Osseous Tissue and Bone

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Osseous Tissue and Bone
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Osseous Matrix - The osseous matrix is composed
of two components 1. Organic Component
Chiefly osteocollagenous fibers These fibers are
difficult to see in ground bone preparations.
Fibers are held together with a special glue
consisting of glycosaminoglycans (a
glycoprotein). This organic material tends to be
acidophilic due to the small amount of
chondroitin sulfate present.     2. The
Inorganic Component The inorganic component is
principally calcium phosphate crystals. These
crystals are deposited within the cement between
the osteocollagenous fibers. Typically, bony
matrix is deposited in layers or lamellae 3 to 7
um thick. The presence of discrete layers around
the central canal is due to the fact that
collagen fiber is laid down spirally in layers
that are at 90 degrees of orientation to each
other.
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The cells of osseous tissue include 1.
Osteoblasts a. Associated with bone formation,
these cells are located where new bone is
forming, e.g., in the periosteum. b. The cells
vary in shape from cuboidal to pyramidal. c.
Osteoblasts often appear stratified as in an
epithelium. d. The nucleus is large with a
single prominent nucleolus. The cytoplasm
contains abundant ribosomes. These organelles are
responsible for the synthesis of the proteins of
the bony matrix. e. Osteoblasts contain the
enzyme alkaline phosphatase used to calcify the
osseous matrix.
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2. Osteocytes - Basically, an osteoblast that
has been enclosed within the bony matrix in a
space called the lacuna. a. The single nucleus
is darkly staining. b. The space occupied by
the osteocyte, the lacuna, has an irregularly
oval shape. Fine cytoplasmic processes of
osteocytes travel some distance into the
thread-like tunnels called canaliculi. The
canaliculi extend between lacunae of osteocytes
and the Haversian canals. c. In developing
bone, the cytoplasmic processes from one
osteocyte make contact with the processes from
adjoining osteocytes. In mature bone, the
processes are withdrawn almost completely. d.
In mature bone the empty canaliculi remain as
passageways for the diffusion of nutrients and
wastes between bone and blood.
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3. Osteoclasts a. Giant, multinuclear cells
which vary greatly in shape. b. They are found
on the surfaces of osseous tissue usually in
shallow depressions called Howships lacunae.
c. The cytoplasm contains lysosomal vacuoles.
d. Under E.M. the cell surface facing the
osseous matrix shows numerous cytoplasmic
projections and microvilli described as a ruffled
border. e. It is thought that osteoclasts
arise by fusion of uninucleated osteoprogenitor
cells or from fused monocytes which emigrate from
the blood. f. The osseous matrix facing the
osteoclast appears demineralized. This and other
studies have led to the assumption that the
osteoclast is involved in bone resorption. When
bone resorption activity ceases in a region of
bone (e.g., following the healing of a fracture)
osteoclasts are not apparent
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4. Chondrocytes are found in the hyaline
cartilage of epiphyseal growth plates of growing
bones and at the articular ends of bone.
Hyaline Cartilage contains fiber in its outer
layer, the perichondrium. Chondroblasts arise
from the perichondrium. As the cartilage ages,
the cells appear in clusters of 2, 4 or 8. The
matrix consists primarily of chondroitin sulfate.
Hyaline cartilage can be found at the articular
ends of long bones, as well as the trachea,
bronchi and larynx.
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Periosteum a fibrous sheath covering a long
bones shaft (diaphysis) but not the articulating
surfaces. The periosteum consists of two layers
1. A dense fibrous, vascular, outer layer 2.
An inner more loosely arranged connective tissue
containing a. elastic fibers osteoblasts
and osteoprogenitor cells for growth and
repair b. coarse Sharpeys fibers which
anchor the periosteum to the underlying
bone Functions of the periosteum include a.
Anchors tendons and ligaments to bone b. Allows
passage of blood vessels, lymphatics and nerves
into and out of the bone c. Participates in
growth (appositional) and repair through the
activities of the osteoprogenitor cells.
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Compact Bone located around the diaphysis.
Osseous tissue found immediately below the
periosteum. The most significant feature of
compact bone is the Haversian system. Each
Haversian system consists of 1. Concentrically
arranged lamellae of calcified matrix (each
lamella is about 3 to 7 um thick). 2. Hair -
like canaliculi cross lamellae and connect the
lacunae containing living osteocytes with a
central Haversian canal. 3. The longitudinal
Haversian canals branch or anastomose freely with
each other. 4. Volkmanns canals enter compact
bone from the endosteal and periosteal surfaces
and join the Haversian canals at right angles.
Blood vessels, lymphatics and nerves pass through
Volksmanns canals, Haversian canals and all of
their branches (anastomoses). As a result, there
is a very extensive transport system within
compact bone.
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COMPACT BONE-Haversian canals

• Compact bone consists of bone matrix laid down in
  concentric layers called lamellae. The lamellae
  surround a central Haversian canal(Hc) which
  contains blood vessels, nerves and lymphatics.
  Very delicate thread-like lines can be seen
  connecting the black osteocytes (oc)with the
  Haversian canal.

Haversian Canals
oc
oc
oc
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Spongy Bone is commonly seen within the
epiphyses and immediately beneath the endosteum
of the diaphyses of long bones. It is also found
sandwiched between two layers of compact bone in
a flat bone.     Spongy bone consists of
trabeculae or plates forming an interconnected
network. The trabeculae are comprised of a
varying number of lamellae with lacunae
containing osteocytes. In prenatal spongy
bone and in regions of regenerating bone, the
lamellae cannot be distinguished since the
osteocollagenous fibers are arranged in an
irregular fashion. This is called woven bone.
In older spongy bone, the fibers between the
lamellae are arranged at 90 degrees to each
other. This arrangement allows the lamellae to be
distinguished from one another.
Hematopoietic (blood-forming) tissue can be found
within the spaces of spongy bone especially
within the epiphyses.
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Development and Growth of Bone Bone has special
characteristics which will influence how bone
grows and develops. The solid,
mineralized matrix of bone is permeated with an
extensive system of canaliculi which extend from
each lacuna to the surfaces of bone and the canal
system. The fluid in these spaces and surfaces
permit an exchange of metabolites and gases
between the blood system and the bone cells.
This allows the survival of osteocytes even
though they are encased in a bony capsule. Bone
is a richly vascular tissue. At a distance of 0.5
mm, the canalicular system becomes highly
inefficient in supplying osteocytes with food and
oxygen. The extensive network of Haversian and
Volkmanns canals bring the blood close enough to
the osteocytes so that the diffusion distance
through the canaliculi does not exceed 0.5 mm for
any osteocyte. Bone can only grow
appositionally, i.e., a process in which new bone
is added to one surface. Bone cannot grow
interstitially, from within by the division of
bone cells because of the presence of rigid
calcium salts within the matrix. The
architecture of bone is not static. Bone is
constantly being destroyed and reformed in
different regions simultaneously. It is a
constantly reforming tissue.
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Stages in Bone Healing