BONE

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BONE
Kharkov National Medical University
Department of histology, cytology and embryology
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BONE as a Tissue (single) as an
Organ (many tissues)
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Types of Bone Tissue
1.(Classification)

• According to its architecture
• Spongy (cancellous) and Compact (dense)
• According to its fine structure
• Primary (woven) and Secondary (lamellar)
• According to its histogenesis
• Intramembranous and Endochondral

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• Compact bone is also called dense bone. 2.
  Compact bone is composed of closely packed
  osteons. An osteon is also called a Haversian
  system.
• Spongy bone is also referred to as cancellous
  (trabecular) bone. The mineralized tissue is seen
  as plates and bars. Trabeculae are the bars seen
  in the spongy bone. Hollow marrow spaces are
  also present.
• Immature bone is woven bone.
• It is non-lamellar bone or bundle bone.

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Functions of Bone 3.

• Supports soft tissues
• Protects vital organs (cranium, thoracic cavity)
• Contains bone marrow
• Reservoir of Ca, PO4 to maintain constant
  concentrations in body fluids
• Allows body to move

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General structure 4.

• Cells
• - Osteoprogenitors (stem cells)
• Osteoblasts
• Osteocytes
• Osteoclasts
• Bone matrix
• - Organic components- osteoid (50) type I
  collagen fibers
• and ground substance
• - Inorganic components - Calcified material,
  lacunae, canaliculi, Calcium hydroxyapatite
• Periosteum and Endosteum
• Bone marrow

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Main Cells
5.1.Osteoblasts are bone-forming cells

• Synthesize organic components of matrix (collagen
  fibers type I, proteoglycans, glycoproteins.)
• Collagen forms osteoids strands of spiral
  fibers that form matrix
• Influence deposit of Ca, PO4.
• Active vs inactive osteoblasts
• Estrogen, PTH stimulate activity

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Osteoblasts
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2.Osteocytes 6.

• Mature bone cells that sit in lacunae with
  filopodia in the canaliculi
• Gap junctions between osteocytes provide
  nutrition (15 cells in a row)
• Maintain bony matrix long living cells
• Stimulated by calcitonin inhibited by PTH

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Osteocyte with cytoplasmic extensions (filopodia)
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Osteocytes with filopodia inside of the
Canaliculae of Lacunae
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3.Osteoclasts 7.
are bone-resorptive cells

• Derived from monocytes engulf bony material
• Active osteoblasts stimulate osteoclast activity
• Large, branched, motile, multinucliated cells
  lying in Howships lacunae
• Secrete enzymes that digest matrix

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Osteoclasts
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Histology of Compact Bone
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Osteon (Haversian system) 8.

• The whole complex of concentric lamellae of the
  bone surrounding a canal containing blood
  vessels, nerves, and loose connective tissue is
  called a Haversian system, or osteon .
• Lacunae containing osteocytes are found between
  , and occasionally within, the lamellae. In each
  lamella, collagen fibers are parallel to each
  other.
• Haversian canals communicate with marrow cavity,
  periosteum, other canals through Volkmanns canals

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Haversian system or 9. osteon is

• a Structural
• and functional unit
• of the Compact Bone

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OSTEONS
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10.Periosteum. The outer surface of the
bones is covered by a double-layered coat of
connective tissue. The outer fibrous layer of
periosteum is dense connective tissue. The inner
or osteogenic layer is a loose CT containing
osteoprogenitor cells.Endosteum lines the marrow
cavity. It is condensed reticular tissue that
contain bone and blood cell precursors.
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Bone Formation 11.

• Ossification - the process by which bone is
  formed
• Bone formation occurs in four situations
• Formation of bone in an embryo
• Growth of bones until adulthood
• Remodeling of bone
• Repair of fractures

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Bone Development 12.

• Bone may develop directly from mesenchyme or by
  the replacement of cartilage (indirectly).
• The process of replacing other tissues with bone
  is called ossification

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OSSIFICATION 13.

• begins at the 6th week of embryo development and
  continues up to age of 25
• Types
• Intramembranous ossification
• flat bones of skull, mandible and clavicle
  Endochondral ossification
• most of the bones in the body formed
• in the place of hyaline cartilage

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Intramembranous (direct) Ossification

• Intramembranous ossification -bone develops from
  mesenchyme.
• Mesenchymal cells start to secrete the organic
  components (primary ossification center).
  Differentiate into osteoblasts. Further
  differentiation into osteocytes.
• Spicule formation
• Formation of spongy and compact bone.

