► forms theforms the strong and rigidstrong and rigid endoskeletonendoskeleton
to which skeletal muscles are attached toto which skeletal muscles are attached to
permit movement.permit movement.
► acts as aacts as a calcium reservoircalcium reservoir and isand is
important in calcium homeostasis.important in calcium homeostasis.
► heavyheavy and its architecture is optimallyand its architecture is optimally
arranged to provide maximumarranged to provide maximum strengthstrength forfor
the least weight.the least weight.
Bone as a tissueBone as a tissue
►composed of cellscomposed of cells
► predominantly collagenous ECMpredominantly collagenous ECM
type I collagentype I collagen
becomes mineralised by the deposition ofbecomes mineralised by the deposition of
calcium hydroxyapatitecalcium hydroxyapatite
giving the bone considerable rigidity andgiving the bone considerable rigidity and
CELLS of the bone:CELLS of the bone:
► derived from aderived from a
primitive mesenchymalprimitive mesenchymal
(stem) cell called the(stem) cell called the
phagocytic cellsphagocytic cells
derived from thederived from the
cell linecell line
► synthesise osteoid and mediate itssynthesise osteoid and mediate its
► they are found lined up along bonethey are found lined up along bone
► housed in lacuna (cavity) within matrixhoused in lacuna (cavity) within matrix
► represent inactive osteoblasts trappedrepresent inactive osteoblasts trapped
within formed bonewithin formed bone
► they may assist in nutrition of bone.they may assist in nutrition of bone.
► phagocyticphagocytic cellscells
which are capable ofwhich are capable of
eroding bone anderoding bone and
which are important,which are important,
along with osteoblasts,along with osteoblasts,
in the constantin the constant
turnover andturnover and
► Intramembranous Ossification: Mesenchymal cells develop intoIntramembranous Ossification: Mesenchymal cells develop into
osteoblasts and synthesize bony matrix, becomes osteocytes trappedosteoblasts and synthesize bony matrix, becomes osteocytes trapped
in the bone matrix. Osteoclasts are multinucleated, phagocytotic cellsin the bone matrix. Osteoclasts are multinucleated, phagocytotic cells
releases hydrolases into the matrix to decalcify the bony matrix.releases hydrolases into the matrix to decalcify the bony matrix.
Other important structures:Other important structures:
► Canaliculi – provide passages forCanaliculi – provide passages for
circulation of ECF & diffusion ofcirculation of ECF & diffusion of
metabolites between lacunae andmetabolites between lacunae and
vessels of Haversian canals.vessels of Haversian canals.
►CT layersCT layers
Periosteum – covers external surfacePeriosteum – covers external surface
Endosteum – covers internal surfaceEndosteum – covers internal surface
Areas within bones:Areas within bones:
► Most bones have aMost bones have a
dense rigid outer shelldense rigid outer shell
ofof compact bonecompact bone,,
► and a centraland a central
cancellouscancellous zone ofzone of
thin interconnectingthin interconnecting
narrow bonenarrow bone
Bone cortex – COMPACT boneBone cortex – COMPACT bone
SPONGY / CANCELLOUS boneSPONGY / CANCELLOUS bone
► a network of interconnecting strutsa network of interconnecting struts
orientated in a position to provide theorientated in a position to provide the
maximum strength for the minimum mass.maximum strength for the minimum mass.
►They are composed of lamellar bone withThey are composed of lamellar bone with
scanty lacunae containing osteocytes.scanty lacunae containing osteocytes.
The trabeculaeThe trabeculae TT have a thin external coating ofhave a thin external coating of
endosteum containing flat inactive osteoblasts.endosteum containing flat inactive osteoblasts.
Long bone section:Long bone section:
(Spongy) bone –(Spongy) bone –
porous withporous with
(collagenous tissue)(collagenous tissue)
► Compact Bone –Compact Bone –
calcified or hardcalcified or hard
bone (periosteum)bone (periosteum)
► Medullary cavity –Medullary cavity –
contains the bonecontains the bone
marrow (endosteum)marrow (endosteum)
Seatwork: by partnersSeatwork: by partners
(use 1 yellow paper only)(use 1 yellow paper only)
► Outline the sequence of events during boneOutline the sequence of events during bone
formation or OSTEOGENESIS.formation or OSTEOGENESIS.
