Chapter 6- Bones and Skeletal Tissue

Bones (osseous tissue)

organ made up of several different tissues (bone, cartilage, dense connective tissue, adipoe, and nervous tissue) working together

skeletal system

framework of the bones and their cartilage

Functions of bones

-support

-protection

-movement

-mineral and growth factors storage

-blood cell formation/hematopoiesis

-fat storage

-hormone production: ostecalcin

Function of Bones: Support

vital organ support

Function of Bones: Protection

brain, spinal cord, vital organ protection

Function of Bones: Movement

levers for muscle action

Functions of Bones: Mineral and growth factor storage

Calcium and phosphorus and growth factors reservoir

Functions of Bones: blood cell formation/Hematopoiesis

occurs in red marrow cavities

Functions of Bones: fat storage

energy source stored in bone cavities

Function of bones: hormone production/osteocalcin

secreted by bones helps to regulate insulin secretion, glucose levels and metabolism

axial skeleton

along axis of body (skull, vertebral column, rib cage)

appendicular skeleton

Bones of upper and lower limbs

Girdles attaching limbs to axial skeleton

Division of skeletal system

axial and appendicular

Bone Classification

long bones, short bones, flat bones, irregular bones

long bones

longer than they are wide (ex. limbs)

short bones

Cube-shaped bones (ex. wrist and ankle)

Sesamoid bones within tendons (e.g., patella)

flat bones

thin and flattened, usually curved (ex. sternum, scapula, ribs, skull bones)

irregular bones

complicated shapes

(ex: vertebrae and hip bones)

3 levels of bone structure

1. Gross

2. Microscopic

3. Chemical

components of bones

1. Compact Bone

2. spongy bone

compact bone

-strongest, provides protection and support

-dense outer layer on every bone, appears smooth and solid

spongy bone

-lightweight and provides tissue support

-honeycomb stricture of small, needle-like pieces of bone called trabeculae

-open spaces between trabeculae are filled with red or yellow bone marrow

Gross Anatomy of short, irregular and flat bones

-thin plates of spongy bone (diploe) covered by compact bone

-periosteum: covers the outside of compact bone

-endosteum: covers the inside of compact bone

-bone marrow is scattered throughout spongy bone; no defined marrow cavity

-articular cartilage: hyaline cartilage covers movable joint surfaces

composition of long bones

1. shaft or diaphysis

2. bone ens or epiphyses

3. membranes (periosteum, endosteum)

Gross anatomy of a long bone

diaphysis, epiphysis, epiphyseal line

Diaphysis

compact bone surrounding a central cavity filled with yellow bone marrow

Epiphyses

compact bone (external) and spongy bone (internal) covered by articular cartilage

epiphyseal line

demarcation between diaphysis and epiphysis, is a remnant of childhood epiphyseal plate where bone growth occurs

Periosteum

-white, double-layered membrane that covers external surfaces except joints

-anchor for tendons and ligaments

-contains nerve and blood vessels that exit through nutrient foramens

1. fibrous layer

2. osteogenic layer

fibrous layer of periosteum

outer layer consisting of dense irregular connective tissue consisting of Sharpey's fibers that secure to bone matrix

osteogenic layer

inner layer of osteogenic stem cells

Endosteum

-thin membrane of connective tissue lining the bone cavity

-covers trabeculae of spongy bone

-lines canals that pass through compact bone

-contains osteogenic stem cells

-capillaries in endosteum supply nutrients

bone markings

projection, depression, opening

Bone Markings: Projections

outward bulge of bone

May be due to increased stress from muscle pull or is a modification for joints

Bone Markings: Depressions

groove can severe as passageways for vessels and nerves, or is a modification for joints

Bone Marking: Openings

hole or canal in bone that are passageways for blood vessels and nerves

Types of projections

where muscles and ligaments attach

-Crest

-Spine

-Tuberosity

-trochanter

-line

-tubercle

-epicondyle

-process

Crest

narrow ridge of bone, prominent

spine

sharp, slender, often pointed projection

Tuberosity

large round projection

Trochanter

large, blunt, irregularly shaped process (only found on femur)

