Skeletal System Overview

Cartilage and Bone Formation

The human skeleton initially forms as cartilage, which is gradually replaced by bone during development. This process allows for flexibility in early growth and provides a framework for ossification.

• Cartilage: A resilient, semi-rigid connective tissue found in areas requiring flexibility (e.g., joints, rib cage).

• Ossification (osteogenesis): The process by which bone tissue replaces cartilage or other connective tissue.

• Types of Cartilage: Includes hyaline, elastic, and fibrocartilage, each with distinct locations and functions in the body.

• Bone Growth occurs via two mechanisms:

  • Appositional growth: New matrix is laid down on the surface of cartilage or bone.

  • Interstitial growth: New matrix is formed within the cartilage, increasing its length.

Functions of Bones

Bones serve several major functions essential to the human body:

• Support: Provide structural framework for the body.

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

• Movement: Serve as levers for muscles to act upon.

• Mineral Storage: Reservoir for minerals, especially calcium and phosphate.

• Blood Cell Production: Hematopoiesis occurs in red bone marrow.

• Triglyceride Storage: Yellow bone marrow stores fat.

Classification of Bones

Divisions of the Skeleton

• Axial Skeleton: Consists of the skull, vertebral column, and rib cage.

• Appendicular Skeleton: Includes the limbs and girdles (shoulder and pelvic girdles).

Bone Shapes

• Long Bones: Longer than wide, with a shaft and two ends (e.g., femur, humerus).

• Short Bones: Cube-shaped (e.g., carpals, tarsals).

• Flat Bones: Thin, flat, and often curved (e.g., sternum, skull bones).

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Bone Structure - Gross Anatomy

General Structure

• All bones have a dense outer layer called compact bone and a spongy inner layer called spongy (cancellous) bone.

• Short, irregular, and flat bones have thin plates of spongy bone covered by compact bone, sandwiched between connective tissue membranes.

• Periosteum: Outer membrane covering compact bone.

• Endosteum: Membrane lining the internal bone surfaces.

Long Bone Anatomy

• Diaphysis: Tubular shaft forming the long axis, composed of compact bone surrounding the medullary cavity.

• Epiphyses: Ends of long bones, consisting of compact bone externally and spongy bone internally.

• Epiphyseal Plate: Growth plate located between diaphysis and epiphysis in growing bones.

• Periosteum: Double-layered membrane covering the external surface, containing osteogenic stem cells.

• Nutrient Foramen: Opening for blood vessels and nerves.

Bone Marrow

• Red Bone Marrow: Site of hematopoietic (blood cell) formation, found in trabecular cavities of spongy bone and flat bones.

• Yellow Bone Marrow: Fat storage, found in the medullary cavity of long bones in adults.

Bone Markings

• Projections: Sites of muscle and ligament attachment (e.g., tuberosity, crest).

• Surfaces: Form joints (e.g., head, facet).

• Depressions: Openings for passage of blood vessels and nerves (e.g., fossa, foramen).

Bone Structure - Microscopic Anatomy

Bone Cells

• Osteoprogenitor (osteogenic) cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells that secrete bone matrix (osteoid).

• Osteocytes: Mature bone cells that maintain bone matrix and act as mechanosensors.

• Bone-lining cells: Flat cells on bone surfaces, help maintain matrix.

• Osteoclasts: Large, multinucleated cells responsible for bone resorption (breakdown).

Compact Bone Structure

• Osteon (Haversian system): Structural unit of compact bone, consisting of concentric lamellae around a central canal.

• Canals:

  • Central (Haversian) canal: Runs through the core, contains blood vessels and nerves.

  • Perforating (Volkmann's) canals: Connect blood vessels and nerves of periosteum to those in the central canal.

  • Lacunae: Small cavities containing osteocytes.

• Lamellae:

  • Interstitial lamellae: Fill gaps between osteons.

  • Circumferential lamellae: Extend around the entire circumference of the diaphysis.

Spongy Bone

• Lacks osteons but has trabeculae aligned along lines of stress.

• Contains irregular lamellae and osteocytes connected by canaliculi.

Bone Structure – Chemical Composition

• Organic Matter: Includes osteoid (collagen fibers and ground substance), providing flexibility and tensile strength.

• Inorganic Matter: Mainly hydroxyapatite (mineral salts, primarily calcium phosphate), providing hardness and resistance to compression.

Bone Development

Ossification (Osteogenesis)

Ossification is the process of bone tissue formation, beginning in the second month of embryonic development and continuing into adolescence.

• Endochondral Ossification: Bone forms by replacing hyaline cartilage. Most bones below the skull are formed this way.

• Intramembranous Ossification: Bone develops from fibrous membranes (e.g., cranial bones, clavicles).

Postnatal Bone Growth

• Long bones grow in length at the epiphyseal plate through four zones:

  1. Proliferation (growth) zone: Rapid cell division increases bone length.

  2. Hypertrophic zone: Older chondrocytes enlarge, lacunae erode and enlarge.

  3. Calcification zone: Matrix calcifies, chondrocytes die, blood vessels invade.

  4. Ossification zone: New bone forms as osteoblasts deposit bone matrix.

• Growth in thickness (appositional growth) occurs by osteoblasts in the periosteum adding new bone matrix to the surface.

Hormonal Regulation of Bone Growth

• Growth hormone: Stimulates epiphyseal plate activity in childhood.

• Thyroid hormone: Modulates growth hormone activity.

• Sex hormones: Promote growth spurts and eventual closure of the epiphyseal plate at puberty.

Sample Questions and Applications

• Fracture in the shaft of a bone: A break in the diaphysis.

• Non-function of the skeletal system: Communication is not a primary function; main functions include support, protection, movement, mineral storage, and blood cell production.

• Bone cell responsible for osteoid matrix production: Osteoblasts initiate ossification and secrete the osteoid matrix.

Summary Table: Types of Bone Cells

Key Equations and Terms

• Hydroxyapatite formula:

• Ossification:

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.