Anatomy: Skeletal System

Skeletal System Overview

Introduction

The skeletal system is a fundamental organ system in the human body, providing structure, protection, and facilitating movement. It is composed of bones, cartilage, ligaments, and joints, and plays a crucial role in hematopoiesis and mineral storage.

• Support and Movement: The skeleton provides a rigid framework that supports the body and permits movement through articulation with muscles.

• Protection: Bones protect vital organs, such as the brain (skull), heart and lungs (rib cage), and spinal cord (vertebrae).

• Marrow Functions: Bone marrow is responsible for the formation of blood cells, a process known as hematopoiesis.

• Storage: Bones store fat (yellow marrow) and essential minerals, including calcium and phosphorus.

Function of Bones

Key Functions

• Support: Provides the structural framework for the body.

• Movement: Acts as levers for muscles to produce movement.

• Protection: Shields internal organs from injury.

• Hematopoiesis: Formation of blood cells occurs in red bone marrow.

• Storage:

  • Fat is stored in yellow marrow.

  • Minerals such as calcium and phosphorus are stored and released as needed.

Organization of the Skeleton

Axial vs. Appendicular Skeleton

The human skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Composed of the skull, vertebral column, and thoracic cage (ribs and sternum). It supports and protects the organs of the head, neck, and trunk.

• Appendicular Skeleton: Includes the limbs and girdles that attach them to the axial skeleton.

Appendicular Skeleton Components

• Pectoral Girdle:

  • Clavicles (collarbones)

  • Scapulae (shoulder blades)

• Upper Limb:

  • Arm (humerus)

  • Forearm (radius and ulna)

  • Hand (carpals, metacarpals, phalanges)

Types of Bone Tissue

Compact (Cortical) Bone vs. Spongy (Cancellous) Bone

Bones are composed of two main types of tissue: compact and spongy bone, each with distinct structure and function.

Feature	Compact (Cortical) Bone	Spongy (Cancellous) Bone
Density	Denser	Less dense, open network
Location	Covers most bones, forms outer layer	Found at ends of long bones, inside flat bones
Structure	Tightly packed osteons	Lace-like trabeculae
Function	Better storage of minerals	Contains marrow, supports hematopoiesis

Bone Anatomy

Long Bone Structure

Long bones, such as the femur and humerus, have a specialized structure that supports their function.

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

• Epiphyses: The ends of the bone, containing spongy bone surrounded by compact bone.

• Articular Cartilage: Covers the ends of bones at joints, reducing friction and absorbing shock.

• Periosteum: A dense connective tissue membrane covering the outer surface of bones, involved in bone growth and repair.

• Medullary Cavity: Central cavity within the diaphysis, containing yellow marrow (fat storage in adults).

• Epiphyseal Line: Remnant of the growth plate, where bone lengthening occurred during development.

Microscopic Anatomy

• Osteon (Haversian System): The structural unit of compact bone, consisting of concentric lamellae (layers) surrounding a central canal.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Volkmann's Canals: Perpendicular channels that connect osteons and facilitate blood and nerve supply.

• Lacunae: Small spaces housing osteocytes (bone cells).

• Canaliculi: Tiny channels that connect lacunae, allowing nutrient and waste exchange.

Bone Marrow

• Red Marrow: Site of hematopoiesis (blood cell formation), found in spongy bone of infants and certain adult bones.

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

Summary Table: Bone Anatomy

Region	Main Features
Diaphysis	Compact bone, medullary cavity (yellow marrow)
Epiphyses	Spongy bone, articular cartilage, red marrow (in infants)
Periosteum	Outer covering, contains blood vessels and nerves
Endosteum	Inner lining of medullary cavity

Key Terms and Definitions

• Hematopoiesis: The process of blood cell formation, occurring in red bone marrow.

• Osteon: The fundamental functional unit of compact bone.

• Trabeculae: The lattice-like network of spongy bone.

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

• Epiphyseal Line: The remnant of the growth plate in adult bones.

Example: Femur Anatomy

The femur, a long bone in the thigh, demonstrates all major bone structures: a diaphysis (shaft), epiphyses (ends), articular cartilage, periosteum, and a medullary cavity containing yellow marrow.

Additional info: The images and slides provided are consistent with standard college-level Anatomy & Physiology content, focusing on the skeletal system's structure and function.