Functions of the Skeletal System

Overview

The skeletal system provides the framework for the body and serves several essential functions necessary for survival and movement.

• Support: Bones form the structural framework that supports the body and maintains its shape.

• Protection: Vital organs are protected by bones (e.g., the skull protects the brain, the rib cage protects the heart and lungs).

• Movement: Bones act as levers and points of attachment for muscles, enabling movement.

• Mineral Storage: Bones store minerals such as calcium and phosphorus, which can be released into the bloodstream as needed.

• Blood Cell Production: Hematopoiesis occurs in the red bone marrow, producing red and white blood cells and platelets.

• Energy Storage: Yellow bone marrow stores lipids as an energy reserve.

Bone Shapes

Classification of Bones by Shape

Bones are classified according to their shapes, which relate to their functions.

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

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

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

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

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

Bone Markings

Surface Features of Bones

Bones have various markings that serve as attachment points for muscles, ligaments, and tendons, or as passageways for nerves and blood vessels.

• Foramen: A hole through which nerves and blood vessels pass.

• Process: A projection or bump for muscle attachment.

• Fossa: A shallow depression.

• Crest: A prominent ridge.

• Condyle: A rounded articular surface.

Long Bone Structure

Anatomy of a Typical Long Bone

Long bones have a distinct structure that supports their function in movement and weight-bearing.

• Diaphysis: The shaft of the bone, composed mainly of compact bone.

• Epiphysis: The ends of the bone, containing spongy bone and red marrow.

• Metaphysis: The region between diaphysis and epiphysis, including the growth plate.

• Medullary Cavity: The central cavity containing yellow marrow.

• Articular Cartilage: Covers the epiphysis, reducing friction at joints.

Bone Matrix

Composition and Function

The bone matrix is the intercellular substance of bone tissue, providing strength and flexibility.

• Organic Component: Mainly collagen fibers, providing tensile strength.

• Inorganic Component: Hydroxyapatite (calcium phosphate crystals), providing hardness.

Formula for Hydroxyapatite:

Cells of Bones

Types and Functions

Bone tissue contains several specialized cell types, each with distinct roles.

• Osteoblasts: Bone-forming cells that secrete the bone matrix.

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoclasts: Bone-resorbing cells that break down bone matrix.

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

Compact Bone

Structure and Function

Compact bone is dense and forms the outer layer of all bones, providing strength for weight-bearing.

• Osteons (Haversian Systems): Cylindrical structures containing a central canal surrounded by concentric lamellae.

• Canaliculi: Small channels connecting osteocytes for nutrient exchange.

Spongy Bone

Structure and Function

Spongy bone, also called cancellous bone, is found at the ends of long bones and inside flat bones.

• Trabeculae: Network of bony struts providing structural support and housing marrow.

• Red Bone Marrow: Located within trabeculae, site of hematopoiesis.

Bone Marrow

Types and Functions

Bone marrow is a soft tissue found within bone cavities, essential for blood cell production and energy storage.

• Red Marrow: Produces blood cells; found in spongy bone of flat bones and epiphyses of long bones.

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

Periosteum

Structure and Function

The periosteum is a dense connective tissue membrane covering the outer surface of bones.

• Outer Fibrous Layer: Provides protection and attachment for tendons and ligaments.

• Inner Cellular Layer: Contains osteoprogenitor cells for bone growth and repair.

Endosteum

Structure and Function

The endosteum is a thin membrane lining the internal surfaces of bones, including the medullary cavity.

• Contains Osteoprogenitor Cells: Important for bone growth, repair, and remodeling.

Bone Formation and Growth

Overview of Ossification

Bone formation, or ossification, is the process by which bone tissue is created. Growth occurs through two main mechanisms.

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

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

Process of Endochondral Ossification

Steps in Cartilage Replacement

Endochondral ossification is the process by which most bones are formed, especially long bones.

1. Development of Cartilage Model: Mesenchymal cells differentiate into chondrocytes, forming a cartilage template.

2. Growth of Cartilage Model: Cartilage model grows in size.

3. Development of Primary Ossification Center: Blood vessels invade, osteoblasts replace cartilage with bone in the diaphysis.

4. Development of Medullary Cavity: Osteoclasts break down bone to form the cavity.

5. Development of Secondary Ossification Centers: Occurs in the epiphyses after birth.

6. Formation of Articular Cartilage and Epiphyseal Plate: Cartilage remains at joint surfaces and growth plate for continued growth.

Example: The femur develops through endochondral ossification, starting as a cartilage model in the embryo.