The special connective tissue cartilage, bone and blood – each have unique cells and extracellular matrices that allow specialized functions.
• Cartilage is a soft skeletal tissue. It is not rigid like bone.
• It is found more abundantly in vertebrate embryo because most of the bones forming skeleton of the adult are cartilaginous in the early stage.
Structure of cartilage
• A typical cartilage consists of cartilage cells and ground substance (matrix).
(i) Cartilage cells or chondrocytes : They are present in a fluid-filled space, called lacunae within the cartilage ground substance. Young cartilage cells (chondroblasts) are small and flattened whereas mature cartilage cells (chrondrocytes) are large and rounded.
(ii) Ground substance (matrix) : It essentially consists of water, proteoglycans, some lipid, collagen, non-collagenous protein and fibres. The core protein is aggrecan. Carbohydrates are glucosaminoglycans (GAG) including chondroitin sulphate, keratin sulphate and hyaluronic acid.
Chondrocytes remain alive, even though there are no blood vessels in the cartilage matrix, because they receive oxygen and nutrients by diffusion through ground substance from surrounding blood vessels. This diffusion can only occur because cartilage matrix is not calcified.
Types of cartilage
• The cartilage is divided into three types as shown in the table below :
• Bone is a solid, rigid and strong connective tissue which gives support to various organs.
• A dried bone is a bone which has been exposed to drying conditions for a long time so that all living cells die, decay and disappear. The bone then contains only inorganic matrix.
• A decalcified bone is obtained when a bone is kept in dilute acid (HCl or HNO3) for long hours due to which calcium, magnesium, potassium and sodium salts of inorganic part of matrix get dissolved whereas organic part of matrix remains intact.
Structure of mammalian bone
• It consists of four parts :
(i) Periosteum : Periosteum is a thick and tough sheath of collagen that forms an envelop around the bone. Bundles of periosteal collagen fibres, called Sharpey’s fibres, penetrate the bone matrix to provide a firm connection between the two. The periosteum contains blood vessels and bone-forming cells, the osteoblasts which produce new bone material.
(ii) Matrix : The matrix is tough, containing both inorganic and organic substances. It consists of ossein protein (30%-35%) and mineralisation occurs by calcium phosphate salts (65%-70%). The Haversian canals, a characteristic feature of the mammalian bones, are present in the matrix. It contains an artery, a vein, a lymph vessel, a nerve, and some bone cells packed in connective tissue. These canals are interconnected by transverse channels called Volkmann’s canals. The matrix of the bone are present as layers
called lamellae. The lamellae are of four types:
–– Haversian lamellae : These lamellae occur around the Haversian canals. A Haversian canal with its surrounding lamellae and osteocytes constitute a cylindrical unit of bone called
Haversian system or osteon. Haversian systems are absent in spongy bones of mammals.
–– Interstitial lamellae : These lamellae occur between the Haversian systems.
–– Outer circumferential lamellae (outer concentric lamellae) : These lamellae occur inner to periosteum.
–– Inner circumferential lamellae (inner concentric lamellae) : These lamellae occur outer to endosteum.
(iii) Endosteum : It is present outer to the bone marrow cavity and comprises of white fibrous tissue and osteoblasts (bone forming cells). Because of presence of osteoblasts in both periosteum and endosteum, above can grow both from outside and inside i.e., growth of bone is bidirectional.
(iv) Bone marrow : In long bones, a cavity is present inner to endosteum called as bone marrow cavity. Marrow cavity of long bones is filled with a soft neurovascular connective tissue (areolar, adipose and blood vascular) called bone marrow. Bone marrow is of two types:
Types of bone cells
Osteoblasts : Secrete collagen fibres and matrix (ground substance) of bone and are responsible for the calcification of the matrix. They retain the ability to divide and communicate by thin cytoplasmic processes which form gap junctions. When an osteoblast is completely surrounded by the matrix, it is called an osteocyte.
Osteocytes : Inactive bone cells or bone forming cells. They are responsible for maintaining the matrix and can both secrete and resorb matrix.
Osteoclasts : Formed by fusion of monocytes, destroy bone matrix. They release lysosomes, organic acids and hydrolytic enzymes to break down, the bone matrix.