The Skeletal System Overview

The Skeletal System: Two Major Divisions

• Axial Skeleton: Includes the skull, vertebral column, and rib cage. Provides structural support and protection for vital organs.
• Appendicular Skeleton: Consists of the bones of the limbs, pectoral girdle (shoulder), and pelvic girdle (hip). Enables movement and locomotion.

Functions of Bones

• Support: Provides a framework for the body, allowing for upright posture and movement.
• Protection: Encases vital organs such as the brain, heart, and lungs, safeguarding them from injury.
• Movement: Serves as attachment points for muscles, facilitating locomotion and other movements.
• Mineral Storage: Acts as a reservoir for essential minerals, particularly calcium and phosphorus, which are crucial for various bodily functions.

Compact vs. Spongy Bone

• Compact Bone: Dense and solid, found in the outer layer of bones. Provides strength and rigidity. Contains osteons (Haversian systems) as its structural unit.
• Spongy Bone: Less dense and porous, found inside the compact bone layer. Provides flexibility and reduces bone weight. Contains trabeculae, a network of bony struts that provide support.

Long Bone Structure

• Diaphysis: The shaft of the bone, composed mostly of compact bone.
• Epiphysis: The ends of the bone, composed mostly of spongy bone covered with a thin layer of compact bone.
• Metaphysis: The region between the diaphysis and epiphysis, where bone growth occurs during childhood and adolescence.
• Trabeculae: Interconnecting bony struts found in spongy bone, providing support and reducing weight.
• Periosteum: A fibrous membrane that covers the outer surface of the bone, containing blood vessels and nerves, aiding in bone growth and repair.
• Endosteum: A thin membrane lining the medullary cavity, containing osteogenic cells and playing a role in bone remodeling.
• Medullary Cavity: The central hollow space within the diaphysis, filled with bone marrow.
• Epiphyseal Plate: A layer of hyaline cartilage found in the metaphysis during childhood and adolescence, responsible for lengthwise growth.
• Epiphyseal Line: A remnant of the epiphyseal plate that appears after growth ceases, marking the former site of bone growth.

Compact Bone Histology

• Central Canal (Haversian Canal): A longitudinal channel running through the center of each osteon, containing blood vessels and nerves.
• Lacunae: Small cavities within the bone matrix that house osteocytes.
• Canaliculi: Tiny canals that connect lacunae to each other and to the central canal, allowing for nutrient and waste exchange.
• Lamellae: Concentric rings of bone matrix surrounding the central canal, providing strength and rigidity.
• Circumferential Lamellae: Lamellae located at the outer and inner surfaces of the bone, running parallel to the long axis of the bone.
• Interstitial Lamellae: Remnants of old osteons that have been partially broken down during bone remodeling.
• Concentric Lamellae: Lamellae arranged in concentric circles around the central canal, contributing to the overall strength of the osteon.
• Perforating (Volkmann’s) Canals: Transverse channels that connect central canals to each other and to the periosteum and endosteum, providing pathways for blood vessels and nerves.
• Osteons (Haversian Systems): The basic structural unit of compact bone, consisting of a central canal surrounded by concentric lamellae, lacunae, canaliculi, and osteocytes.

Bone Cells

• Osteogenic (Osteoprogenitor) Cells: Undifferentiated stem cells found in the periosteum and endosteum. They can differentiate into osteoblasts.
• Osteoblasts: Bone-building cells that synthesize and secrete osteoid, the organic matrix of bone. They are derived from osteogenic cells.
  • Alkaline Phosphatase: An enzyme produced by osteoblasts that is crucial for the mineralization of bone, facilitating the precipitation of calcium phosphate crystals.
• Osteocytes: Mature bone cells that reside within lacunae and maintain the bone matrix. They are derived from osteoblasts.
• Osteoclasts: Large, multinucleated cells responsible for bone resorption. They secrete enzymes and acids that break down bone matrix, releasing calcium and other minerals into the bloodstream.

Bone Extracellular Matrix

• Organic Components: Collagen fibers (provide tensile strength and flexibility), proteoglycans (resist compression and bind water), and other organic molecules.
• Inorganic Components: Calcium phosphate crystals (provide rigidity and hardness).

Bone Formation (Ossification)

• Intramembranous Ossification: Bone formation within a mesenchymal membrane. Occurs in flat bones like the skull, clavicle, and mandible.
• Endochondral Ossification: Bone formation by replacement of a hyaline cartilage model. Occurs in most bones of the body, including long bones.

Bone Growth

• Interstitial Growth (Lengthwise Growth): Growth that occurs at the epiphyseal plate in long bones.
  • Zone of Resting Cartilage: Contains inactive chondrocytes.
  • Zone of Proliferation: Cartilage cells actively divide and grow.
  • Zone of Hypertrophy: Cartilage cells enlarge and mature.
  • Zone of Calcification: Cartilage matrix calcifies.
  • Zone of Ossification: Calcified cartilage is replaced by bone by osteoblasts.
• Appositional Growth (Widthwise Growth): Growth that occurs at the periosteum, adding new bone tissue to the outer surface of the bone. This process contributes to the thickening of bones.

Blood Calcium Regulation

• Parathyroid Hormone (PTH): Secreted by the parathyroid glands when blood calcium levels are low. It stimulates osteoclasts to resorb bone, releasing calcium into the bloodstream.
• Calcitonin: Secreted by the thyroid gland when blood calcium levels are high. It inhibits osteoclast activity and promotes calcium deposition into bone.
• Calcitriol (Vitamin D3): Increases calcium absorption in the intestines and promotes bone resorption.
  • Hypocalcemia: Low blood calcium levels, leading to muscle cramps, tremors, and tetany.
  • Hypercalcemia: High blood calcium levels, leading to weakness, fatigue, and kidney stones.

Hormones Involved in Bone Growth

• Estrogen and Testosterone: Promote bone growth during puberty, but later inhibit bone growth and stimulate bone remodeling.
• Growth Hormone: Stimulates bone elongation during childhood and adolescence.
• Calcitriol (Vitamin D3): Promotes intestinal calcium absorption, which is essential for bone growth and remodeling.

Bone Fractures

• Compound (Open) Fracture: A fracture where the broken ends of the bone protrude through the skin.
• Simple (Closed) Fracture: A fracture where the broken ends of the bone do not protrude through the skin.
• Complete Fracture: A fracture in which the bone is broken completely.
• Incomplete Fracture: A fracture in which the bone is only partially broken.
• Displaced Fracture: A fracture in which the broken ends of the bone are not aligned.
• Non-Displaced Fracture: A fracture in which the broken ends of the bone are still aligned.

Bone Remodeling and Repair

• Remodeling: A continuous process of bone breakdown (resorption) and bone formation (deposition) that helps maintain bone strength and adapt to changing loads.
• Repair: A complex process that restores bone continuity after a fracture.
  • Hematoma Formation: Blood clots form at the fracture site.
  • Soft Callus Formation: Fibrocartilage is produced by fibroblasts and chondroblasts.
  • Hard Callus Formation: Osteoblasts replace fibrocartilage with bone.
  • Bone Remodeling: The hard callus is remodeled into a more normal bone shape.

Explore the two major divisions of the skeletal system: axial and appendicular. Learn about the vital functions of bones such as support, protection, movement, and mineral storage. Dive deeper into the differences between compact and spongy bone.