Chapter 6: The Skeletal System – Structure, Function, and Cellular Anatomy

The Skeletal System Overview

Components of the Skeletal System

The skeletal system is a complex framework that provides structure, protection, and support for the human body. It consists of bones, cartilages, ligaments, and connective tissues.

• Axial skeleton: Includes the skull, thorax (rib cage and sternum), and vertebral column. It forms the central axis of the body and consists of 80 bones.

• Appendicular skeleton: Comprises the limbs (upper and lower) and limb girdles (pectoral and pelvic). It contains 126 bones and is responsible for movement.

• Cartilages: Structures such as costal cartilages, articular cartilages, and intervertebral discs provide flexibility and cushioning.

• Ligaments and connective tissue: Ligaments connect bones to other bones, while connective tissues like the periosteum cover and protect bone surfaces.

Functions of the Skeletal System

Major Functions

The skeletal system performs several essential functions necessary for survival and movement:

• Support: Provides a rigid framework for the attachment of soft tissues and organs.

• Mineral storage: Stores calcium and phosphate in the bone matrix; yellow bone marrow stores lipids for energy reserves.

• Blood cell production: Red bone marrow is the site of hematopoiesis (production of blood cells).

• Protection: Protects vital organs (e.g., skull protects the brain, thorax protects the heart and lungs, pelvis protects reproductive organs).

• Movement and leverage: Bones act as levers that muscles pull on to generate body motion.

Classification of Bones

Types of Bones

Bones are classified based on their shapes and functions:

• Flat bones: Thin and relatively broad (e.g., parietal bone, roof of skull, sternum, ribs, scapulae).

• Long bones: Longer than they are wide (e.g., humerus, bones of the limbs except carpals and tarsals).

• Irregular bones: Complex shapes (e.g., vertebrae, pelvis, some skull bones).

• Short bones: As wide as they are long (e.g., carpal and tarsal bones).

Bone (Osseous) Tissue

Composition and Properties

Bone tissue is a specialized connective tissue with unique structural and functional properties:

• Cells: The primary cell type is the osteocyte, which resides in spaces called lacunae.

• Matrix:

  • About 2/3 of bone mass is calcium phosphate salt ($Ca_3(PO_4)_2$), providing strength against compression but making bone inflexible and brittle.

  • About 1/3 is collagen fibers, which contribute flexibility and strength against tension, allowing bones to bend slightly under force.

  • The combination of minerals and collagen makes bone strong, slightly flexible, water-resistant, and resilient to bending forces.

• Remodeling and repair: Bone is a dynamic tissue, capable of remodeling and repair due to its cellular and vascular properties.

Bone Cells

Types and Functions

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

• Osteocytes: Mature bone cells that are amitotic (do not divide). They reside in lacunae and maintain the protein and mineral content of the bone matrix.

• Osteoblasts: Bone-forming cells that produce and secrete proteins and increase local mineral concentration to form new bone. Once surrounded by matrix, they become osteocytes.

• Osteoclasts: Large, multinucleated cells derived from immune cells. They break down bone matrix (osteolysis) using acids and enzymes, playing a key role in bone resorption and remodeling.

Summary Table: Bone Types and Examples

Bone Type	Shape/Description	Examples
Flat Bones	Thin, broad	Parietal bone, sternum, ribs, scapulae
Long Bones	Longer than wide	Humerus, femur, radius, ulna
Irregular Bones	Complex shapes	Vertebrae, pelvis, some skull bones
Short Bones	As wide as long	Carpals, tarsals

Example: Bone Matrix Composition

• Calcium phosphate ($Ca_3(PO_4)_2$): Provides compressive strength.

• Collagen fibers: Provide tensile strength and flexibility.

• Clinical relevance: Imbalances in these components can lead to brittle bones (osteoporosis) or overly flexible bones (osteomalacia).

Additional info: The notes above are based on the provided slides and expanded with standard academic context for clarity and completeness.