Skeletal tissues are composed of solid matrix and cells. It includes notchordal tissues. Cartilages and bones. It forms endoskeletal elements in chordates. It protects and supports the soft part of the body.
In protochordates the notochord is a rod-like structure supporting various parts of the body. In vertebrates, it is more or less replaced by the vertebral column. The notochordal tissue has numerous tightly packed vacuolar cells. The vacuoles are filled with a fluid to make the tissue turgid. Surrounding the vacuolar cells a single layer of peripheral cells there present. The peripheral cells secret an inner elastic sheath which is further surrounded by thick band of outer chordal sheath formed of connective tissue.
The cartilages are harder than the muscles and softer than the bones. In other words, they are called soft bones. But structurally these differ from the bones.
A cartilage is made up of a protein called chondrin secreted by the chondrioblasts or chondriocytes. These cells are present in one to four in numbers in fluid-filled lacunae. The lacunae are arranged irregularly in the matrix, to which the chondrioblasts secret. The surface of the cartilage is covered by a layer of connective tissue called perichondrium. Four types of cartilages are distinguished as follows:
(a) Hyaline Cartilage:
Its matrix is homogeneous, translucent and soft. It is bluish white in colour in fresh condition. It is devoid of any fibres hence it is glass like. It is found in trachea, nasal septum, larynx and thyroid gland.
(b) Fibrous Cartilage:
Its matrix is made up of white collagenous fibres. It is found in inter-vertebral disc of vertebral coloumn.
(c) Elastic Cartilage:
This cartilage contains a large number of yellow elastic fibres in matrix. It is highly flexible. Ear pinna, epiglottis and tip of nose possess this cartilage.
(d) Calcified Cartilage:
It is hard. Its matrix contains calcium salts. It is found in the suprascapula and at the head of humerus and femur in adult frogs.
Bone is also called osseous tissue. It is the hardest connective tissue. It is composed of osteocytes (bone cells) and hard matrix containing calcium salts and proteins. The proteins of the bone matrix called oseein. Ostgenic collagen and elastic fibres are embedded in the matrix. The bone cells which secret ossein called osteoblast and the bone cells, which dissolve bones, called osteoclast. The osteocytes are spider-shaped mature bone cells. The bone cell lie in a fluid filled space called lacunae. Bones are of two types- compact bone and spongy bone.
In mammalian long bones, there is a central cavity called marrow cavity. It is made up of mostly haematopoietic tissue and a few apidose tissues. The haematopoietic tissue forms bloods cells.Bone marrows are of two types – yellow marrow and red marrow. Yellow marrow is made up of apidose tissue and found in marrow cavity of long bones. The red bone marrow lies in spongy bones. The wall of a long bone comprises from outside periosteum, periosteal lamellae, Haversian system and endosteal lamellae.
It is an outermost covering of bones. It is absent at articular surface of long bones. It is made up of fibrous connective tissue. It is supplied with blood and lymphatic vessels. It provides surface for attachment of muscles.
(ii) Periosteal Lamellae:
These are concentric layers of bony deposits below the peristeum. It is pierced by strands of fibrous tissue called perforating fibres of sharpey.
(iii) Haversian System:
It is also called osteon. It is the structural and functional unit of compact bones. The wall of compact bones is pierced by numerous longitudinal tunnels called Haversian canal. The Haversian canals are connected by transverse canals called Volkmann’s canal. The canals allow the blood vessels and nerve fibres to pass through them. There are concentric layers of bony deposits around each Haversian canal called Haversian lamellae. Between two adjacent lamellae there are fluid filled spaces called lacunae. Lacunae are arranged in circle around each Haversian canal. Each lacuna communicates with other lacuna through fine canals called canalculi. Each lacuna contains an osteocyte and canaliculi contain process of osteocytes.
Each Haversian system is made upon of Haversian canal, Haversian lamellae, Osteocytes in lacunae with intercommunicating canaliculi. Each Haversian system is separated from other by a thin line called cementing lining of Ebner.
These are concentric layers bony deposits on outer side of the endosteum.
It lines up the marrow cavity. It is made up of reticular cells. Spongy bone- It is also called cancellous bone. It is found in ends of long bones, inner part of flat bones. This bone lacks Haversian system and marrow cavity.
Functions of Bone:
- It forms skeletal framework of tbody and provides shape to the body.
- It protects delicate and vital organs.
- It is a reservoir of calcium ions.
- Harmful heavy metals (Pb) are absorbed and deposited in bone and teeth.
- Locomotion is possible through skeletal system.
- Bone marrow forms blood cells.