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Intramembranous Ossification

• Condensation of mesenchyme into trabeculae
• Osteoblasts on trabeculae lay down osteoid tissue
  (uncalcified bone)
• Calcium phosphate is deposited in the matrix
  forming bony trabeculae of spongy bone
• Osteoclasts create marrow cavity
• Osteoblasts form compact bone at the surface
• Surface mesenchyme produces periosteum

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Remodeling is secondary bone formation

• Osteoclasts erode the primary bone matrix, blood
  vessels, nerves and lymphatics invade the cavity
  and osteogenic cells develop in osteoblasts and
  osteocytes, which create concentric lamellae and
  osteons.
• Remodeling helps reshape growing bones to adapt
  to changing loads.

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INTRAMEMBRANOUS OSSIFICATION 14.

• 1.Mesenchyme cells differentiate into
  osteoblasts. Osteogenic islands.
• 2.Osteoblasts secrete osteoid matrix. Osteoblasts
  surround themselves with bone matrix, forming
  osteocytes. It is osteoid stage.
• 3.Osteoid becomes mineralized through
  crystallization of Ca salts using enzyme
  alkaline phosphatase and is called primary
  ossification center(OC). It is ossification stage
  and formation of spicules.
• 4.Blood vessels begin to grow spicules that meet
  and fuse together.
• Woven Bone (primary spongy bone).
• 5. Osteoclasts erode the primary bone matrix. It
  is remodeling or Secondary bone formation.

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Endochondral (indirect)Ossification 1

• Bone develops from pre-existing model
• perichondrium and hyaline cartilage
• Formation of primary ossification center (OC) and
  marrow cavity in shaft of model
• bony collar developed by osteoblasts
• chondrocytes swell and die
• stem cells give rise to osteoblasts and clasts
• bone laid down and marrow cavity created

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Endochondral Ossification 2

• Secondary ossification centers and marrow
  cavities are formed in the ends of the bone
• same process
• Cartilage remains as articular cartilage and
  epiphyseal (growth) plates
• growth plates provide for increase in length of
  bone during childhood and adolescence
• by early twenties, growth plates are gone and
  primary and secondary marrow cavities united

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• Endochondral bone formation 15.
• 1.Cartilage model
• 2.The periosteal bone collar (perichondral
  ossification)
• 3.Proliferation,hypertrophy,calcification of the
  cartilage. Formation of primary marrow cavity
• and Periosteal bud- small cluster of blood
  vessels
• 4. Primary ossification center
• 5.Secondary ossification center
• 7.Secondary bone formation and remodeling
• 8.Bone growth in length and girth

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Structure of the epiphyseal plate
16. 1. Zone of reserve cells (resting
cartilage) A thin layer (3-6 cells wide) of
small, randomly oriented chondrocytes adjacent to
the bony trabeculae on the articular side of the
growth plate. 2. Zone of proliferation
Chrondrocytes are stacked in prominent rows,
mitotic figures and the cartilage matrix becomes
more basophilic 3. Zone of maturation No
mitoses gradual cellular enlargement. 4. Zone of
hypertrophy Chrondrocytes and their lacunae
increase in size. 5. Zone of calcification
Deposition of minerals in the matrix surrounding
the enlarged lacunae causing cell death. 6. Zone
of ossification Osteoblasts deposit bone matrix
on the exposed plates of calcified cartilage. 7.
Zone of resorption Osteoclasts absorb the oldest
bone spicules.
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Bone Growth and Remodeling

• Bones increase in length
• interstitial growth of epiphyseal plate
• epiphyseal line is left behind when cartilage
  gone
• Bones increase in width appositional growth
• osteoblasts lay down matrix in layers on outer
  surface and osteoclasts dissolve bone on inner
  surface
• Bones remodeled throughout life
• Wolffs law of bone architecture of bone
  determined by mechanical stresses
• action of osteoblasts and osteoclasts
• greater density and mass of bone in athletes or
  manual worker is an adaptation to stress

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BONE GROWTH in LENGTH

• on epiphyseal side chondrocytes continually
  reproduce by epiphyseal plate
• on diaphyseal side chondrocytes swell and become
  surrounded by calcified matrix, chondrocytes die,
  bone replaces chondrocytes
• bone is eroded away by osteoclasts, marrow cavity
  forms
• epiphyseal plate remains constant until 18 years
  in females,20 years in males
• epiphyseal plate becomes ossified after 25 years
• It is stop growth of the bone.

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BONE GROWTH in girth(DIAMETER)

• Compact bone grows in thickness by proliferation
  and differentiation of osteoprogenitor cells in
  the inner layer of the periosteum and due to
  deposition of new ossified tissue on the outer
  surface of the bone.
• Marrow cavity increases in diameter
• Balance between osteoblasts and osteoclasts
  depends from Hormonal regulation.