The Skeletal System: Structure, Function, and Organization

The Skeletal System

Overview and Components

The skeletal system is a complex organ system that provides the framework for the human body. It consists of bones, cartilages, joints, ligaments, and connective tissues that stabilize or connect bones.

• Bones of the skeleton: The rigid structures that form the body's framework.

• Cartilages, joints, ligaments, and connective tissue: These structures stabilize, connect, and allow movement between bones.

Functions of the Skeletal System

Major Roles

• Support: Provides structural support for the entire body and serves as attachment points for soft tissues and organs.

• Storage of minerals & lipids: Bones act as reservoirs for minerals such as calcium (Ca2+) and phosphorus (P). Yellow bone marrow stores lipids for energy reserves.

• Blood cell production: Red bone marrow is the site of production for red blood cells, white blood cells, and other blood elements.

• Protection: Skeletal structures surround and protect soft tissues and vital organs.

• Leverage: Bones function as levers, changing the size and direction of forces generated by skeletal muscles, resulting in body movement.

Osseous Tissue Characteristics

Bone Tissue Structure

Bone is a supporting connective tissue containing specialized cells in a solid matrix. The primary cell type is the osteocyte.

• Matrix composition:

  • Calcium salts (mainly calcium phosphate, Ca3(PO4)2) account for about two-thirds of bone weight.

  • Extracellular protein fibers (mainly collagen) make up about one-third of bone weight.

Classifying Bones by Shape

Bone Types

Bones are classified based on their shapes, which relate to their functions.

• Long bones: Longer than they are wide (e.g., femur, phalanges).

• Short bones: About as wide as they are long (e.g., carpal bones).

• Flat bones: Thin and relatively broad (e.g., ribs, parietal bone).

• Irregular bones: Complex shapes that do not fit other categories (e.g., vertebrae).

Structures of a Long Bone

Gross Anatomy

• Diaphysis: The central shaft of the bone, surrounding the marrow (medullary) cavity filled with bone marrow.

• Epiphyses: The expanded ends of the bone, covered with articular cartilage and articulating with adjacent bones at joints.

• Types of bone tissue:

  • Compact bone (dense bone): Relatively solid, forms the wall of the diaphysis.

  • Spongy bone (cancellous bone): An interlacing network of bony rods separated by spaces, fills the epiphyses, and lines the marrow cavity.

Layerings of a Long Bone

Periosteum and Endosteum

• Periosteum: Covers the outer surface of bone.

  • Has an inner cellular layer and an outer fibrous layer.

  • Isolates bone from other tissues and attaches with fibers of tendons and ligaments.

• Endosteum: Covers inner surfaces and spongy bone of the marrow cavity.

  • Functions during bone growth and repair.

Microscopic Features of Bone (Additional info)

Osteocytes and Bone Matrix

• Osteocytes: Mature bone cells located in lacunae, which are small pockets between sheets of calcified matrix called lamellae.

• Canaliculi: Small channels in the matrix that contain cytoplasmic extensions of osteocytes, connecting lacunae to blood vessels for nutrient and waste exchange.

• Osteon (Haversian system): The functional unit of compact bone, consisting of concentric layers (lamellae) surrounding a central canal containing blood vessels.

Additional info: These microscopic features are essential for bone strength, nutrient delivery, and waste removal.