SKELETAL CARTILAGE

General comments

1. Water - the primary constituent; spring back after compressed (resiliency)

2. Collagen fibers - form a structural mesh

3. Proteoglycans - central protein (hyaluronic acid); GAGs (chondrotin sulfate)

4. Perichondrium - dense connective tissue membrane; vascularized, nourishes

Hyaline cartilage

1. Histology - only collagen fibers

2. Function - support with flexibility and resilience

3. Location - most abundant type (figure 6.1 p. 166)

-a. Embryonic skeleton - made mostly of hyaline cartilage

-b. Epiphyseal plate - for growth of long bones

-c. Articular cartilage - covers the articular (joint) surface of bones

-d. Costal cartilage - joins ribs to sternum

-e. Laryngeal cartilage - cartilage plates form the larynx

-f. Tracheal/bronchial cartilage - reinforces respiratory passage way

-g. Nasal cartilage - supports the external nose

Elastic cartilage

1. Histology - like hyaline but with more elastic fibers

2. Function - able to stand up to repeated bending

3. Location – include

-a. External ear – pinna or auricle

-b. Epiglottis - guards opening to larynx

Fibrocartilage

1. Histology - parallel rows of chondrocytes alternating with collagen fibers

2. Function - highly compressible with great tensile strength

3. Location – include

-a. Menisci - of knees, pad like

-b. Intervertembral disc - between the vertebrae of the spine

Growth of cartilage

1. Appositional growth - chondroblast in perichondrium lie new matrix outside

2. Interstitial growth - chondrocytes in the lacunae lay down new matrix within

FUNCTION OF BONES

Support

1. Leg bones - pillars which support the trunk

2. Rib cage - support thoracic wall

Protection

1. Skull - protects brain

2. Vertebrae - form spine which protects spinal cord

3. Rib cage - protects organs in the thoracic cavity

Movement

1. Levers - attached to muscles by tendons

Mineral storage

1. Calcium - most important minerals stored in

2. Phosphate – also very important

Blood cell formation

1. Hematopoiesis - occurs in the marrow cavities of certain bones

2. Red marrow – found in different places, depending on age

-a. Newborns – medullary cavities; all areas of spongy bone

-b. Adults – head of femur and humerus; flat bones and some irregular bones

BONE HISTOLOGY

Types of bone tissue

1. Compact bone – dense outer layer of every bone; looks smooth

2. Spongy bone – honeycomb; small spines on flat pieces trabeculae; marrow

Types of cells

1. Osteogenic cells – stem cells, develop into osteoblast

2. Osteoblast – bone building cells

3. Osteocytes – mature bone cells; maintain; nutrient exchange

4. Osteoblast – from fusion of many monocytes; break down

BONE STRUCTURE

Long bone

1. Diaphysis – cylindrical shaft composed of compact bone

2. Medullary cavity – within the shaft; usually contains yellow (fat) bone marrow

3. Epiphysis – two expanded ends; compact bone exterior; spongy bone interior

-a. Articular cartilage – on exterior joint facing part of the epiphysis articular

-b. Epiphyseal line – remnant epiphyseal plate; between diaphysis and epiphysis

4. Periosteum – double layered outer covering; blood vessels, nerves, and lymphatics

-a. Fibrous layer – outer layer composed of dense irregular connective tissue

-b. Osteogenic layer – abutting the bone surface; osteoblast and osteoclast cells

-c. Sharpey’s fibers – collagen fibers; fibrous layer into bone matrix; secure to bone

-d. Functions – nutrition (blood vessel); innervation; attachment of tendons

5. Endosteum – internal bone surface; spongy bone; canals; osteoblast, osteoclast

Short, irregular, and flat bones

1. Compact bone – external

-a. Periosteum – covers exterior compact bone

2. Spongy bone – inside

-b. Endosteum – covers the trabeculae

-c. Bone marrow – also in spongy bone

3. Diploe – spongy bone in flat bones

Compact bone

1. Osteon (Haversian system) – elongated cylinders, parallel to the axis of the bone

2. Lamella – concentric tubes of bone matrix, makes up osteon; lamellar bone

3. Haversian (central) canal – through center of each osteon; blood supply, nerves

4. Volkmann canals – perpendicular; periosteum to Haversian to medullary cavity

5. Osteocytes – spider shaped mature bone cells

6. Lacunae – small cavities which hold the osteocytes; junction of the lamella

7. Canaliculi – connect osteocytes, Haversian canal; transport of nutrients and waste

Spongy bone

1. Trabeculae – irregularly shaped lamella; osteocytes connected by canaliculi

2. Nutrients – diffuse from the marrow through the canaliculi to the osteocytes

Composition of bone

1. Organic components – cells (osteo-); osteoid

2. Osteoid – organic matrix of the bone; about one third of matrix

-a. Collagen fibers – tensile strength; opposite direction in consecutive lamella

