Skeletal TissueChapter 7

Types of Bone Tissue

• Compact bone– Dense or solid

appearance

• Cancellous or spongy bone– Open spaces

filled with needle-like bone structures

Structural Classification of Bones

• Long bones– typically longer than wide– Have shaft w/ heads at

both ends– contains mostly compact

bone– Ex: femur, humerus, ulna,

radius • Short bones

– Cube or box-shaped– Mostly spongy bone– Ex: carpals & tarsals

• Flat bones– Thin, flattened– Usually curved surface– Thin layer of compact

bone covering spongy bone

– Ex: ribs, skull, sternum, scapulae

– Bone marrow aspirations occur here

• Irregular bones– Various shapes/sizes– Do not fit into other

categories– Ex: vertebrae, facial bones– Sesamoid bones: occur

singularly (ex: patella)

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Long Bones – In More Detail

• Diaphysis– Shaft of the bone– Hollow, compact bone

• Epiphyses – Ends of the long bones– Points of muscle attachment– Stability to joints– Spongy bone filled with red marrow– Epiphyseal plate: area between diaphysis and

epihyses (“growth plate”)

Long Bones – In More Detail• Articular cartilage

– Hyaline cartilage that covers joint surfaces

• Periosteum– Dense, white fibrous membrane that covers

bone (excepts joint surfaces)– Tendon fibers interlace with these fibers

creating a firm attachment

• Medullary (marrow) cavity– Hollow space in diaphysis of long bones– Filled with yellow marrow (CT rich in fat)

• Endosteum – Epithelial membrane that lines the medullary

cavity

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Bone Tissue

• Connective tissue• Consists of cells, fibers, extracellular

matrix– Matrix predominates – Matrix hard and calcified– High content of collagen fibers

Composition of Bone MatrixExtracellular bone matrix can be

subdivided into two components:– Inorganic salts– Organic matrix

1. Inorganic salts– Hydroxyapatite – specialized chemical

crystals of calcium and phosphate• Needle-like; found btwn spaces of collagen

fibers; oriented to reduce stress

– Deposition of these chemicals = calcification

– Mg, Na, sulfate, F are also found in bone

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Composition of Bone Matrix2. Organic matrix

– Collagenous fibers– Ground substance

• Protein & polysaccharide mixture• Provides support & adhesion for cellular &

fibrous elements• Necessary for growth & repair • Chondroitin sulfate – important component of

ground substance • Glucosamine and chondroitin sulfate required

for bone & cartilage repair and maintenance

Microscopic Structure of Compact Bone

• Structural unit of compact bone = osteon or Haversian system– Each osteon surrounds a cannal & runs

lengthwise– Bone cells “cemented” within these

units– Structure permits delivery and removal

of nutrients and wastes• Structures within each osteon:

– Lamellae, lacunae, canaliculi, Haversian canal

Osteon structure• Lamellae: concentric, cylinder-shaped

layers of calcified matrix• Lacunae (“little lakes”): small spaces filled

with tissue fluid which hold bone cells• Canaliculi: very small canals connecting

lacunae together• Haversian canal: extend lengthwise through

the center of an osteon– contains blood vessels, lymphatic vessels &

nerves• Volkmann’s canal: (not part of osteon)

transverse canals containing nerves and blood vessels– Carry blood from exterior surface of bone to

osteons

Microscopic Structure of Cancellous (spongy) Bone

• No osteons• Needle-like bony spicules – trabeculae

– Bone cells found within the trabeculae

• Spongy bone usually lies between two layers of compact bone– Diploe– Ex: skull bones

• Bony spicules arranged along lines of stress– Varies for different types of bones

Types of Bone Cells1.Osteoblasts – bone-forming cells

– synthesize & secrete a specialized organic matrix = osteoid; important part of ground substance

– Osteoid serves as the framework for calcium & phosphate deposits = accumulation of mineralized bone

