HUMAN ANATOMY

LECTURE SIX

BONES

FUNCTIONS OF THE SKELETAL SYSTEM

• Support (bones, cartilage, ligaments, tendons) - supporting framework for soft tissues

- maintains body shape

• Protection - internal organs, soft tissues

• Movement - levers for muscles to produce body movement

• Mineral storage - matrix of bone stores a variety of minerals that are released as needed (Ca2+ and P3-). Fat stored in marrow cavities.

• Blood cell production - bone marrow gives rise to RBC, WBC and platelets

COMPONENTS OF THE SKELETAL SYSTEM

• Bone• Cartilage

- Hyaline

- Fibrocartilage

- Elastic

BONE SHAPES• Long - long and skinny

- typical ‘bone’- eg: upper and lower limbs

• Short - as tall as wide - eg: wrists and ankles

• Flat - thin- act as attachment site for muscles- eg: ribs, sternum, skull, scapula

• Irregular - unusual shaped, notches and ridges

- eg: vertebrae, facial bones, pelvis

• Sesamoid – small, flat- associated with tendons, near joints- eg: patella (knee cap)

LONG BONE STRUCTURE• Diaphysis - central shaft of compact

bone surrounding marrow• Epiphysis - end of the bone formed of

cancellous bone• Epiphyseal Plate (growth plate)

- between diaphysis and epiphysis- hyaline cartilage- present during growth

• Epiphyseal Line - when growth stops cartilage of plate replaced by bone

• Medullary Cavity – full of marrow

• Periosteum - dense connective tissue covering- contains blood vessels and nerves

• Endosteum – connective tissue lining of medullary cavity

FLAT, SHORT, IRREGULAR BONES

• no diaphysis or epiphysis

• sandwich of cancellous between compact bone

BONE HISTOLOGY

• Matrix - collagen fibers in a ground substance of proteoglycan held together by hydroxyapatite (calcium phosphate salts)- very strong but brittle

• Bone Cells - osteoblasts- osteocytes- osteoclasts- stem cells or osteoprogenitor cells

BONE MATRIX

• If mineral is removed - bone is too bendy

• If collagen is removed - bone is too brittle

BONE CELLSOsteoblast (bone formers)

• Formation of bone through ossification or osteogenesis

• Make and release collagen and organic components of matrix

• As osteoblasts are surrounded by hard bone matrix they become into osteocytes

• Found in periosteum and endosteum

Osteocyte (mature bone cells)

• Found within lacunae (spaces) between layers of lamellae of matrix

• Canaliculi – canals occupied by osteocyte cell processes

• Maintain surrounding matrix - continually dissolving and rebuilding

• Can change into osteoblast or osteoprogenitor cells if necessary for bone repair

Osteoclasts (bone breakers)

• Release enzymes the breakdown bone matrix on inner edge nearest bone marrow - reabsorption

- recycle minerals back into body

- at same time osteoblasts are rebuilding from the outer edge

Osteoprogenitor Cells (stem cells)

• Become osteoblasts

• Important to bone repair

• Located in inner layer of periosteum and endosteum

TYPES OF BONECancellous (Spongy) Bone• Formed from trabeculae - interconnecting rods or plates of bone

- spaces filled with marrow and blood vessels- have several lamellae with osteocytes

• Found at ends of long bones and areas of little stress

Compact (Haversian)Bone• Forms diaphysis of long bones and

thinner surfaces of other bones

• Haversian system or Osteon composed of:

- Haversian canals - run parallel,

surrounding blood vessels

- canals surrounded by rings of

concentric lamellae with

osteocytes

• Volkmann’s or perforating canals run perpendicularly to Haversian

- contain blood vessels

• Interstitial lamellae - fill spaces between osteons

• Circumferential lamellae - run around outer area of bone

BONE DEVELOPMENTOssification - formation of new bone by osteoblasts

- 2 types of ossification/bone remodeling:

• Intramembranous ossification - takes place in connective tissue membrane (in areas without cartilage)

• Endochondral ossification – takes place in cartilage

Once bone remodeling occurs – formation type cannot be distinguished

INTRAMEMBRANOUS OSSIFICATION

• Takes place in connective tissue membrane within the layers of dermis

• Forms flat bones of skull, jaw, clavicle = dermal bones

1) Begins in ossification centre - osteoblasts deposit bone matrix in trabeculae (spicules)

- osteoblasts get caught and become osteocytes

2) Blood vessels grow and get trapped by the spicules

3) First bone is spongy – trapped blood vessels form the osteon – develops into compact bone

