Functions of Bone and the Skeletal System

Major Functions of the Skeletal System

The skeletal system is essential for providing structure, protection, and support to the body. Bones, along with associated cartilages, form the skeleton and perform several key functions:

• Support: Bones provide a rigid framework that supports the body and cradles soft organs.

• Protection: Bones protect vital organs (e.g., the skull protects the brain, the rib cage protects the heart and lungs).

• Assistance in Movement: Bones serve as levers for muscles to act upon, enabling movement.

• Mineral Storage and Release: Bones store minerals such as calcium and phosphorus, releasing them into the bloodstream as needed.

• Blood Cell Production (Hematopoiesis): Occurs in red bone marrow, producing red and white blood cells.

• Triglyceride Storage: Occurs in yellow bone marrow, serving as an energy reserve.

Types of Bones

Classification by Shape

Bones are classified based on their shapes and surface markings, which relate to their functions:

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Nearly equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and often curved (e.g., sternum, cranial bones). Between the external and internal tables of compact bone is a layer of spongy bone called the diploë.

• Irregular Bones: Complex shapes (e.g., vertebrae, some facial bones).

• Sesamoid Bones: Small bones embedded in tendons (e.g., patella).

• Sutural (Wormian) Bones: Small bones located in the sutures between certain skull bones.

Anatomy of a Bone

Structure of a Typical Long Bone

Long bones have a characteristic structure that supports their function:

• Diaphysis: The shaft or central part of the bone.

• Epiphyses: The ends of the bone, which include the epiphyseal plate (growth plate).

• Metaphyses: Regions between the diaphysis and epiphyses, containing the epiphyseal plate until bone growth stops.

• Articular Cartilage: Hyaline cartilage covering the joint surfaces.

• Periosteum: A dense connective tissue membrane covering the outer surface of the bone, attached by perforating (Sharpey's) fibers.

• Medullary (Marrow) Cavity: Central cavity containing bone marrow.

• Endosteum: Thin membrane lining the medullary cavity.

Other bone types have outer plates of compact bone covering an inner core of spongy bone.

Surface Markings of Bones

Types and Functions

Surface markings are features adapted to specific functions, such as attachment points for muscles or passageways for nerves and blood vessels.

• Depressions and Openings: Allow passage of soft tissues and form joints (e.g., fissure, foramen, fossa, sulcus, meatus).

• Processes: Projections that form joints or serve as attachment points for connective tissue (e.g., condyle, facet, head, crest, epicondyle, line, spine, trochanter, tubercle, tuberosity).

Histology of Bone Tissue

Composition and Cell Types

Bone tissue contains abundant matrix surrounding widely separated cells. The matrix consists of:

• Mineral Salts: Mainly hydroxyapatite, deposited in a framework of collagen fibers, providing hardness.

• Collagen Fibers: Give bone flexibility and tensile strength.

Major cell types in bone tissue:

• Osteogenic Cells: Stem cells that give rise to other bone cells.

• Osteoblasts: Bone-forming cells responsible for bone matrix production.

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoclasts: Cells that resorb bone tissue during remodeling.

Types of Bone Tissue

• Compact Bone: Dense and organized into osteons (Haversian systems), providing strength and support.

• Spongy Bone: Consists of trabeculae, providing support and housing bone marrow.

Blood and Nerve Supply of Bone

Vascularization

Bones have a rich blood supply, especially those containing red bone marrow. Blood supply is provided by arteries and accompanying veins:

• Periosteal Arteries: Supply the outer compact bone.

• Nutrient Arteries: Enter through the nutrient foramen to supply the medullary cavity.

• Epiphyseal Arteries: Supply the epiphyses.

Nerves accompany blood vessels and are important for pain sensation and regulation of blood flow.

Bone Formation (Ossification)

Processes of Bone Formation

Ossification is the process by which bone forms. It occurs in four principal situations:

• During embryonic development

• Growth of bones until adulthood

• Remodeling of bone throughout life

• Repair of fractures

Two main types of ossification:

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of the skull).

• Endochondral Ossification: Bone forms by replacing hyaline cartilage (e.g., long bones).

Key steps in endochondral ossification include development of the cartilage model, growth and calcification, and formation of ossification centers.

Bone Growth in Length and Thickness

Epiphyseal Plate and Appositional Growth

Bone lengthening occurs at the epiphyseal plate, which consists of four regions:

• Zone of resting cartilage

• Zone of proliferating cartilage

• Zone of hypertrophic cartilage

• Zone of calcified cartilage

Bone thickening occurs via appositional growth, where new bone tissue is added to the outer surface.

Growth in length usually completes by age 25.

Bone Remodeling

Continuous Renewal of Bone Tissue

Bone remodeling is a continuous process in which old bone is replaced by new bone tissue. It involves:

• Osteoclasts: Resorb (break down) bone tissue.

• Osteoblasts: Form new bone tissue.

Fractures and Bone Repair

Types and Healing Process

Fractures are breaks in bone, classified as open (compound), closed (simple), comminuted, greenstick, impacted, Pott's, Colles', and stress fractures.

Bone repair involves several steps:

1. Formation of a fracture hematoma

2. Formation of a fibrocartilaginous callus

3. Conversion into a bony callus

4. Remodeling of the callus

Normal bone growth, remodeling, and repair depend on the presence of minerals (e.g., calcium, phosphorus), vitamins (e.g., A, C, D), and hormones (e.g., growth hormone, sex hormones).

Exercise and Bone Tissue

Effects of Mechanical Stress

Bone has the ability to alter its strength in response to mechanical stress. Increased stress leads to increased deposition of mineral salts and production of collagen fibers. Removal of mechanical stress can cause weakening and loss of bone mass.

Bone mass is maintained by weight-bearing activities such as walking and resistance training.

Aging and Bone Tissue

Changes with Age

Until young adulthood, bone formation exceeds bone resorption. After middle age, bone resorption outpaces deposition, leading to decreased bone mass.

• Demineralization: Loss of calcium from bone matrix, making bones more brittle and susceptible to fracture.

• Decreased Rate of Protein Synthesis: Loss of collagen fibers reduces bone strength.

Osteoporosis is a condition characterized by reduced bone mass and increased risk of fractures, especially in older adults.

Key Terms and Definitions

• Hematopoiesis: Production of blood cells in red bone marrow.

• Epiphyseal Plate: Growth plate in long bones where lengthening occurs.

• Osteon: Structural unit of compact bone.

• Osteoblast: Bone-forming cell.

• Osteoclast: Bone-resorbing cell.

• Periosteum: Membrane covering the outer surface of bone.

• Endosteum: Membrane lining the medullary cavity.

Summary Table: Types of Bones by Shape

Key Equations

• Bone Remodeling Rate:

• Calcium Homeostasis:

Additional info: Some context and definitions have been expanded for clarity and completeness.