BONE DEVELOPMENT

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BONE DEVELOPMENT Evolution of bone Experiments
with calcified cartilage Bone povided rigid
support New features introduced with advent of
bone 1. Canalicular system means of
diffusive communication boundaries open to
tissue spaces enabled metabolism in osseus
tissue
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2. Internal vascularity Canalicular system
effective only up to 0.2mm from tissue
fluid Vascular supply in matrix
solution Vessels in Haversian canals and
Volkmanns canals Interaction of
canalicular system and vascularity ? bone can
live indefinitely
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3. Appositional growth new type Interstitia
l growth not possible Thickness ? same plan
as cartilage Elongation ? new type of
appositional growth Epiphyseal plate
(disk) grows from outer edge, inner
edge replaced by bone
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4. Reconstruction Local destruction with
bone reformed After birth new matrix
deposited in layers During
reconstructions Haversian systems formed
(some sites) Parallel lamellae (periosteal
and endosteal) laid down last
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HISTOGENESIS OF BONE / OSTEOGENESIS Two types of
bone formation Intramembranous bone
formation Intracartilaginous bone formation or
(Endochondral bone formation) Types refer to
environment where development occurs Types of
formation are generally similar
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Generalities Specialized cells of bone
development Osteoblast From mesenchymal
cells medium sized cells Associate in
continuous layer along edge of forming
bone Cuboidal shape deeply basophilic with
special granules in cytoplasm for ground
substance Paler cells depleted
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Collagenous fibers of matrix are
osteocollagenous fibers Cytoplasm contains
alkaline phosphatase (deposition of
matrix) Osteoblasts delicate processes
along fibers and surface of forming
bone Process forshadow canaliculi Become
trapped in matrix Become Osteocytes Bone
forming role ceases except lacunar capsule
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OSTEOCLASTS Giant cells of bone
development Large mulitnucleated cells up to 100
µ several to dozens of nuclei Pale to
acidophilic, foamy cytoplasm Found where bone
matrix is being resorbed In regions of
calcified cartilage they are called
chondroclasts Occupy shallow pits (Howships
lacunae) Fringe by matrix (striate border?)
ruffled border
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Role of Osteoclast in matrix absorption Phosphatas
e enzymes released by cell Hydroxyapatite seen in
osteoclasts Osteoclasts ? phagocytic Osteoclasts
release enzymes to dissolve bone matrix Matrix
remnants in cytoplasm Parathyroid hormone
promotes resorption Calcitonin from thyroid
parafollicular cells inhibits Parathormone and
calcitonin counterbalance
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Osteoclast origin Mononuclear hemopoietic
progenitor cells CFU-GM ? neutrophilic
granulocytes and monocytes CFU-M ?
monocytes (CFU Colony Forming Unit) CFU-GM
and CFU-M related to macrophages Product of
cell fusions rather than nuclear divisions
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Tissue Participants Primitive Marrow Vascular
mesenchyme provides osteoblasts and blood
vessels Formation of spongy bone Perichondrium
of cartilage bone ? vascular bud into bone
model Perichondrium becomes periosteum Carti
lage breaks down as bud advances Tissue
differentiation ? primitive marrow and
osteoblasts
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Periosteum Inner layer of primitive
periosteum Acquires osteogenic ability Sheet of
osteoblasts from on inner surface Deposition of
bone matrix Early bone is spongy Later bone
deposited is compact Reabsorption along surface
of periosteum-bone contact
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Area of resportion
Replacement by bone
Cartilage growth
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Endosteum Peripheral layer of marrow tissue is
osteogenic endosteal lamellae Surface of
endosteum is also osteogenic Site of extensive
resorption of bone
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Cartilage Temporary fetal skeleton Develops
rapidly ? can keep up with rapid fetal growth
(bone cannot) Gives rise to unique epiphyseal
plates Growth in length of bone Cartilage
proliferates Cartilage destroyed Bone
deposited on remnants of bone
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OSTEOGENESIS
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