Bone pdf - D. Talib - Muhadharaty

BONE

Is mineralized and specialized connective tissue confers mechanical and
metabolic functions to the skeleton

Bone is composed of intercellular calcified material, (matrix),
and 3 cell types:

 osteocytes (Gr. Osteon, bone +cytos, cell), which are found in

cavities (lacunae) within the matrix,

 osteoblasts (Osteon, bone + blasts, germ), which synthesize the

organic components of the matrix,

 osteoclast (Osteon, bone + klastos, broken), which are

multinucleated giant cells involved in the resorption and
remodeling of bone tissue.

Bone functions are:

1. Supports fleshy structures.

2. Protects vital organs such as those in the cranial and thoracic

cavities.

3. Harbors the bone marrow, where blood cells are formed.

4. Acts as a reservoir of calcium, phosphate, and other ions that can be

released or stored in a controlled fusion to maintain constant
concentrations of these important ions in body fluids.

5. Form a system of levers that multiply the forces generated during

skeletal muscle contraction and transform them into bodily
movement.

Schematic drawing of the wall of a long-bone diaphysis showing
3 types of lamellar bone: Haversian system and outer and inner
circumferential lamellae. The protruding Haversian system on the
left shows the orientation of collagen fibers in each lamella. At
the right is a Haversian system showing lamellae, a central blood
capillary, and many osteocytes with their processes.

metabolites are unable to diffuse through the calcified matrix of
the bone, the exchanges between osteocytes and blood capillaries
depend on communication through the canaliculi (L. canalis,
canal), which are thin, cylindrical spaces that perforate the
matrix. All bones are lined on both internal and external surfaces
by layers of tissue containing osteogenic cells

endosteum on the

internal surface and periosteum on the external surface

because if its hardness, bone is difficult to section with the
microtome, and special technique that permits the observation of
the cells and organic matrix is based on the decalcification of bone

preserved by standard fixatives. The mineral is removed by
immersion in solution containing calcium-chelating substance

(Ethyline diamine tetra acetic acid)
[EDTA] the decalcified tissue is then
embedded, sectioned and stained.

BONE CELLS

Osteoblasts: osteoblasts are responsible for the synthesis of
the organic component of bone matrix (type collagen,
proteoglycans, and glycoproteins).

 Depositions of the inorganic component of bone depend on

the presence of osteoblasts.

 Osteoblasts are exclusively located at the surfaces of bone

tissue,  side  by  side,  in  a  way  that  resembles  simple
epithelium.  When  they  are  actively  engaged  in  matrix
synthesis.

 Osteoblasts have cuboidal to columnar shape and basophilic

cytoplasm when their synthesizing activity declines, they
flatten and cytoplasmic basophilia declines.

 Some osteoblasts are gradually surrounded by newly

formed matrix and become osteocytes. During this process a
space called a lacuna is formed.

 lacuna are occupied by osteocytes and their extension along

with a small amount of extra cellular no calcified matrix.

 Osteocytes

Osteocytes  which  derive  from  osteoblast  lie  in  the  lacunae
situated  between  lamella  of  matrix.  Only  one  osteocyte  is
found  in  each  lacuna,  the  thin  cylindrical  matrix  canaliculi
house  cytoplasmic  processes  of  osteocytes  processes  of
adjacent cells make contact via gap junction and molecules are
passed  via  this structures  from cell to  cell

this exchange can

provide nourishment for a chain of about 15 cells

Schematic  drawing  of  2  osteocytes  and  part  of  a  Haversian
system.  Collagen  fibers  of  contiguous  lamellae  are  sectioned  at
different  angles.  Note  the  numerous  canaliculi  that  permit
communication between lacunae and with the Haversian canals.
Each lamella consists of multiple parallel arrays of collagen fibers.
In adjacent lamellae, the collagen fibers are oriented in different
directions.  The  presence  of  large  numbers  of  lamellae  with
differing  fiber  orientations  provides  the  bone  with  great
strength,despite its light weight.

When compared with osteoblasts the flat, almond-shaped
osteocyte exhibit a significantly reduce rough endoplasmic

reticulum and Golgi complex and more condensed nuclear
chromatin.

These cells are actively involved in the maintenance of bony
matrix and their death is followed by resorption of this matrix

 Osteoclasts

While  bone  matrix  is  deposited  by  osteoblasts,  it  is  eroded  by
osteoclasts,  these  large  (20  to  100Mm  in  diameter)  multi-
nucleated  (2  to  50  nuclei),  cells  are  a  type  of  macrophage.  Like
other microphages they develop from monocytes that originated
in the hemopoitic  tissue of the bone marrow.

These precursor cells are released into the bloodstream and
collect at sites of bone reabsoption where they fuse to form the
multinucleated osteoclasts. They are found close association with
the surface of bone, often in shallow excavations known as
Howships lacunae.

Pdf by zahraa jalil