Intramembranous ossificationIntramembranous ossification
Endochondral ossificationEndochondral ossification
Bone Formation orBone Formation or
► Fetal development of bone occurs in twoFetal development of bone occurs in two
Endochondral ossification -Endochondral ossification - cartilage bonescartilage bones
Intramembranous ossification -Intramembranous ossification - membranemembrane
► Both of which involve replacement ofBoth of which involve replacement of
primitive collagenous supporting tissue byprimitive collagenous supporting tissue by
Endochondral OssificationEndochondral Ossification
requires a pre-existing cartilage model
► embryonal type
► continuously growingcontinuously growing
followed by deposition of bone matrix.
cartilage model is progressively replacedcartilage model is progressively replaced
by bone.by bone.
►Occur in formation of long bones, vertebrae, pelvisOccur in formation of long bones, vertebrae, pelvis
and bones of the base of the skull.and bones of the base of the skull.
1.1. A hyaline cartilage model starts with a boneA hyaline cartilage model starts with a bone
Cartilage model - a small model of the longCartilage model - a small model of the long
bone is first formed in solid hyaline cartilage.bone is first formed in solid hyaline cartilage.
Undergoes appositional growth to form anUndergoes appositional growth to form an
elongated, dumb-bell shaped mass of cartilageelongated, dumb-bell shaped mass of cartilage
1.1. At diaphysis: Bone collar undergoAt diaphysis: Bone collar undergo
intramembranous ossification and forms anintramembranous ossification and forms an
calcified cartilage plate.calcified cartilage plate.
Intramembranous ossification in diaphysis of theIntramembranous ossification in diaphysis of the
cartilage model of long bone.cartilage model of long bone.
► Chondrocytes enlarge greatly, resorbing theChondrocytes enlarge greatly, resorbing the
surrounding cartilage so as to leave only slendersurrounding cartilage so as to leave only slender
perforated trabeculae of cartilage matrix.perforated trabeculae of cartilage matrix.
► This cartilage matrix then becomes calcified andThis cartilage matrix then becomes calcified and
the chondrocytes degenerate leaving large,the chondrocytes degenerate leaving large,
interconnecting spaces.interconnecting spaces.
► Perichondrium develops osteogenic potential andPerichondrium develops osteogenic potential and
assumes the role of periosteum.assumes the role of periosteum.
► The periosteum then lays down a thin layer ofThe periosteum then lays down a thin layer of
bone around the surface of the shaft.bone around the surface of the shaft.
► Primitive mesenchymal cells and blood vesselsPrimitive mesenchymal cells and blood vessels
invade the spaces left within.invade the spaces left within.
► The primitive mesenchymal cells differentiate intoThe primitive mesenchymal cells differentiate into
osteoblasts and blood-forming cells of the boneosteoblasts and blood-forming cells of the bone
► The osteoblasts form a layer of cells on theThe osteoblasts form a layer of cells on the
surface of the calcified remnants of the cartilagesurface of the calcified remnants of the cartilage
matrix and commence the formation of irregular,matrix and commence the formation of irregular,
woven bone.woven bone.
3.3. Nutrient vessels penetrate the diaphysisNutrient vessels penetrate the diaphysis
after osteoclasts invaded the area, theafter osteoclasts invaded the area, the
osteoprogenitor cells follow the movementosteoprogenitor cells follow the movement
of blood vessels.of blood vessels.
4.4. At diaphyseal area: invading osteoblastsAt diaphyseal area: invading osteoblasts
undergo matrix synthesis (undergo matrix synthesis (primary
Secondary ossificationSecondary ossification
centers appear at thecenters appear at the
swellings of theswellings of the
cartilage modelcartilage model
(epiphyses). During(epiphyses). During
their expansion andtheir expansion and
remodeling, theremodeling, the
primary and secondaryprimary and secondary
ossification centersossification centers
produce cavities thatproduce cavities that
are gradually filled withare gradually filled with
bone marrow.bone marrow.
► In the secondary ossification centers, cartilageIn the secondary ossification centers, cartilage
remains in two regions:remains in two regions:
1. articular cartilage,1. articular cartilage, which persists throughoutwhich persists throughout
adult life and does not contribute to bone growthadult life and does not contribute to bone growth
2. epiphyseal cartilage,2. epiphyseal cartilage, also called thealso called the
epiphyseal plate,epiphyseal plate, which connects the epiphyseswhich connects the epiphyses
to the diaphysis, is responsible for the boneto the diaphysis, is responsible for the bone
elongation, this plate disappears in adults, which iselongation, this plate disappears in adults, which is
why bone growth ceases in adulthood.why bone growth ceases in adulthood.
