Types of Bones in the Human Body

Classification of Bones

• Flat bones are thin, flattened, and curved, with two parallel layers of compact bone surrounding a layer of spongy bone; examples include most skull bones, scapula, sternum, and sacrum
• Irregular bones do not fit into any other category, contain foramina for soft tissue and neurovascular structures, and include vertebrae, hip bone, and some skull bones
• Sesamoid bones are small, rounded, and embedded in muscle tendons near joints; examples include patella and pisiform

Method of Formation

• Ossification (osteogenesis) is the process of bone tissue development, beginning in utero and completing around 21 years of age
• During fetal development, three germ layers (endoderm, mesoderm, and ectoderm) are present
• Mesenchyme develops from mesoderm and forms the embryonic skeleton, which undergoes either intramembranous or endochondral ossification
• Osteoblasts converge on the mesenchymal/cartilaginous models to initiate ossification

Bone Structure

• Bones are covered in dense connective tissue called periosteum (outer) and endosteum (inner)
• Spongy (cancellous) bone is a deep, airy layer of bone that is highly vascularized
• Central canal contains nerves, lymphatics, and blood vessels
• Lamellae are cylindrical plates of bone surrounding the central canal
• Lacunae are small cavities between lamellae that house osteocytes

Classification by Shape

• Long bones are tubular in shape, composed mostly of compact bone, and include femur, humerus, ulna, tibia, and clavicle
• Short bones are roughly cuboid or round in shape, contain a thin layer of compact bone surrounding spongy bone, and include tarsal and carpal bones
• Flat bones are thin, flattened, and curved, with two parallel layers of compact bone surrounding a layer of spongy bone
• Irregular bones do not fit into any other category, contain foramina for soft tissue and neurovascular structures, and include vertebrae, hip bone, and some skull bones
• Sesamoid bones are small, rounded, and embedded in muscle tendons near joints; examples include patella and pisiform

Clinical: Fracture of Bone

• A fracture is a breakage in bone due to injury, stress, or disease
• Types of fractures include simple, compound, comminute, and incomplete

Cartilage and Joints

• Cartilage is a flexible connective tissue found in multiple organ systems, composed of chondrocytes, collagen fibers, and ground substance rich in proteoglycan and elastin fibers
• Types of cartilage include hyaline, elastic, and fibrocartilage
• Joints are connections or unions of two or more bones or cartilages
• Joints can be classified into fibrous, cartilaginous, and synovial joints

Musculo-Skeletal System

• The musculo-skeletal system includes skeletal muscle, tendons, ligaments, bones, joints, and articulating cartilages
• The skeletal system can be divided into axial skeleton (skull, vertebral column, and thoracic cage) and appendicular skeleton (upper and lower limb bones)

Bone Functions

• Bones provide support for the body and its cavities, protect vital organs, and serve as a mechanical basis for movement
• Bones store salts, supply new blood cells, and are highly specialized living tissue
• Bone tissue is composed of cells and fibers (organic component) and calcium salts (inorganic component)