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Bone Functions

Structural framework for support of the skeleton

Protection of organs (brain, spinal cord)

Levers for muscles

Reservoir for minerals (stores 99% calcium and phosphates)

Bone Matrix

EC matrix (ground substance and fibres) of bone consists of inorganic and organic components that have calcified

Inorganic material (65%)

Mainly calcium phosphate, calcium carbonate and magnesium, sodium, potassium, bicarbonate, fluoride, citrate, sulphate, and hydroxide
Gives bone both hardness and rigidity

Organic component (35%)

Mostly type I collagen (95%)
Gives bone slight flexibility
And ground substance containing glycosaminoglycans with proteins (proteoglycans)
Gives bone resilience

Bone Cells

Osteoprogenitor cells

From embryonic mesenchyme

Differentiate into osteoblasts

Most active during period of intense bone growth

Found in inner cellular layer of periosteum (osteogenic layer),

Lining Haversian canals,
And in endosteum (lining of medullary cavity)

Osteoblasts

Responsible for formation and growth of new bone

Found on surfaces of existing bone tissue or calcified cartilage

Where they deposit new bone matrix (osteoid), which has no minerals

Later, mineralisation occurs, and tissue is new bone

Osteocytes

Elongated cells with cytoplasmic processes

Maintain bone tissue and store minerals

As secreted bone matrix accumulates, each osteoblast surrounded by matrix

Cell now known as osteocyte, and space it occupies called a lacuna

Radiating out from lacuna are narrow spaces (canaliculi), which have cytoplasmic processes of osteocytes

Allow transfer of nutrients, metabolites, and wastes between osteocytes and blood

Osteoclasts

Large, motile, multinucleated cells (150 µm diameter)

Contain up to 50 nuclei

Break down and reabsorb bone

Have bone marrow precursor (macrophage progenitor cell)

In presence of bone, osteoclast precursors fuse to produce multinucleated osteoclast

Occupy shallow depressions (Howship’s lacunae)

Ruffled border (infolded plasma membrane) of osteoclast inc surface area plasmalemma involved in resorption of bone

Periosteum

Vascular, fibrous layer

Surrounds bone, except over articular surfaces

Outer layer

Dense irregular collagenous fibres with some elastic fibres
Distributes vascular and nerve supply to bone

Inner layer

Cellular (osteogenic layer, osteoprogenitor cells), provides new bone
Central medullary cavity of bone lined with endosteum, thin CT of osteoprogenitor cells and osteoblasts

From outer layer of periosteum, bundles of collagenous fibres (Sharpey’s) penetrate underlying bone at intervals,

To attach periosteum, especially at sites of attachment of tendons and ligaments

Bone Matrix Development

  1. First made as osteoid (collagen and GAG’s with no minerals)
  2. Becomes mineralised (immature, primary or woven bone)
    First bone in development and in repair after fractures
  3. Starts to remodel as adult form (mature, secondary, lamellar)
    Most calcium impregnated
    Collagenous fibres arranged in tight, concentric layers or lamellae (12µm thick)
    Laid down around blood vessels during development to form Haversian systems or osteons

Mature bone organisation

Dense (compact)

Usually at edge of bone

Has Haversian systems (osteons)

Complex of 4 to 20 concentric, circular lamellae surrounding central (Haversian) canal (20-100µm diameter)

Canal contains blood vessels, with unmyelinated nerve fibres, loose CT, and osteogenic and osteoblast cells that line lumen of canal

Osteocytes are in lacunae located within or between lamellae

Arranged as cylindrical tubes parallel to longitudinal axis of long bone

Second arrangement of lamellae as lamellar fragments found between osteons, interstitial lamellae

Remnants of older, partially reabsorbed Haversian systems

Third arrangement are circumferential lamellae

Several rings of bone around the entire bone, immediately beneath periosteum
These thick lamellae not organised into osteons

Outer circumferential lamellae are just deep to the periosteum, forming outermost region of diaphysis

Contain Sharpey’s fibres anchoring periosteum to bone

Inner circumferential lamellae, adjacent to outer circumferential lamellae, completely encircle marrow cavity

Radiating from lacunae are many canaliculi (small channels)

Haversian canals have communications with marrow cavity, periosteum, and each other

Via transverse Volkmann’s canals, run at right angles to long axis of bone

Each osteon has thin cement line, composed mostly of calcified ground substance with some collagen fibres

Spongy (cancellous) bone

Not organised into Haversian systems but a meshwork of thin bars (lamellae) or trabeculae of bone lining marrow cavity

The many spaces within bone filled with bone marrow

Trabeculae have osteocytes in lacunae that are nourished by diffusion from the marrow cavity

Blood and nerve supply

Bones have good blood supply

Vessels (e.g. periosteal vessels) penetrate cortical bone of diaphysis of long bones
And divide into branches that enter Haversian systems

