The connective tissues (skeletal tissues)
Bone
Bone is specialized connective tissues composed of intercellular
calcified material the bone matrix and 3 different cell types:
osteocytes (which are found activities )lacunae within the matrix
, osteoblast which synthesized the organic components of matrix
and osteoclast which are multinucleated giants cells involved in
resorption in bone tissues.
Since metabolites are unable to diffuse through the
calcified matrix of bone the exchanges between
osteocytes and blood depend on cellular communication
through canaliculi thin cylindrical spaces that perforate
matrix .
All bones are lined on both internal and external surfaces
by layers of tissues containing osteogenic cells
endosteum on the inner surface and periosteum on the
outer.
Bone cells
1-osteoblasts:
Osteocytes are responsible for the synthesis of the
organic components of bone matrix type I collagen
(proteoglycans, glycoprotiens).
2-osteocytes :
Which derived from osteoblasts lie in the lacunae situated
between lamella of matrix. Only one osteocytes found in
each lacunae the thin cylindrical canaliculi cytoplasmic
processes of osteocytes processes of adjacent cells make
contact via gap junction and nutrients are pass through
this way to the cells.
When compared with osteoblasts the flat , almond shape
,reduced rough endoplasmic reticulum and Golgi
complex and more condensed nuclear chromatin death of
osteocytes is followed by resorption of this matrix.
3-Osteoclasts:
Are very large, branched motile cells, have 5-50 or more
nuclei , the branched are irregular and vary in both
thickness and shape , osteoclasts are derived from the
fusion of blood derived monocytes and belong to the
mononuclear phagocytes system , osteoclaste are
acidophilic cytoplasm , some rough endoplasmic
reticulum , mitochondria ,and Golgi complex are found in
addition to the great number lysosomes present with the
cell , osteoclasts secrete acid, collagens, and other
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proteolytic enzymes that are attack the bone matrix
librates the calcified ground substances
Periosteum and endosteum:
Each bone is ensheathed by a tough , vascular fibrous
layer the peroisteum its cover the bone except the
articular surfaces , peroisteum have 2 layers outer layer
(collage nous fibers) with a small components of elastic
fibers , the inner layer is highly vascular and cellular
(osteogenic layer ) that give rise the concentration rings
of new bone.
The thin poorly defined cellular layer that lines the
medullary cavity of the shaft and the spaces of spongy
bone is the endosteum .
Bone formation or development
It is a complex process in which cartilage model serves as the
precursor of the bone (endochondral ossification) that is mean, the
continued growth of long bones is dependent on the presence of
Epiphyseal cartilage through out the growth period.
As the diaphyseal marrow cavity enlarges, a distal zonation can be
recognized in the cartilage at either end of the cavity., they are
(beginning with that most distal to the diaphyseal center of
ossification and proceeding toward that center ):1- Zona of reserve cartilage, which exhibits no cellular proliferation
or active matrix production.
2- Zona of proliferation, which is adjacent to the zona of reserve
cartilage in the direction of the diaphysis ,where the cartilage cells
undergo division and are organized into distinct column, these cells are
larger than those in the reserve zona and are actively producing matrix.
3-Zona of hypertrophy, which contains cartilage cells that are greatly
enlarged ,their cytoplasm is clear, the glycogen normally accumulates
in.
4-Zona of calcified cartilage, in which the enlarged cells begin to
degenerate and
matrix becomes calcified.
5-Zona of resorption , which is the zona nearest the
diaphysis ,here a small blood vessels and connective tissue
invade the region occupied by the dying chondrocytes and
therefore whole region appear as a honeycomb
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(Bone formation)
Types of bone:
There are 2 type of bone primary , immature ,or woven
bone and secondary mature or lamellar bone primary
bone is the first bone tissue to appear in emperyonic
development and in fracture and the other repair
processes it is characterized by random of fine collagen
fibers in contrast to organized lamellar disposition of
collagen in secondary bone primary bone is temporary
and is replaced in adults by secondary bone. Gross
observation of bone in cross section shows dense areas
with out cavities –corresponding to compact bone and
areas with numerous interconnecting in cavities
corresponding to cancellous (spongy) bone in long bones
the bulbous end called epiphyses are composed of spongy
bone covered by thin layer of compact bone the
cylindrical part diaphyses is composed of compact bone
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with small components of spongy bone on its inner
surfaces around the bone marrow cavity , short bones
usually have a core of spongy bone completely
surrounding by compact bone the flat bones that form the
calvarias have 2 layers of compact bone called plates
separated by layer of spongy bone called the dipole the
cavities of spongy bone and the marrow cavities in the
diaphyses of long bones contain bone marrow of which
there are 2 kinds red bon marrow in which blood cells are
forming and yellow bone marrow composed of fat cells.
Primary bone tissue:
1-Is the first bone tissue to appear is primary bone , it is
temporary and replaced in adults by secondary bone
tissue except in a very few places in the body ex: near the
structures of the flat bones of skull , in tooth sockets , and
in some tendons.
In addition to the irregular array of collagen fibers other
features of primary bone tissue are a smaller mineral
content and higher proportion of osteocytes than in
secondary bone tissue
( Compact bone)
Secondary bone tissue
Usually found in adults , it is show collagen fibers
arranged in lamella , that are parallel to each other or
concentrically organized a round a vascular canal, the
whole complex of concentric lamellae of bone
surrounding a canal containing blood vessels , nerve and
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loose connective tissues is called haversian system or
osteon , lacunae containing osteocytes are found between
and occasionally within the lamellae in each lamellae
collagen fibers are parallel to each other surrounding each
haversian system is a deposit of a amorphous material
called the cementing substance that consist of mineralized
matrix with few collagen fibers.
In compact bone the lamellae exhibit typical organization
consisting of the haversian system , outer circumferential
lamellae ,inner circumferential lamellae and interstitial
lamellae ,each haversian system is a long often
cylindrical parallel to the long axis of the diaphysis it
consist of central canal surrounded by concentric lamellae
each endosteum lined canal contains blood vessels ,
nerves and loose connective tissues the haversian canal
communicate with bone marrow cavity .
Bone repair:
when a bone is broken the following cellular events
immediately follow to repair the damage they involve not
only bone but also adjacent soft tissues.
1-torn blood vessels in the area of fracture cause hemorrhage
with the formation of blood clot
2-circulation to osteon in and near the fracture site is disrupted
causing death of osteocytes and necrosis of the bone fragments .
3-fibroblast and a new capillaries , largely from the priosteum
invade the clot to form the so called granulation tissue.
4-as the granulation tissue become more fibrous temporary
cartilage bars are formed that are later replaced by bone this
newly formed fibrous connective tissue cartilage bridge between
the ends of the fractured bone is called the callus it serves to
stabilize and bind together the bone fragments for healing.
5-As the callus is forming the inner osteogenic cells of
periosteum became active and lay down a sleeve of bone around
the callus extending some distance beyond the fractured ends.
6-periosteal buds invade the callus and produce spongy bone
which replaces the temporary cartilage a process similar to
endochondral bone formation.
7-the newly bone spongy bone is transformed in to compact bone
, lamellar bone .
8- by absorption the protruding surfaces and excess bone are
removed and the broken bone is essentially returned to its normal
contour and morphology.
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