- surrounding connective tissue forms the periostium

Intramembranous Ossification

ENDOCHONDRIAL OSSIFICATION• Found in bones at base of skull and the rest of the skeleton• Takes place in cartilage that has general bone shape - replaced during

growth1) Chondrocytes increase in number and size

- lacunae expand - matrix begins to calcify and chondrocytes die

2) Blood vessels grow into perichondrium (surrounding cartilage)- inner perichondrial cells become osteoblasts

3) Bone development begins around cartilage4) Osteoclasts break down the calcified cartilage - osteoblasts line up on edge of

remaining matrix and form spongy bone which spreads towards ends5) Osteoclasts begin to break down inner trabeculae to form the marrow cavity6) Now epiphysis begin to calcify and fill with spongy bone• Articular cartilage - cap of cartilage remaining on the end of the bone to prevent contact with other bones• Epiphyseal plate – separates epiphysis from diaphysis

Endochondral Ossification

Appositional Growth (growth in diameter)• New bone lamellae forms over existing bone or

connective tissue• Osteoblasts deposit new bone matrix• Produces a series of layers - form circumferential

lamellae

While osteoblasts are creating new bone on the outer surface the osteoclasts are breaking down old bone at the inner surface - around marrow cavity (at a slower rate)

BONE GROWTH

Growth in Length• Occurs at epiphyseal plate - on diaphyseal side• Chrondrocytes increase in number and line up in

columns parallel to the long axis on the bone - causing elongation of the bone

• Cartilage matrix calcifies• Osteoclasts remove dead chondrocytes - replaced

with osteoblasts • Bone formation on surface of calcified cartilage

FACTORS AFFECTING BONE GROWTH

Size and shape of bone is determined genetically but can be affected by nutrition and hormones

• Nutrition- Lack of calcium, protein and other nutrients during growth and development can cause bones to be small

- Vitamin D:

- Necessary for absorption of calcium from intestines

- Lack can cause rickets, osteomalacia

- Vitamin C:

- Necessary for collagen synthesis by osteoclasts

- Lack can cause scurvy, slow healing, loss of teeth

• Hormones

- growth hormone from anterior pituitary stimulates interstitial

cartilage growth and appositional growth

- thyroid hormone is required for growth of all tissues

- sex hormones such as estrogen and testosterone cause growth at

puberty and also closure of epiphyseal plates (cessation of growth)

BONE REMODELING• Occurs in all bones• Removing existing bone

(osteoclasts) and replacing it with new bone (osteoblasts)

• For growth - at epiphyseal plate

• Responsible for:- changing bone shape- adjustment of bone due to

stress- bone repair- Ca2+ regulation

BONE REPAIR1) Hematoma formation - mass of

blood that forms a clot2) Callus formation - mass of tissue a

fracture site that connects with broken ends of boneInternal repair (ossification)- blood vessels extend into hematoma- macrophages clean up debris- osteoclasts breakdown dead tissue- fibroblasts produce collagen- chondroblasts produce cartilage

within the collagen- osteoblasts invade and form new

bone (cancellous bone first)

External: collar forms that stabilizes the two pieces of bone

3) Callus Ossification

- callus replaced by cancellous

bone

4) Bone Remodeling

- replacement of cancellous

and damaged material into

compact bone

BONE FRACTURES

• Open (compound) - break with open wound, bone may be sticking out of wound

• Closed (simple) - skin not perforated

• Incomplete - doesn’t extend across the bone

• Complete - extends across entire bone

• Greenstick - incomplete fracture on convex side of the curve of the bone

• Hairline - incomplete where two sections of bone do not separate - common in skull fractures

• Comminuted - complete with break in more than two pieces

BONE FRACTURES• Impacted - one fragment is driven

into the cancellous portion of the other fragment

• Classified according to direction of fracture:

- linear - parallel to long axis

- transverse - at right angles

- spiral - twisted

- oblique - any other angle

- denate - rough, broken ends

- stellate - radiating out from a

central point

AGING ON BONES• Bone matrix decreases - become brittle due to lack of collagen

and less hydroxyapetite

• Bone mass decreases

• Osteoporosis – reduction in overall quality of bone tissue

• Osteopenia - inadequate ossification

• Osteomalacia – softening of bones due to calcium depletion

• Increased bone fractures

• Bone loss causes deformity, loss of height, pain, stiffness

- stooped posture

- loss of teeth

OTHER CLINICAL ISSUES• Rickets - growth retardation due to nutritional deficiencies (calcium,

phosphate, vitamin D)• Giantism – overproduction of growth hormone causing abnormally

increased size due to growth at epiphyseal plates of long bones• Dwarfism - abnormally small due to improper growth at epiphyseal

plates