Zones of Bone GrowthZones of Bone Growth
1)1) Resting zoneResting zone - hyaline cartilage without morphological changes- hyaline cartilage without morphological changes
in the cells.in the cells.
2)2) Proliferative zoneProliferative zone -- chondroblasts mitotically divides and formchondroblasts mitotically divides and form
columns of stacked cells parallel to the long axis of the bone.columns of stacked cells parallel to the long axis of the bone.
3)3) Hypertrophic cartilage zoneHypertrophic cartilage zone - large chondrocytes with- large chondrocytes with
glycogen-filled cytoplasm. The resorbed matrix is reduced to thinglycogen-filled cytoplasm. The resorbed matrix is reduced to thin
septa between the chondrocytes.septa between the chondrocytes.
4)4) SSimultaneous with the death of chondrocytes in theimultaneous with the death of chondrocytes in the calcifiedcalcified
cartilage zonecartilage zone ,, the thin septa of cartilage matrix becomethe thin septa of cartilage matrix become
calcified by deposition of hydroxyapatite (bone mineralization).calcified by deposition of hydroxyapatite (bone mineralization).
5)5) Ossification zoneOssification zone -- endochondral bone tissue appears. Bloodendochondral bone tissue appears. Blood
capillaries and osteoprogenitor cells formed by mitosis of cellscapillaries and osteoprogenitor cells formed by mitosis of cells
originating from the periosteum invade the cavities left by theoriginating from the periosteum invade the cavities left by the
chondrocytes. The osteoprogenitor cells form osteoblasts, whichchondrocytes. The osteoprogenitor cells form osteoblasts, which
are distributed in a discontinuous layer over the septa of calcifiedare distributed in a discontinuous layer over the septa of calcified
cartilage matrix. Ultimately, the osteoblasts deposit bone matrixcartilage matrix. Ultimately, the osteoblasts deposit bone matrix
over the calcified cartilage matrix .over the calcified cartilage matrix .
Intramembranous OssificationIntramembranous Ossification
►no hyaline cartilage model
►occurs within 'membranes' of condensed,occurs within 'membranes' of condensed,
primitive mesenchymal tissue.primitive mesenchymal tissue.
►mineralization of matrix by osteoblasts
►direct replacement of mesenchyme by bonedirect replacement of mesenchyme by bone
► occurs in the bones of the vault of the skull,occurs in the bones of the vault of the skull,
the maxilla and most of the mandiblethe maxilla and most of the mandible
1.1. Mesenchymal cellsMesenchymal cells osteoblastsosteoblasts
begin synthesis and secretion of osteoid atbegin synthesis and secretion of osteoid at
multiplemultiple centres of ossification.
1.1. Osteoblasts (trapped in lacunae)Osteoblasts (trapped in lacunae)
fine cytoplasmic extensions shrink to form thefine cytoplasmic extensions shrink to form the
fine processes contained within the canaliculi.fine processes contained within the canaliculi.
1.1. Osteoprogenitor cells at the surface of theOsteoprogenitor cells at the surface of the
centres of ossification undergo mitoticcentres of ossification undergo mitotic
division to produce further osteoblastsdivision to produce further osteoblasts
which lay down more bone.which lay down more bone.
4.4. Fusion of adjacent ossification centres toFusion of adjacent ossification centres to
form bone (spongy in gross appearance).form bone (spongy in gross appearance).
““woven bone” – collagen fibres of developingwoven bone” – collagen fibres of developing
bone are randomly arranged in interlacingbone are randomly arranged in interlacing
4.4. Progressive remodelling of woven boneProgressive remodelling of woven bone
into lamellar bone by osteoclasticinto lamellar bone by osteoclastic
5.5. Osteoblastic deposition to form matureOsteoblastic deposition to form mature
compact or trabecular bone.compact or trabecular bone.
6.6. The primitive mesenchyme remaining in theThe primitive mesenchyme remaining in the
network of developing bone differentiates intonetwork of developing bone differentiates into
bone marrow.bone marrow.