the bone

II- Bone.
Bone is one of the hardest tissue of the body, is a specialized connective
tissue composed of intercellular calcified material, the bone matrix, two
types of bones, they are:
(a)- Compact bone
• Is largely composed of cylindrical units of bone
structure called Haversian systems, each consists
of a concentric lamellae of bone matrix surrounding
acentral canal called Haversian canal, Osteoblasts
or osteocytes, lying each in a lacunae. The lacunas
are connected together by fine Canaliculi.
• Note that other bone lamellae and some lacunae are
present a mong the Haversian systems, and are not
arranged around Haversian canals such as these are
called Non-Haversian systems .



(b)-Spongy bone
Is very similar to compact bone except
that the tissue is arranged as trabeculae
or spicules; numerous interconnecting
marrow spaces of various size are
present between the bone tissue the
matrix of the bone is lamellated .There
are three designated cell areassociated
with this tissue :
1-Osteoblasts, are recognized
by their cuboidal or polygonal
shape and their aggregations
single layer of cells.
2-Osteocytes, are differentiated
osteoblasts,occupies spaces
or lacuna.
3-Osteoclast, rest directly on
the surface of the bone where
resorption is take place as a
result of its activity a shallow
bay called Howships lacunae
is formed

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 ):-
• Zona of reserve cartilage, which exhibits no cellular
proliferation or active matrix production.
• 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 colums, these cells are larger than those in the
reserve zona and are actively producing matrix.
• Zona of hypertrophy, which contains cartilage cells that are
greatly enlarged ,their cytoplasm is clear,the glycogen
normally accomulates in.
• Zona of calcified cartilage, in which the enlarged cells
begin to degenerate and the matrix becomes calcified
• 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
therefor whole region appear as a honeycomb .