-b. Ground substances – proteoglycans and glycoproteins

3. Inorganic material – 2/3^rd matrix; calcium salts; mostly calcium phosphates

OSTEOGENESIS

Bony skeleton formation: intermembranous ossification

1. Flat bones – most skull bones and clavicles

2. Fibrous connective tissue membrane – mesenchymal cells forms the; week 8

3. Ossification center – forms centrally; mesenchymal to osteoblast

4. Bone matrix – osteoid secreted in by osteoblast; mineralizes

-a. Osteocytes – what the trapped osteoblast become

5. Woven bone formation – osteoid forms trabeculae; blood vessels enclosed

6. Periosteum forms – vascular mesenchyme condenses; exterior face of woven bone

7. Bone collar – trabeculae deep to periosteum thickens; replaced by lamellar bone

8. Red marrow forms – from vascular tissue trapped by trabeculae

Bony skeleton formation; endochondral ossification

1. Most bones – formed this way; use a hyaline cartilage model

2. Primary ossification center – starts in the middle of the diaphysis; 2 months

-a. Bone collar forms – osteoblast secrete osteoid against the diaphysis

-b. Calcification – of chondrocytes in middle of diaphysis

-c. Cavitation – can’t receive nutrients; die; matrix deteriorates; cavities are opened

-d. Periosteal bud – blood vessels, lymphatics, nerves; osteoblast secrete osteoid

-e. Spongy bone – forms, initially

-f. Medullary cavity – forms as osteoclast break down spongy bone

3. Secondary ossification center – in the epiphyses; shortly before birth

-a. Epiphyses ossification – same pattern; osteoclast do destroy the spongy bone

-b. Epiphyseal plate – growth plate; remains between diaphysis and epiphysis

-c. Articular cartilage – also remnants of hyaline cartilage

Postnatal bone growth: increase in length

1. Epiphyseal plate – where this occurs

2. Interstitial growth – of the cartilage on the diaphysis side of epiphyseal plate

3. Zones – mimic bone development in fetus

-a. Growth zone – rapidly dividing; push epiphysis away from diaphysis

-b. Transformation zone – cartilage calcifies and dies; matrix deteriorates

-c. Osteogenic zone – deteriorated area becomes covered with spongy bone

4. Medullary cavity – extends as osteoclast break down the spongy bone

5. Remodeling – accompanies longitudinal growth; right proportions; appositional

Postnatal bone growth: increase in width

1. Appositional growth – the type of growth; form the outside

-a. Osteoblast – deep to the periosteum secrete new bone

-b. Osteoclast – in the endosteal surface break bone down

Hormonal regulation of bone growth

1. Growth hormone – infancy, childhood, most important; epiphyseal plate growth

2. Thyroid hormones – modulate growth hormone; skeleton has right proportions

3. Sex hormones – growth spurt of adolescence; masculization and feminization

BONE HOMEOSTASIS

Bone remodeling

2. Bone deposition – as follows

-a. Osteoblast – secretes osteoid; appears as a thin gauze like matrix

-b. Calcification – osteoid matures in 10 - 12 days; calcification is sudden

3. Bone resorption – as follows

-a. Osteoclast – giant multinucleated cells believed; possibly related to macrophages

-b. Lysosomal enzymes – secreted by osteoclast break down the organic matrix

-c. Acids – also secreted by osteoclast; calcium salts to soluble form; leach out

Control of remodeling: hormonal mechanism

1. Blood calcium homeostasis – not bone centered; avoid neuromuscular problems

2. Parathyroid hormone (PTH) – found on posterior of thyroid

-a. Low blood calcium level – stimulates the release of parathyroid hormone

-b. Osteoclast – are stimulated to break down bone to release calcium to blood

3. Calcitonin – from special cells in thyroid gland

-a. High blood calcium level – ↑calcitonin; ↑calcium deposition in bone

Control of remodeling: mechanical stress

1. Bone centered – unlike hormonal control; serves the need of the skeleton

2. Stress – from both muscle pull and the force of gravity

3. Wolff’s law – bone deposition will occur in response to mechanical stress

5. Mechanism – uncertain; maybe due to electric current when bone is deformed

Fractures

1. Classification – several different systems; simplest system

-a. Comminuted – fragmented into three or more pieces

-b. Epiphyseal – epiphysis from diaphysis; along epiphyseal plate

-c. Spiral – ragged break; twisting; sports injury

-d. Compression – crushed; usually porous bone; vertebrae

-e. Depression – broken bone pressed inward; skull

-f. Greenstick – incomplete break; children

3. Reduction – the realignment of broken bone ends

-a. Closed reduction – using the hand to join broken ends

-b. Open reduction – surgically joining the ends; pins and wires

4. Immobilization – by a cast or traction

5. Stages of bone healing – include

-a. Hematoma formation – broken vessels result; mass of clotted blood

-b. Fibrocartilaginous callus – granulation tissue fibroblast secrete collagen fibers

-c. Bony callus formation – week 3 osteoblast gradually; spongy bone which replaces

-d. Remodeling – osteoclast and osteoblast remodel to pre-fracture appearance

Homeostatic imbalances

1. Osteoporosis – bone resorption over takes bone deposition

-a. Proportion – of inorganic and organic components of the matrix remains same

-b. Fractures – common; bone more porous

-c. Estrogen level – maintain normal bone density of skeleton; menopause in women

-d. Treatment – calcium and Vitamin D; estrogen replacement therapy

2. Osteomalacia – osteoid is formed but not calcified; bones soft and deform

3. Rickets – similar in children; bowed legs; lack of dietary calcium or vitamin D

4. Paget’s disease – localized abnormal deposition of woven bone; deformities; virus