Skeletal System: Structure, Function, and Bone Development

Skeletal System Overview

Appendicular vs Axial Skeleton

The human skeleton is divided into two main parts: the axial and appendicular skeletons, each with distinct components and functions.

• Axial Skeleton: Consists of the bones of the skull, thorax, and vertebral column.

• Appendicular Skeleton: Includes the bones of the limbs, pectoral girdle, and pelvic girdle.

• Divisions: The axial skeleton provides central support, while the appendicular skeleton facilitates movement.

Functions of the Skeletal System

The skeletal system performs several vital functions necessary for survival and movement.

• Support: Provides structural framework for the body.

• Mineral & Lipid Storage: Stores minerals (e.g., calcium, phosphorus) and lipids in bone marrow.

• Blood Cell Production: Produces red blood cells (RBCs), white blood cells (WBCs), and platelets in red bone marrow.

• Protection: Surrounds and protects delicate tissues and organs (e.g., brain, heart).

• Leverage: Acts as levers to change the magnitude and direction of forces generated by skeletal muscles.

Bone Structure and Classification

Bone Classifications

Bones are classified based on their shape and structure, which relate to their function.

• Flat Bones: Thin and broad (e.g., skull, ribs).

• Structural Bones: Small bones found between the flat bones of the skull.

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

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

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

• Short Bones: Boxlike in appearance (e.g., carpal bones).

Long Bones

Long bones have a unique structure that supports movement and weight-bearing.

• Epiphysis: Expanded area at each end of the bone; articulates with another bone and is covered with articular cartilage (hyaline cartilage).

• Diaphysis: Shaft of the bone; contains the medullary cavity and is separated from the epiphysis by the metaphysis.

• Metaphysis: Narrow zone between the diaphysis and epiphysis; site of bone growth during development.

• Medullary Cavity: Central cavity in the diaphysis; contains bone marrow (yellow or red).

• Compact Bone: Dense bone forming the outer layer.

• Spongy Bone: Found within the epiphyses; contains red bone marrow.

Compact Bone vs Spongy Bone

Bones contain two main types of osseous tissue: compact and spongy bone, each with distinct characteristics.

• Compact Bone:

  • Forms the walls of the diaphysis.

  • Dense and strong; provides support.

• Spongy Bone (Cancellous or Trabecular Bone):

  • Consists of a network of trabeculae (branching plates).

  • Reduces bone weight and resists forces from multiple directions.

  • Contains red bone marrow in spaces between trabeculae.

  • Most bones have a layer of compact bone covering spongy bone.

Microscopic Structure of Bone

Osteon

The osteon is the fundamental functional unit of compact bone, providing strength and structure.

• Structure: Cylindrical units parallel to the long axis of the bone.

• Function: Provides great strength when compressed; resists bending and fracturing.

• Spongy Bone: Lacks osteons; instead, forms trabeculae (branched networks).

Cartilage Cells

Cartilage is a flexible connective tissue with specialized cells and matrix.

• Mesenchymal Cells: Stem cells for all connective tissue.

• Chondroblasts: Arise from mesenchymal cells; produce ground substance and fibers (matrix).

• Chondrocytes: Mature cartilage cells; live inside lacunae.

How Does Cartilage Receive Nutrients?

Cartilage is avascular, so it relies on diffusion for nutrient delivery.

• Ground Substance: Hydrophilic solutes in the matrix draw nutrients into the cartilage.

• Diffusion: Nutrients diffuse from surrounding tissues into the matrix.

Types of Cartilage

There are three main types of cartilage, each with unique properties and locations.

• Hyaline Cartilage:

  • Most common type; provides support and flexibility.

  • Contains fine collagen fibers in the matrix.

  • Locations: Ends of bones, ribs, nose, trachea.

• Elastic Cartilage:

  • Contains elastic fibers as well as collagen.

  • Locations: Outer ear, epiglottis, larynx.

• Fibrocartilage:

  • Contains very large collagen fibers; extremely durable.

  • Locations: Intervertebral discs, joint capsules, ligaments.

Bone Cells and Bone Matrix

Bone Cells

Bone tissue contains several types of cells, each with specialized functions.

• Osteogenic Cells: Mesenchymal stem cells found in the inner layer of periosteum, endosteum, and vascular passageways; divide to produce osteoblasts.

• Osteoblasts: Immature bone cells responsible for bone formation; produce new bone matrix (osteoid) and initiate calcification.

• Osteocytes: Mature bone cells surrounded by bone matrix; maintain bone tissue and communicate via canaliculi.

• Osteoclasts: Large, multinucleated cells that resorb and recycle bone matrix; derived from white blood cell lineage.

Bone Matrix

The bone matrix is a composite of organic and inorganic components, providing strength and flexibility.

• Organic Matrix: Collagen fibers (provide flexibility and tensile strength).

• Inorganic Matrix: Hydroxyapatite (calcium phosphate) crystals (provide hardness).

• Composition: Salts make up about 70% of the matrix.

Bone Development and Growth

Endochondral Ossification

Endochondral ossification is the process by which most bones form from a hyaline cartilage template.

• Occurs in long bones and most other bones of the body.

• Steps:

  1. Cartilage model forms and grows.

  2. Blood vessels invade, bringing osteogenic cells.

  3. Osteoblasts form bone collar and begin ossification in the diaphysis (primary ossification center).

  4. Cartilage in the center calcifies and is replaced by bone.

  5. Secondary ossification centers form in the epiphyses after birth.

  6. Epiphyseal plates allow for continued growth until adulthood.

Bone Growth

Bones grow in length and diameter through different mechanisms.

• Appositional Growth: Addition of new layers to the outer surface; increases bone diameter.

• Epiphyseal Plate: Site of lengthwise growth; cartilage is replaced by bone as growth continues.

• As bone grows, osteoclasts remove matrix from the inner surface, enlarging the medullary cavity.

Intramembranous Ossification

Intramembranous ossification forms flat bones directly from mesenchymal tissue.

• Occurs in the skull, mandible, clavicle, and some sesamoid bones.

• Mesenchymal cells differentiate into osteoblasts, which secrete bone matrix.

• Osteoid calcifies and hardens, forming trabeculae (spongy bone).

• Compact bone forms on the outer surfaces.

Bone Marrow

Bone marrow is a soft, netlike mass of connective tissue found within bones.

• Red Bone Marrow: Site of blood cell production; found in spongy bone and medullary cavities of certain bones.

• Yellow Bone Marrow: Stores fat; found in the medullary cavity of long bones.

Major Stages in Bone Development (Table)

The following table summarizes the major stages in bone development and their distinguishing features.

Stage	Distinguishing Features
Cartilage Model Development	Hyaline cartilage forms the initial template for bone
Primary Ossification Center Formation	Osteoblasts replace cartilage in the diaphysis; bone collar forms
Medullary Cavity Formation	Osteoclasts break down spongy bone to create the cavity
Secondary Ossification Center Formation	Osteoblasts replace cartilage in the epiphyses
Epiphyseal Plate Formation	Cartilage remains at the ends of bones for continued growth
Bone Maturation	Epiphyseal plates close; bone growth ceases