General structure of bone and cartilage
Bone is a rigid form of connective tissue which forms the endoskeleton of the vertebrates. Bone is a living hard and strong structure. It consists of a hard ground substance or matrix in which the cells of the bone are embedded. In an adult human being the matrix consists of about 65% inorganic matter mainly consisting of calcium phosphate and calcium carbonate and about 35% organic substances mainly proteins and collagen. The cells are embedded in the matrix.
The bone is surrounded by a membrane made of connective tissue known as periosteum. This is a very tough membrane and adds to the strength of the body. The periosteum is provided with blood vessels and nerves to keep the cells alive and allow growth. Below the periosteum is a layer of bone forming cells known as the osteoblast cells. These osteoblast cells secrete the so called bony substance that helps in increasing the thickness of the bone. The increase in thickness of bone means the increase in diameter of the bone through the cross section. The activity of the osteoblast cells is also very important and necessary in the repairing process of a fracture or any other damage to the bone. Actually the osteoblast cells are the rapidly dividing cells and the progeny form the osteocytes which are the actual cells of bone.
Bone contains numerous round canals known as the Haversian canals. Each of these canals is surrounded by concentrically arranged boney plates called Haversian lamellae. These haversian lamellae consist of calcified matrix in which the collagen fibers are embedded. Present between the adjacent lamellae are spaces called lacunae that contain bone cells or osteocytes. From the lacunae radiate fine fibers or canaliculi in all directions. The Haversian canal together with the lamellae, lacunae and canaliculi constitute the haversian system of the bone.
The outside layer of the bone is dense and hard and is known as the compact bone. Inside the compact bone is a loose spongy bone with spaces in between it. The difference between the compact and spongy bone is the ratio of bone space to bone substance. The ration is greater in the case of loose spongy bone and lower in case of tough compact bone. The loose spongy bone forms the red bone marrow within which the important red blood cells are produced. This is found mainly in the sternum, ribs, vertebrae and the heads of the long bones. The shaft of mature long bones is filled with soft fatty yellow marrow not capable of producing blood cells.
Cartilage is softer than the bone and is also a type of connective tissue. It forms some very important parts of the skeleton in all vertebrates. In higher vertebrates it is much reduced in amount because in most parts of the skeleton it has been replaced by the bones. Cartilage consists of tough elastic and homogenous matrix containing numerous spaces or lacunae in which are embedded nucleated cells or chondrocytes which help in increasing the diameter of the cartilage. One or two cells usually lie in each lacuna. A dense layer of connective tissue, the perichondrium, covers the surface of the cartilage. The cells are nourished by lymph through osmosis. Blood vessels do not enter cartilage. There are three main types of cartilage;
Hyaline cartilage: It is the most abundant type. The matrix is tough, homogenous, and translucent. The lower part of the ribs, cartilage of larynx, trachea and bronchi are the examples of hyaline cartilage in human beings.
Fibrous cartilage: The matrix of such cartilage contains collagen fibers. This is found in intervertebral discs and also in the places of attachment of tendons and ligaments.
Elastic cartilage: In this type of cartilage the matrix is elastic. Elastic cartilage is found in external ear and epiglottis.