Bones are rigid organs that form part of the endoskeleton of vertebrates.
They function to move, support, and protect the various organs of the body, produce red and white blood cells, and store minerals.
Bone tissue is a type of dense connective tissue. Bones come in a variety of shapes and sizes and have a complex internal and external structure.
Composition and Properties:
1. Matrix Material:
Organic Components: Primarily collagen, which provides tensile strength and flexibility.
Inorganic Components: Primarily calcium phosphate, which provides hardness and resistance to compression.
2. Bone Cells:
Osteoblasts: Cells responsible for bone formation.
Osteocytes: Mature bone cells that maintain the bone matrix.
Osteoclasts: Cells involved in the resorption of bone tissue.
Structure of Bone
Bones are rigid connective tissue organs that make up the skeletal system.
They are composed of specialized cells, extracellular matrix, and minerals.
The structure of a bone can be divided into two parts: the macroscopic and microscopic structures.
Macroscopic Structure of Bone:
1. Diaphysis:
The long, cylindrical portion of a long bone.
Composed primarily of compact bone, providing strength and rigidity.
2. Epiphyses:
The rounded ends of a long bone.
Mostly made of spongy bone, with an outer layer of compact bone.
3. Metaphysis:
The region between the diaphysis and epiphysis where bone growth occurs during childhood and adolescence.
Contains the epiphyseal plate (growth plate) in growing individuals.
4. Periosteum:
A dense, fibrous membrane covering the outer surface of bones, except at joints.
Contains blood vessels, nerves, and osteogenic cells essential for bone growth, repair, and nutrition.
5. Endosteum:
A thin, vascular membrane lining the inner surface of bones, including the medullary cavity and trabeculae of spongy bone.
Contains osteogenic cells involved in bone growth and repair.
6. Medullary Cavity:
A hollow space within the diaphysis of long bones.
Filled with yellow bone marrow, which stores fat and serves as an energy reserve.
Microscopic Structure of Bone:
1. Compact Bone:
Also known as cortical bone, it is dense and forms the outer layer of bones.
Consists of tightly packed concentric rings called lamellae, surrounding a central Haversian canal.
Haversian canals contain blood vessels and nerves, providing nutrients and oxygen to the bone cells.
2. Spongy Bone:
Also known as cancellous or trabecular bone, found mainly in the epiphyses of long bones and the interior of other bones.
Consists of a network of interconnected trabeculae, creating a porous structure.
The spaces within spongy bone are filled with red bone marrow, where blood cell production occurs.
Functions:
Support: Bones provide a framework for the attachment of muscles and other tissues.
Protection: Bones such as the skull and rib cage protect vital organs from injury.
Movement: Muscles are attached to bones, which act as levers to produce movement.
Mineral Storage: Bones serve as a reservoir for minerals, particularly calcium and phosphorus.
Blood Cell Production: The marrow in the cavities of bones is the site of red blood cell production (hematopoiesis).
Bone Growth and Remodeling:
1. Growth:
Bones grow in length due to activity at the epiphyseal plate (growth plate).
Bones grow in diameter through the addition of bone tissue at the surface.
2. Remodeling:
A lifelong process where bone tissue is continuously replaced and reshaped.
Influenced by mechanical stress and hormonal changes.
Types of Bones
1. Long Bones:
Longer than they are wide, functioning as levers for movement.
Examples: Femur, tibia, fibula, humerus, radius, ulna.
2. Short Bones:
Cube-shaped, providing support and stability with little movement.
Examples: Carpals (wrist bones), tarsals (ankle bones).
3. Flat Bones:
Thin, flattened shape, protecting underlying organs or providing large surface areas for muscle attachment.
Examples: Skull bones, scapula, sternum, ribs.
4. Irregular Bones:
Complex shapes that fulfill various functions, such as protection and support.
Examples: Vertebrae, sacrum, coccyx, bones of the pelvis (ilium, ischium, pubis).
5. Sesamoid Bones:
Small, round bones embedded within tendons, helping to reduce friction.
Example: Patella (kneecap).
Health and Disease of Bones
1. Osteoporosis:
A condition characterized by decreased bone mass and increased susceptibility to fractures.
2. Arthritis:
Inflammation of the joints, which can affect the surrounding bone structure.
3. Fractures:
Breaks in the bone often caused by stress or impact.