Small myelinated and unmyelinated nerves follow blood vessels from periosteum into Haversian canals

Periosteum contains pain fibres which makes it sensitive to injury, e.g. the acute pain felt when striking the tibia

Histogenesis of bone development

Bone development is mesodermal

If earlier tissue is membrane-like (e.g. sheet of mesenchyme or loose CT)

It is intramembranous bone formation

If bone replaces cartilage that is largely reabsorbed before bone formed

It is endochondral (intracartilaginous) bone development

Intramembranous bone formation

Mesenchyme -> bone directly

Locations: most flat bones such as skull, mandible, clavicle

Events in intramembranous bone formation

Occurs in vascularised mesenchymal CT membrane

Some mesenchymal cells differentiate into osteoprogenitor cells and then osteoblasts

Which secrete osteoid (collagen, GAG, no minerals)
This region is primary ossification centre

Calcification by deposition of hydroxyapatite crystals of calcium and phosphate follows osteoid formation

Osteoblasts trapped in matrices become osteocytes in lacunae

Other cells arrive via blood and fuse to form multinucleated osteoclasts which reabsorb bone

As sponge-like network of bony trabeculae/spicules is established (primary, woven, immature bone)

Vascular CT in their interstices transformed into bone marrow

Addition of trabeculae to periphery inc size of forming bone (appositional growth)

Remodeled to secondary (lamellar) bone
Large bones have several ossification centres, which fuse with each other to form single bone

Areas of mesenchymal tissues that remain uncalcified differentiate into periosteum and endosteum of developing bone

Endochondral bone formation

  1. Miniature hyaline cartilage model formed in region where bone is to grow within embryo
  2. Cartilage model continues to grow appositionally and interstitially and serves as structural scaffold for bone development, is reabsorbed, and replaced by bone

Locations: long, short bones, pelvis, vertebrae

Events in endochondral bone formation

Primary ossification centre in diaphysis

Perichondrium midway at diaphysis of cartilage invaded by capillaries

Inner cells of perichondrium differentiate into osteoblasts

Which lay down thin collar of bone (subperiosteal) around midsection of cartilage plate

Osteoblasts surround remnants of calcified cartilage and enclose them, first with osteoid then calcified bone

Thus cartilage spicules replaced by bone spicules that coalesce to form spongy bone
This area becomes primary ossification centre of long bone, which expands -> both epiphyses as medullary cavity

Osteoclasts begin resorbing calcified cartilage/calcified bone complex enlarging marrow cavity

As this continues, cartilage of diaphysis replaced by bone
Except epiphyseal plates (responsible for continued growth for 18-20 years)

Secondary ossification centres in epiphysis

Centre of epiphyseal cartilage gives way to spongy bone similar to that in diaphysis

Osteoprogenitor cells invade cartilage of epiphysis, differentiate into osteoblasts, begin secreting matrix on cartilage scaffold
Eventually cartilage of epiphysis replaced with bone

Differences between 2 ossification centres are that

A bony collar not formed
No definitive marrow cavity
Cartilage on articular surface and epiphyseal plate remains hyaline cartilage

Developing bone region at epiphyseal plate

Area between shaft and epiphysis is epiphyseal plate

Proliferation occurs at epiphyseal aspect, replacement by bone takes place at diaphyseal side of plate

Centre of plate -> diaphysis, 5 zones:

Zone of reserve cartilage: chondrocytes distributed throughout matrix mitotically active producing hyaline cartilage
Zone of proliferation: chondrocytes rapidly proliferate and form stacks of isogenous cells that parallel direction of bone growth
Zone of maturation and hypertrophy: chondrocytes mature, hypertrophy and accumulate glycogen in cytoplasm
Zone of calcification and cell death: hypertrophied chondrocytes die and cartilage matrix becomes calcified, impregnated with hydroxyapatite crystals of calcium and phosphorus
Zone of ossification: blood vessels invade spaces left by dying chondrocytes carrying osteoprogenitor cells from periosteum and differentiate into osteoblasts which release matrix that becomes calcified on surface of calcified cartilage, some osteoblasts entrapped as osteocytes and bone spicules formed. Coalescence of spicules creates spongy bone. Resorption of spongy bone by osteoclasts in centre of diaphysis enlarges medullary cavity

Growth

Length

Long bones grow in length as result of interstitial growth of epiphyseal cartilage

Growth continues until early adulthood when epiphyseal plates close and growth in length ceases

Also, primary and secondary ossification centres fuse

Width

Growth of diaphysis in girth takes place by appositional growth from surface and resorption by osteoclasts of inner part of shaft so that marrow space can be enlarged

Bone remodelling

Continual remodelling occurs in life in response to forces

Bone deposited where there is traction and resorbed where there is exerted pressure

In young, bone development > bone resorption, new Haversian systems developed faster than old ones being resorbed

As adult, new bone development = bone resorption