Line

small, round process

Tubercle

small round process

Epicondyle

Raised area on or above a condyle

process

any bony prominence

surfaces that form joints

head, facet, condyle

head

bony expansion carried on a narrow neck

facet

smooth, nearly flat articular (joint) surface

Condyle

rounded articular projection with a corresponding depression (fossa)

openings and depressions

fissure, foramen, meatus, fossa, notch, sinus

fissure

Narrow, slitlike opening

Foramen

large round opening through bone

Meatus

canal-like passageway

Fossa

shallow, basin-like depression in a bone, often serving as an articular surface

notch

indentation at the edge of a structure

sinus

Cavity within a bone, filled with air and lined with mucous membrane

Cells of bone tissue

1. Osteogenic cells

2. Osteoblasts

3. Osteocytes

4. Bone-lining cells

5. Osteoclasts

osteogenic cells

-Also called osteoprogenitor cells

-Mitotically active stem cells in periosteum and endosteum

-When stimulated, they differentiate into osteoblasts or bone-lining cells

-Some remain as osteogenic stem cells

Osteoblasts

-Bone-forming cells that secrete unmineralized bone matrix called osteoid

-osteoid is made up of collagen and calcium-binding proteins

-collagen makes up 90% of bone protein

-osteoblasts are actively mitotic

Ostyocyte

-mature bone cell in lacunae that no longer divide

-maintain bone matrix and act as stress or strain sensors

-respond to mechanical stimuli such as increased force on bone or weightlessness

-involved in bone remodelling

bone lining cells

-flat cells on bone surfaces believed to also help maintain matrix (along with osteocytes)

Periosteal cells: line external bone surfaces

endosteal cells: lines internal surfaces

osteoclasts

-break down bone

-from the white blood lineage

-giant, multinucleate cells

-cells have ruffled borders that serve to increase surface area for enzyme degradation of bones

Compact bones

hard outer shell of the bone

consists of

1. osteon

2.canals and canaliculi

3.interstitial and circumferential lamellae

Osteon

-structural unit of compact bone

-elongated cylinder that runs parallel to long axis of bone

-acts as tiny weigh-bearing pillars

-consist of several rings of bone matrix called lamellae

-lamellae contain collagen fibers that run in a different directions in adjacent rings

Canals

-central canal run through the core of osteon

-contains blood vessels and nerve fibers

-perforating canals: canals that lined with endosteum that occurs at right angles to central canal

-connect blood vessels and nerves of periosteum, medullary cavity and central canal

cavities and connections

lacunae & canaliculi

Lacunae (compact bone)

small cavities in bone that contain osteocytes

Canaliculi (compact bone)

Hairlike canals that connect lacunae to each other and the central canal

-communication

-movement of nutrients and waste

chemical composition of bone

-extracellular matrix that surrounds distantly separated cells

-ECM

~15% water

~30% collagen

~55% crystallized mineral salts

-hydroxyapatite

Formation of bony skeleton

-up to about week 8, fibrous membranes and hyaline cartilage of fetal skeleton are replaced with bone tissue

-cartilage remains in areas requiring flexibility

-articular cartilage

-at the epiphyseal plate

Clacification

hardening of bone and cartilage due to calcium deposition, occurs during normal bone growth in youth

Bone Development

Human bones grow until about age 25

Ossification

formation of the bony skeleton

4 situations where bones form

1. During embryological and fetal development

2. When bones grow before adulthood

3. When bones remodel

4. When fractures heal

types of ossification

intramembranous and endochondral

endochondrial ossification

-replaces hyaline cartilage with bone in the developing embryo and fetus

-form most of skeleton except clavicles

-occurs in epiphyseal plates of long bones as they grow in length

ossification process

1. mesenchymal stem cells specialize into osteoblasts

2. osteoblasts secrete osteoid against the diaphysis creating a bone collar encasing cartilage

