The Skeletal System: Structure, Function, and Anatomy

The Skeletal System

Overview and Learning Objectives

The skeletal system is a fundamental organ system in the human body, providing structure, protection, and support. This section outlines the key learning outcomes for understanding the skeletal system in an Anatomy & Physiology context.

• Identify the organs and tissues that compose the skeletal system.

• Describe several functions of the skeletal system.

• Define terms for the gross anatomy of bones.

• Distinguish the two types of bone marrow.

• Identify features of the microscopic anatomy of bone, including the two types of bone tissue, four types of bone cells, and components of the noncellular matrix.

Components of the Skeletal System

Organs and Tissues

• Bones: Rigid organs that form the framework of the body.

• Cartilage: A flexible connective tissue; serves as a forerunner of bone in embryonic development and covers many joint surfaces.

• Ligaments: Collagenous bands that hold bones together at joints.

• Tendons: Structurally similar to ligaments; attach muscle to bone.

• Bone Marrow: Soft material enclosed in bones, involved in blood cell production and fat storage.

Organization of the Human Skeleton

Bone Count and Divisions

• Adults have 206 bones; infants are born with approximately 300 bones, which fuse during development.

Axial Skeleton (80 bones)

• Skull

• Thoracic cage and sternum

• Vertebral column

Appendicular Skeleton (126 bones)

• Extremities (arms and legs)

• Pectoral girdle (shoulder region)

• Pelvic girdle (hip region)

Functions of the Skeletal System

• Support: Bones of the limbs and vertebral column support the body; mandible and maxilla support teeth.

• Protection: Bones protect vital organs such as the brain, spinal cord, lungs, heart, and pelvic viscera.

• Movement: Bones provide attachment and leverage for muscles, enabling movement.

• Blood Formation (Hematopoiesis): Bones are the major producer of blood cells.

• Storage: Bones serve as the body's main reservoir of calcium and phosphorus; bone marrow stores fat as a reserve fuel.

Gross Anatomy of Bones

Types of Bone Tissue

• Osseous Tissue: The hard, calcified tissue of bone.

• Compact Bone: Dense, solid bone forming the outer shell; organized into osteons.

• Spongy (Cancellous) Bone: Porous lattice found on the interior of bones; contains trabeculae and bone marrow.

Bone Structure

• Diaphysis: The shaft or central part of a long bone.

• Epiphysis: The expanded end of a long bone, which strengthens joints and provides attachment for tendons and ligaments.

• Medullary Cavity: The central cavity of bone shafts where marrow is stored.

• Articular Cartilage: Hyaline cartilage covering the ends of bones at joints.

• Periosteum: A fibrous membrane covering the outer surface of bones.

Microscopic Anatomy of Bone

Bone Cells

• Osteogenic Cells: Stem cells on bone surfaces that give rise to osteoblasts.

• Osteoblasts: Bone-forming cells that synthesize the organic matrix (osteoid); found in a single layer on bone surfaces.

• Osteocytes: Mature bone cells derived from osteoblasts; reside in lacunae and maintain bone tissue.

• Osteoclasts: Large, multinucleated cells that dissolve bone matrix; important for bone resorption and remodeling.

Bone Matrix

• Inorganic Component: About two-thirds of bone matrix; primarily calcium phosphate, providing resistance to compression.

• Organic Component: About one-third of bone matrix; mainly collagen and protein-carbohydrate complexes, providing tensile strength.

Example: A deficiency in calcium leads to rickets, a condition where bones are easily deformed due to insufficient mineralization.

Classification of Bones

• Long Bones: Most bones of the extremities (e.g., femur, humerus).

• Short Bones: Carpals (wrist bones) and tarsals (ankle bones).

• Flat Bones: Skull, ribs, sternum.

• Irregular Bones: Vertebrae.

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

Bone Marrow

• Red Marrow: Hematopoietic tissue responsible for blood cell production; found mainly in flat bones and the ends of long bones.

• Yellow Marrow: Primarily adipose tissue; serves as an energy reserve and is found in the medullary cavity of long bones.

Bone Development and Growth

Ossification

Ossification is the process of bone formation from other tissues. It begins in the first few weeks of fetal development and continues until about 20 years of age.

• Intramembranous Ossification: Formation of flat bones (e.g., skull, clavicle) from mesenchymal tissue.

• Endochondral Ossification: Formation of long bones from a cartilage model.

Bone Fractures and Healing

Types of Fractures

• Stress Fracture: Caused by unusual stress, such as a fall or accident.

• Pathological Fracture: Occurs in bones weakened by disease.

• Open (Compound) Fracture: Bone breaks through the skin.

• Closed (Simple) Fracture: Bone does not break the skin.

Healing Process

1. Formation of a fracture hematoma from broken blood vessels.

2. Deposition of collagen and fibrocartilage forms a soft callus.

3. Soft callus ossifies into a hard callus.

4. Remodeling: Spongy bone is replaced with compact bone over months to years.

Joints (Articulations)

Classification of Joints

• Fibrous Joints (Synarthrosis): Bones are bound by collagen fibers; little to no movement (e.g., skull sutures, tooth-jaw joint).

• Cartilaginous Joints (Amphiarthrosis): Bones are linked by cartilage; limited movement (e.g., pubic symphysis, intervertebral discs, costal cartilage).

• Synovial Joints (Diarthrosis): Most familiar and structurally complex joints (e.g., shoulder, elbow, hip, knee); allow free movement.

Structure of Synovial Joints

• Joint (Articular) Capsule: Encloses the joint cavity; consists of an outer fibrous layer and an inner synovial membrane.

• Synovial Membrane: Secretes synovial fluid for lubrication.

• Articular Disc: Pad of fibrocartilage between bones, improving fit and absorbing shock.

• Meniscus: Crescent-shaped cartilage in the knee, providing stability and shock absorption.

• Bursa: Fluid-filled sacs that cushion muscles and help tendons move smoothly over bones.

Types of Joint Movements

• Abduction/Adduction: Movement away from/toward the midline.

• Elevation/Depression: Raising or lowering a body part.

• Protraction/Retraction: Moving a body part forward or backward on a horizontal plane.

• Circumduction: Circular movement of a limb or eye.

• Rotation: Turning a bone around its own axis (e.g., head, humerus).

• Supination/Pronation: Rotational movement of the forearm; supination turns the palm up, pronation turns it down.

• Dorsiflexion/Plantar Flexion: Upward/downward movement of the foot at the ankle.

• Inversion/Eversion: Turning the sole of the foot inward/outward.

• Opposition: Movement of the thumb to touch the fingertips.

Additional info: The provided images and slides use humor and pop culture references to reinforce anatomical concepts, such as labeling bones and understanding skeletal structure in various poses. These are intended to aid memory and engagement in learning.