The Skeletal System - Introduction and Overview

1. Divisions of the Skeleton

2. Components of the Skeletal System

A. Cartilage

• Support soft tissues
• Provide smooth gliding surfaces at bone articulations (joints)
• Enable development and growth of long bones

B. Bone

1. Structural support for the body
2. Protection of vital organs (e.g., ribcage protects heart and lungs; skull protects brain)
3. Reservoir of calcium and phosphorus (mineral homeostasis)
4. Levers for muscles to act upon, producing movement
5. Container for blood-producing (hematopoietic) cells in the marrow

3. Types of Bone Tissue

• Compact (Cortical) Bone - Dense, forms the outer shell of all bones; surrounds spongy bone
• Spongy (Trabecular / Cancellous) Bone - Consists of bony spicules (trabeculae) enclosing cavities containing blood-forming cells (marrow)

4. Classification of Bones by Shape

Clinical note: Accessory and sesamoid bones are normal variants present in many people, especially in the wrist, hands, ankles, and feet. They must not be mistaken for fractures on imaging. - Gray's Anatomy for Students

5. Blood Supply and Innervation of Bone

• Each bone typically receives one nutrient artery from an adjacent vessel, which enters the internal cavity and supplies the marrow, spongy bone, and inner compact bone.
• The outer layers of compact bone are supplied by vessels from the periosteum.
• The periosteum (fibrous connective tissue membrane covering all bone surfaces except joint surfaces) is richly supplied with sensory nerve fibers and is very sensitive to injury - this is why bone injuries are so painful.
• Bone itself has relatively few sensory nerve fibers.
• A bone stripped of its periosteum will not survive.

6. Development of the Skeletal System (Ossification)

Intramembranous Ossification

• Mesenchymal models of bones undergo direct ossification without a cartilage intermediate
• Forms: flat bones of the skull (calvaria), clavicle, mandible

Endochondral Ossification

• Mesenchyme first forms a cartilage model, which is then replaced by bone
• Ossification centers appear by end of the embryonic period (~56 days)
• Forms: most bones, including all long bones and the appendicular skeleton

"The skeletal system develops from mesenchyme, which is derived from mesoderm and the neural crest. In most bones, such as the long bones in the limbs, the condensed mesenchyme undergoes chondrification to form cartilage models for bone formation." - The Developing Human: Clinically Oriented Embryology

7. Skeletal Maturity and Bone Age

• Bones develop in a predictable sequence throughout childhood and adolescence
• Skeletal maturity typically occurs between ages 20-25 years, though this varies with geography, socioeconomic conditions, genetics, and disease
• Bone age can be assessed by radiographing the non-dominant (left) hand and comparing to standardized reference radiographs

8. Joints (Articulations)

9. Key Clinical Correlates

• Fractures - occur when abnormal load exceeds bone strength, or normal load exceeds weakened bone (e.g., osteoporosis). Healing involves hematoma formation, new vessel ingrowth, collagen deposition, and callus formation by osteoblasts.
• Greenstick fractures - partial cortical fractures in children's developing bones (bends rather than breaks completely)
• Avascular necrosis - death of bone tissue from temporary or permanent loss of blood supply; classic site is the femoral head after femoral neck fracture
• Epiphyseal (growth plate) fractures - more common during growth spurts (ages 7-10 and puberty); may cause asymmetric growth if the plate is damaged

• Gray's Anatomy for Students (9780323934237), pp. 29-34
• The Developing Human: Clinically Oriented Embryology (9780323611541), pp. 901-945