Bones and Skeletal Tissues: Structure, Function, and Classification

Bones and Skeletal Tissues

Introduction

The study of bones and skeletal tissues is fundamental to understanding the human body's support, movement, and protection mechanisms. This topic covers the anatomy, types, growth, and functions of bones and cartilages, which are essential for clinical practice and understanding diseases such as osteoporosis.

Skeletal Cartilages

Basic Structure, Types, and Locations

Skeletal cartilage is a resilient tissue that provides flexibility and support in various parts of the skeleton. It is primarily composed of water and lacks blood vessels and nerves.

• Perichondrium: A dense connective tissue layer surrounding cartilage, providing nutrients and resisting outward expansion.

• Chondrocytes: Specialized cells encased in lacunae within the extracellular matrix.

Types of Cartilage

• Hyaline cartilage:

  • Provides support, flexibility, and resilience.

  • Most abundant type; contains only collagen fibers.

  • Locations: articular (joints), costal (ribs), respiratory (larynx), nasal (nose).

• Elastic cartilage:

  • Similar to hyaline but contains elastic fibers.

  • Locations: external ear, epiglottis.

• Fibrocartilage:

  • Contains thick collagen fibers for great tensile strength.

  • Locations: menisci of knee, vertebral discs.

Growth of Cartilage

Mechanisms of Cartilage Growth

Cartilage grows by two primary mechanisms, allowing for expansion and adaptation during development and repair.

• Appositional growth:

  • Matrix is secreted by cells in the perichondrium against the external surface of existing cartilage.

  • New matrix is laid down on the surface.

• Interstitial growth:

  • Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

  • New matrix is made internally.

Calcification of cartilage can occur during normal bone growth in youth and sometimes in old age, but hardened cartilage is not the same as bone.

Functions of Bones

Major Functions

Bones perform several vital functions necessary for survival and homeostasis.

• Support: Provides structural framework for the body and soft organs.

• Protection: Shields the brain, spinal cord, and vital organs from injury.

• Movement: Acts as levers for muscle action, enabling locomotion.

• Mineral and growth factor storage: Reservoir for calcium, phosphorus, and growth factors.

• Blood cell formation: Hematopoiesis occurs in red marrow cavities of certain bones.

• Triglyceride (fat) storage: Fat stored in bone cavities serves as an energy source.

• Hormone production: Osteocalcin secreted by bones helps regulate insulin secretion, glucose levels, and metabolism.

Classification of Bones

Axial and Appendicular Skeleton

Bones are grouped based on their location in the body:

• Axial skeleton: Long axis of the body, including the skull, vertebral column, and rib cage.

• Appendicular skeleton: Bones of the upper and lower limbs, and girdles attaching limbs to the axial skeleton.

Bone Shapes

Bones are also classified by shape:

• Long bones: Longer than they are wide (e.g., limb bones).

• Short bones: Cube-shaped (e.g., wrist and ankle bones); includes sesamoid bones formed within tendons (e.g., patella).

• Flat bones: Thin, flat, and slightly curved (e.g., sternum, scapula, ribs, most skull bones).

• Irregular bones: Complicated shapes (e.g., vertebrae, hip bones).

Bone Structure

Overview

Bones are organs composed of several tissue types, including osseous tissue, nervous tissue, cartilage, fibrous connective tissue, muscle cells, and epithelial cells in blood vessels. Bone structure can be studied at gross, microscopic, and chemical levels.

Compact and Spongy Bone

• Compact bone: Dense outer layer that appears smooth and solid.

• Spongy bone: Honeycomb-like structure of small needle-like pieces called trabeculae, with spaces filled with red or yellow bone marrow.

Microscopic Anatomy of Bone

Bone Cells

Five major types of bone cells, all derived from the same basic type:

• Osteogenic cells (osteoprogenitor cells): Mitotically active stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid), composed mainly of collagen and calcium-binding proteins.

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as stress sensors.

• Bone-lining cells: Flat cells on bone surfaces, help maintain matrix; called periosteal cells (external) and endosteal cells (internal).

• Osteoclasts: Derived from hematopoietic stem cells; giant, multinucleate cells responsible for bone resorption.

Compact Bone Structure

• Osteon (Haversian system): Structural unit of compact bone; elongated cylinders parallel to bone axis, composed of concentric rings (lamellae).

• Canals and canaliculi:

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

  • Perforating (Volkmann's) canals: Connect blood vessels and nerves of periosteum, medullary cavity, and central canal.

  • Lacunae: Small cavities containing osteocytes.

  • Canaliculi: Tiny canals connecting lacunae, allowing communication and nutrient/waste exchange.

• Interstitial and circumferential lamellae:

  • Interstitial lamellae: Fill gaps between osteons or are remnants of old osteons.

  • Circumferential lamellae: Extend around the entire diaphysis, just deep to periosteum.

Spongy Bone Structure

• Composed of trabeculae arranged along lines of stress for strength.

• No osteons present, but trabeculae contain lamellae and osteocytes interconnected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Includes osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid.

• Osteoid: Makes up one-third of organic bone matrix; consists of ground substance and collagen fibers, providing tensile strength and flexibility.

• Resilience due to sacrificial bonds in or between collagen molecules that stretch and break to dissipate energy and prevent fractures.

Inorganic Components

• Hydroxyapatites (mineral salts): Make up 65% of bone by mass; mainly tiny calcium phosphate crystals in and around collagen fibers.

• Responsible for bone hardness and resistance to compression.

• Bones are half as strong as steel in resisting compression and as strong as steel in resisting tension.

• Mineral composition allows bones to last long after death and provide information about ancient people.

Summary Table: Types of Cartilage

Type	Main Fibers	Locations	Function
Hyaline	Collagen	Joints, ribs, larynx, nose	Support, flexibility, resilience
Elastic	Elastic fibers	Ear, epiglottis	Flexibility
Fibrocartilage	Thick collagen	Knee menisci, vertebral discs	Tensile strength

Summary Table: Bone Cell Types

Cell Type	Function	Location
Osteogenic	Stem cell, forms osteoblasts	Periosteum, endosteum
Osteoblast	Bone formation	Bone surface
Osteocyte	Maintains matrix	Lacunae
Bone-lining	Matrix maintenance	Bone surfaces
Osteoclast	Bone resorption	Resorption bays

Key Equations

• Bone Matrix Composition: $\text{Bone} = \text{Organic Components} + \text{Inorganic Components}$

• Hydroxyapatite Formula: $\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2$

Additional info: These notes expand on the original slides by providing definitions, examples, and tables for clarity and exam preparation.