Definition of bone

The word "skeleton" comes from the Greek word skeleton meaning "dried up". Bone is a specialized connective tissue that has the strength of cast iron and lightness of pinewood. Living bone is not dry, brittle or dead. It is a moist changing, productive tissue that is continually resorbed, reformed and remodeled. The skeletal system consists of bone and other structure that make up the joints of the skeleton. The adult skeletal system consists of many bones (206 bones) but there are about 270 bones at birth and during childhood. Latter, with increasing age, separate bones gradually fuse so the number of bone decreases. The study of the bones is called oestology.

The skeletal system is composed of three types of tissues - bone tissue, cartilages and fibrous connective tissue, which forms the ligaments that hold bones together at the joints.

Functions of the Skeletal System

• Support: It forms the internal framework and support bones of the leg, pelvis and vertebral column and supports the body. All the bone provides support for muscle.
• Protection: It encloses and protection some internal organs such as brain, spinal cord, heart and lungs, reproductive agents.
• Movement: Skeletal muscle attached to and the bone to lever to move the body its part.
• Blood Cell Formation: This process is called hemopoiesis. It occurs in the bone marrow of certain bones. Red bone marrow is the major producer of blood cells. (Leukocytes, erythrocytes and platelets)
• Storage: Skeleton is the body's main mineral reservoir. It stores calcium and phosphate and release them according to body's physiological need.
• Acid - Base Balance: Bone Buffers the blood against excessive ph changes by absorbing alkaline mineral salts.
• Detoxification: Bone tissue removes heavy metal, and other foreign elements from the blood to reduce their toxic effects in nervous system and other tissues.

Bone Tissue

Bone tissue is composed of cells embedded in a matrix of ground substances and fibers. It is more rigid than other tissues because it contains inorganic salts mainly calcium phosphate and calcium carbonate. A network of collagenous fibers in the matrix gives bone tissue its strength and flexibility. Most bones have an outer sheet of compact bone tissue enclosing an interior spongy bone tissue.

Compact bone tissue forms the outer sheet of a bone. It is very hard and dense. It appears to naked eye to be solid but not. Compact bone tissue contains cylinders of calcified bone known as osteons (Haversian system). Osteons are made up of concentric layers called lamellae, which are arranged seemingly in wider and wider drinking straws. In the center of the osteons are central canals (haversian canal), which are longitudinal canals that contains blood vessels, nerves and lymphatic vessels. Central canals, usually have branches called perforating canals /Volkmann's canal that run at right angle to central canal extending the system of nerves and vessels out ward to periosteum and to endosteum. Lacunae (Little spaces) that houses osteocytes (bone cells) are contained in lamella. Radiating from each lacuna are tiny canaliculi containing the slender extensions of the osteocytes where nutrients and wastes can pass to and from central canal.

Spongy (cancellous) Bone tissue is in the form of an open interlaced pattern that withstands maximum stress and supports in shifting stress. The spongy bone consists of lamellae arranged in an irregular lattice of thin column called trabeculae. Trabeculae are tiny spikes of bone tissue surrounded by bone matrix that has calcified. The bone cells receive nutrients from the blood circulation through the blood vessels in the trabeculae. The spongy bone tissue makes up the most of the interior bone tissue of short, flat, and irregular bones.

Bone Cells

Bone contains four types of cells: osteogenic cells, osteoblasts, osteocytes and osteoclasts.

• Osteogenic (osteoprogenitor) cells: these are small spindle shaped stem cell which undergo cell division and develop into osteoblasts. They found mostly in the deepest layer of periosteum and endosteum.
• Osteoblasts are bone-building cells found in the growing portion of bone including periosteum. They are able to synthesize and secrete un-mineralized ground substance, act as pump move calcium and phosphate in and out of bone tissue.
• Osteocytes are the main cell of fully developed bones. They have a cell body that occupies a lacuna. Osteocytes are derived from osteoblasts. They together with osteoclasts play important role of homeostasis by helping to release calcium.
• Osteoclasts are multinuclear giant cell, which are found where bone is resorbed during its normal growth. Osteoclasts are derived from white blood cells called monocytes.

Bone is not completely solid, it has many small spaces between its cells and extracellular matrix components. Depending on the size and distribution of the spaces, the region of the bone may be categorized as compact or spongy. Overall, about 80% of skeleton is compact bone and 20% is spongy bone.

Classification of Bone

Bones are classified by various ways. Bone can be classified on the basis of process of development, on the basis of shape or according to position in the body.

According to shape/morphology (most used classification)

• Long bone: The bones of the arms, legs, hands and feet (but not wrist and ankles) are the long bones. Long bones consist of a shaft and two ends. It is made of compact bone but may contain spongy bone also. Eg: femur, tibia, fibula, humerus, radius, clavicle etc.
• Short bone: Short bone are roughly cube like in shape. They contain mostly spongy bone. The bones of wrist and ankle are short bone (carpal and tarsal bone).
• Flat bone: Flat bones are thin, flattened in appearance. The sternum, ribs and most skull bone are flat bone.
• Irregular bone: A bone that does not meet the preceding classes are called irregular bone. Irregular bone includes the vertebral and the hip bone, bones of face and bones in the base of skull.
• Sesamoid bone: These bones develop in certain tendons and are found where tendons cross the ends of long bones in the limbs; they protect the tendons from excessive wear and tear and often change the angle of the tendons as they pass to their attachments. e.g., the patella or knee cap.
• Pneumatic bones: Those containing air filled cavities are celled pneumatic bone. Maxilla, ethmoid, sphenoid bones are pneumatic bones.