2.Osteoclasts – bone-reabsorbing cells– Giant multinucleated cells– Large number of mitochondria and lysosomes– Responsible for erosion of bone minerals– Break down bone matrix for remodeling &

release of calcium3.Osteocytes – mature bone cells

– Mature, nondividing osteoblasts– Lie within lacunae

Bone Marrow• Bone marrow = myeloid tissue

– Specialized, soft CT• Site of blood cell production• Found within medullary cavities of long

bones & spongy bone• Red marrow

– In infant and children – red marrow dominates

– RBC production– In adults - Ribs, bodies of vertebrae, ends of

long bones, pelvis still contain red marrow– During times of blood loss – yellow red

Bone Marrow

• During aging red marrow becomes yellow

• Marrow cells become saturated with fat

• Inactive during blood cell production

Functions of Bone1. Support

– Shape, alignment, positioning of body parts2. Protection

– Protects organs3. Movement

– Bones and joints act as levers– Muscles attached to bones produce movement

of joints4. Mineral storage

– Calcium and phosphorous – Homeostasis of blood calcium levels

5. Hematopoiesis – Blood cell formation

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Regulation of Blood Calcium Levels

• Bones store approx 98% of body’s calcium

• Roles of calcium in the body:– Transmission of nerve signals– Skeletal & cardiac muscle contractions

• Osteoblasts – remove calcium from blood

• Osteoclasts – release calcium into blood

Mechanisms of Calcium Homeostasis

• ***Parathyroid Hormone*** – parathyroid glands– Ca levels below homeostatic “set point”:

• Osteoclast activity stimulated• Calcium absorbed from urine in renal system• Vitamin D synthesis stimulated increases

absorption of calcium in intestine

• Calcitonin – thyroid gland– High blood Ca levels

• Stimulate osteoblast activity and inhibit osteoclast activity

– Miacalcin: calcitonin nasal spray; used in osteoporosis pts

Development of Bone

• Infant – cartilage replaced by calcified bone matrix– Requires osteoblast & osteoclast activity– Osteogenesis

• Intramembranous ossification: process by which most flat bones are formed within fibrous connective tissue membranes– Ex: skull bones (flat bones) & irregular bones

Endochondral Ossification (fig 7-8 pg. 198)

• Bones formed from cartilage model– Occurs from center to ends– Typical of long bones

1. Cartilage model2. Periosteum ring or collar of bone3. Primary ossification center and entrance of blood

vessels4. Medullary cavity; thickening/lengthing of collar5. Secondary ossification center in epiphyseal

cartilage6. Enlargement of secondary centers; bone growth

moves toward diaphysis7. Epiphyseal plates close

Bone Growth & Resorption

• Bones grow in diameter– Osteoclasts enlarge diameter of medullary

cavity– Osteoblasts in the periosteum build new bone

from the outside– Grow in length (at epiphyseal plates)

• Childhood/adolescence – growth > resorption

• Adulthood – growth = resorption• > 35-40 yrs – growth < resorption

Bone Remodeling

• Formation of new Haversian systems (osteons)– Osteoclasts erode surface of bone

grooves– Blood vessels lie in these grooves– New bone covers these blood vessels

(osteoblast activity)– Grooves canals– New lamallae develops

Bone Fracture Repair

• Fracture = break in the continuity of bone• Vascular damage initiates repair sequence1. Vascular damage hemorrhage blood

clot (fracture hematoma)2. Hematoma resorbed callus

– Specialized repair tissue that binds the broken ends of the fracture together

3. Callus replaced by normal bone tissue– Proper alignment and immobilization necessary

Questions1. Name the two types of bone tissue.2. List the six structural components of a

typical long bone visible to the naked eye.3. What are the two principal chemical

components of bone matrix?4. What disease is characterized by loss of

bone mineral density?5. List and briefly describe the four

structures found within an osteon.6. Name and briefly describe the three

major bone cell types.7. What are the 5 functions of bone?8. Briefly describe the steps in endochondral

ossification.

Clarification

Resorption = destruction

Role of osteoclasts is bone resorption (destruction); erodes bone tissues releasing calcium into the blood