tissue, cartilage, dense connective tissue, epithelium, adipose tissue, and nervous tissue.

A fracture (Any break in a bone.)

is any break in a bone. Fractures are named according to their severity, the shape or position of the fracture line, or even the physician who first described them.a bone may fracture without visibly breaking.

5.The periosteum (per-ē-OS-tē-um; peri- = around)

is essential for bone growth, repair, and nutrition.is a tough connective tissue sheath and its associated blood supply that surrounds the bone surface wherever it is not covered by articular cartilage. It is composed of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer that consists of cells.also periosteum is the membrane that covers bone and consists of connective tissue, osteogenic cells, and osteoblasts;

A long bone such as the humerus (the arm bone)

is one that has greater length than width

Between the concentric lamellae are small spaces called

lacunae (la-KOO-nē = little lakes; singular is lacuna),A small, hollow space, such as that found in bones in which the osteocytes lie.

At first glance, the trabeculae of spongy bone tissue may appear to be

less organized than the osteons of compact bone tissue. However, they are precisely oriented along lines of stress, a characteristic that helps bones resist stresses and transfer force without breaking.

Osteoporosis(os′-tē-Ō-pō-RŌ-sis; -por- = passageway; -osis = condition), Age-related disorder characterized by decreased bone mass and increased susceptibility to fractures, often as a result of decreased levels of estrogens.

literally a condition of porous bones, affects 10 million people a year in the United States (Figure 6.11). In addition, 18 million people have low bone mass (osteopenia), which puts them at risk for osteoporosis. The basic problem is that bone resorption (breakdown) outpaces bone deposition (formation). In large part this is due to depletion of calcium from the body—more calcium is lost in urine, feces, and sweat than is absorbed from the diet. . . Osteoporosis afflicts the entire skeletal system.

In the United States, osteoporosis causes more than 1.5 million fractures a year,

mainly in the hips, wrists, and vertebrae

3.Osteocytes (OS-tē-ō-sīts′; -cytes = cells)

mature bone cells, are the main cells in bone tissue and maintain its daily metabolism, such as the exchange of nutrients and wastes with the blood. Like osteoblasts, osteocytes do not undergo cell division. (Note: The ending -cyte in the name of a bone cell or any other tissue cell means that the cell maintains and monitors the tissue.)

This process is called appositional (exogenous) growth (a-pō-ZISH-o-nal),

meaning growth at the outer surface. Interstitial growth and appositional growth of cartilage

osteonic is a

microscopic tube running the length of the spinal cord in the gray commissure. A circular channel running longitudinally in the center of an osteon (haversian system) of mature compact bone, containing blood and lymphatic vessels and nerves. Also called a haversian (ha-VER-shun) or central canal.

Osteoporosis primarily affects

middle-aged and elderly people, 80% of them women.

The study of bone structure and the treatment of bone disorders is referred to as

osteology (os-tē-OL-o-jē; osteo- = bone; -logy = study of

Each osteon consists of concentric lamellae arranged around an

osteonic (haversian or central) canal

Compact bone tissue is composed of repeating structural units called

osteons, or haversian systems, also (osteons)The basic unit of structure in adult compact bone, consisting of a central (haversian) canal with its concentrically arranged lamellae, lacunae, osteocytes, and canaliculi.

Four types of cells are present in bone tissue

osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts

Stress fractures are quite

painful and also result from disease processes that disrupt normal bone calcification, such as osteoporosis (discussed in Disorders: Homeostatic Imbalances at the end of this chapter). About 25% of stress fractures involve the tibia. Although standard x-ray images often fail to reveal the presence of stress fractures, they show up clearly in a bone scan.

They are connected to the periosteum by

perforating (Sharpey's) fibers

Once the perichondrium starts to form bone, it is known as the

periosteum

The metaphyses

regions between the diaphysis and the epiphyses

In addition to fractures, osteoporosis causes

shrinkage of vertebrae, height loss, hunched backs, and bone pain.

The entire framework of bones and their cartilages , ligaments, and tendons constitute the

skeletal system

Between the trabeculae are spaces that are

visible to the unaided eye. These macroscopic spaces are filled with red bone marrow in bones that produce blood cells, and yellow bone marrow (adipose tissue) in other bones. Both types of bone marrow contain numerous small blood vessels that provide nourishment to the osteocytes

The areas between neighboring osteons contain lamellae called interstitial lamellae

which also have lacunae with osteocytes and canaliculi. Interstitial lamellae are fragments of older osteons that have been partially destroyed during bone rebuilding or growth.

Radiating in all directions from the lacunae are tiny canaliculi (kan-a-LIK-ū-lī = small channels)A small channel or canal, as in bones, where they connect lacunae

which are filled with extracellular fluid. Inside the canaliculi are slender fingerlike processes of osteocytes (see inset at right of Figure 6.3a).