3. cartilage cells hypertrophy in the centre of the cartilage shaft

4. central cartilage in diaphysis calcifies

5. chondrocytes die due to a lack of nutrients, the matrix deteriorates and cavities form

6. cavities are invaded by the blood vessels, nerves, red marrow, osteogenic cells, and osteoclasts from the periosteal bud

7. osteoclasts erode the calcified cartilage matrix

8. remaining clarified cartilage fragment form the earliest spongy bone

9. bones thicken thanks to the cooperative action of the osteblasts and osteoclasts

10. as osteoblasts deposit bone on the outer surface osteoclasts widen the medullary cavity by breaking down the newly-formed spongy bone from within

11. the cartilage along the shaft calcifies and is replaced by bone

12. secondary ossification centres appear in epyshises and the epiphyses ossify

intramembranous ossification

process of bone development from fibrous membranes

intramembranous ossification process

-occurs in flat bones when a fibrous connective tissue membrane is replaced by bone

-bone formed by mesenchymal cells

-forms frontal, parietal , occipital, temporal and clavicle bones

4 steps of ossification

1. ossification centers are formed when mesenchymal cells cluster and become osteoblasts

2. osteoid is secreted, then calcified

3. woven bone is formed when osteoid is laid down around blood vessels, resulting in trabeculae that forms the periosteum

4. lamellar bone replaces woven bone, red marrow appears

post natal bone growth

-bones grow lengthwise via interstitial growth

-bones increase in width through appositional growth

increasing bone length

occurs along the epiphyseal plate

Five zones of epiphyseal plate

1. Zone of resting cartilage

2. Zone of proliferating cartilage

3. Zone of hypertrophic cartilage

4. Zone of calcified cartilage

5. Zone of ossification

resting zone of epiphyseal plate

Area of cartilage on epiphyseal side of epiphyseal plate that is relatively inactive

proliferation zone of epiphyseal plate

area of cartilage that is dividing rapidly

-new cells formed move upward pushing epiphysis away from the diaphysis causing lengthening

hypertrophic zone of epiphyseal plate

region with older chondrocytes closer to diaphysis

-cartilage lacunae hypertrophy and leave large interconnecting spaces

calcification zone of epiphyseal plate

area where the surrounding cartilage matrix calcifies causing the chondrocytes to die, matrix to deteriorate and blood vessels to invade

ossification zone of epiphyseal plate

new bone formation, cartilage is being broken down

-chondrocytes deterioration leaves calcified cartilage at epiphysis-diaphysis junction

-calcified cartilage is eroded by osteoblasts

-ultimately replaced with spongy bone

-medullary cavity enlarged as cartilage is eroded

epiphyseal plate

Growth plate, made of cartilage, gradually ossifies

epiphyseal plate closure

occurs when epiphysis and diaphysis fuse

appositional bone growth

formation of new bone matrix on surface of bone, increase in width of bone

Hormones that regulate bone growth

growth hormone, thyroid hormone, testosterone, estrogen

Growth Hormone (GH)

most important hormone in stimulating epiphyseal plate activity in infancy and childhood

Thyroid Hormone (TH)

modulates activity of growth hormone, ensuring proper proportions

testosterone and estrogen

promote adolescent growth spurts, end growth by inducing epiphyseal plate closure

bone deposition

the addition of minerals and collagen fibers to bone by osteoblasts

Osteiod seam

band of unmineralized bone matrix that marks area of new matrix

calcification front

abrupt transition zone between osteoid seam and older mineralized bone

bone resorption

the removal of minerals and collagen fibers from bone by osteoclasts

-break down bony matrix

-secrete lysosomes and enzymes and acid

control of bone remodeling

-controlled by negative feedback loop

1. maintaining homeostasis of calcium levels in the blood

2. response to mechanical stress