the skeletal tissue

2- The Skeletal Tissues:
These tissues compose the skeleton. They are either cartilage or bone.
I-Cartilage Is a specialized form of connective tissue in which the extracellular matrix has a firm consistency, it is essential for the development and the growth of long bones, It consist of cells (Chondrocytes) and an extensive extracellular matrix composed of fibers and ground substance. The cells are located in cavities called lacuna, also it surrounded by the perichondrium which is a capsule like sheath of dense connective tissue. Three forms of cartilage have evolved,each exhibiting variations matrix composition.

(1)- Hyaline Cartilage. Examine a T. S. of the trachea of the rabbit. Note that the matrix is solid and clear, and that the chondroblasts or chondrocytes are arranged either singly or in-groups of twos, fours or rarely eighths. Note that the single or group of chondroblasts lies in a clear capsule or lacuna and the presence the perichondrium.
































(c) - Elastic cartilage.Has a matrix rich in yellow fibers and surrounded by perichondrium it is found in the ear pinna and epiglottis of mammals.









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 aggregationas 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 .