Bones and Skeletal Tissue: Structure, Function, and Classification

Bones and Skeletal Tissue

Introduction

The skeletal system is a complex framework composed of bones, cartilages, joints, and ligaments. It provides support, protection, movement, and serves as a reservoir for minerals and growth factors. Understanding the structure and function of bones and skeletal tissue is fundamental in Anatomy & Physiology.

The Skeleton

Overview

• Skeleton consists of bones, cartilages, joints, and ligaments.

• Most bones are connected by joints, which are reinforced by ligaments.

• The skeleton accounts for approximately 20% of body mass.

• Two major divisions:

  1. Axial skeleton: Skull, vertebral column, rib cage

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

Skeletal Cartilages

Types and Functions

• Cartilage is initially present in the human skeleton and is later replaced by bone except in areas requiring flexibility.

• Skeletal cartilage is made of highly resilient, molded tissue that consists primarily of water.

• Cartilage contains no blood vessels or nerves.

• Perichondrium: Layer of dense connective tissue surrounding cartilage, helps resist outward expansion and contains blood vessels for nutrient delivery.

• Cartilage is made up of chondrocytes, cells encased in small cavities called lacunae within a jelly-like extracellular matrix.

Types of Cartilage

• Hyaline cartilage: Most abundant, provides support, flexibility, and resilience. Contains collagen fibers only. Found in articular (joints), costal (ribs), respiratory (larynx), and nasal cartilage.

• Elastic cartilage: Similar to hyaline but contains elastic fibers. Found in external ear and epiglottis.

• Fibrocartilage: Thick collagen fibers, great tensile strength. Found in menisci of knee and vertebral discs.

Growth of Cartilage

• Appositional growth: Cartilage-forming cells in perichondrium secrete matrix against external face of existing cartilage.

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

• Calcification of cartilage occurs during normal bone growth in youth and can also occur in old age. Hardened cartilage is not the same as bone.

Functions of Bones

Seven Major Functions

• Support: For body and soft organs.

• Protection: Protects brain, spinal cord, and vital organs.

• Movement: Levers for muscle action.

• 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 as an energy source.

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

Classification of Bones

By Location

• Axial skeleton: Skull, vertebral column, rib cage.

• Appendicular skeleton: Limbs and girdles.

By Shape

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

• Short bones: Cube-shaped (e.g., wrist and ankle bones, sesamoid bones like patella).

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

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

There are 206 named bones in the human skeleton.

Bone Structure

Types of Bone Tissue

• Osseous tissue (bone tissue) predominates, but bones also contain:

  • Nervous tissue

  • Cartilage

  • Fibrous connective tissue

  • Muscle cells

  • Epithelial cells in blood vessels

Levels of Structure

• Gross

• Microscopic

• Chemical

Compact and Spongy Bone

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

• Spongy bone: Made up of a honeycomb of small, needle-like or flat pieces called trabeculae. Open spaces between trabeculae are filled with red or yellow bone marrow.

Structure of Short, Irregular, and Flat Bones

• Consist of thin plates of spongy bone (diploë) covered by compact bone.

• Compact bone sandwiched between connective tissue membranes:

  • Periosteum: Covers outside of compact bone.

  • Endosteum: Covers inside portion of bone.

• Bone marrow is scattered throughout spongy bone; no defined marrow cavity.

• Hyaline cartilage covers area of bone that is part of a movable joint.

Structure of Typical Long Bone

• All long bones have a shaft (diaphysis), bone ends (epiphyses), and membranes.

• Diaphysis: Tubular shaft forms long axis, consists of compact bone surrounding medullary cavity filled with yellow marrow in adults.

• Epiphyses: Ends of long bones, consist of compact bone externally and spongy bone internally.

• Articular cartilage: Covers articular (joint) surfaces.

• Epiphyseal line: Between diaphysis and epiphysis; remnant of childhood bone growth at epiphyseal plate.

Gross Anatomy - Membranes

Periosteum

• White, double-layered membrane covering external surfaces except joints.

• Fibrous layer: Outer layer of dense irregular connective tissue, contains Sharpey's fibers that secure to bone matrix.

• Osteogenic layer: Inner layer abutting bone, contains primitive osteogenic stem cells.

• Contains many nerve fibers and blood vessels that continue to the shaft through nutrient foramina openings.

• Anchoring points for tendons and ligaments.

Endosteum

• Delicate connective tissue membrane covering internal bone surface.

• Covers trabeculae of spongy bone and lines canals that pass through compact bone.

• Contains osteogenic cells that can differentiate into other bone cells.

Hematopoietic Tissue in Bones

• Red marrow: Found within trabecular cavities of spongy bone and diploë of flat bones (e.g., sternum).

• In newborns, medullary cavities and all spongy bone contain red marrow.

• In adults, red marrow is located in heads of femur and humerus, but most active areas are flat bone diploë and some irregular bones (e.g., hip bone).

• Yellow marrow can convert to red if person becomes anemic.

Microscopic Anatomy of Bone

Bone Cells

• Five major cell types, all specialized forms of the same basic cell type:

  1. Osteogenic cells

  2. Osteoblasts

  3. Osteocytes

  4. Bone-lining cells

  5. Osteoclasts

Summary Table: Types of Cartilage

Type	Main Features	Locations
Hyaline	Support, flexibility, resilience; collagen fibers only	Joints, ribs, nose, larynx, trachea
Elastic	Similar to hyaline, but contains elastic fibers	External ear, epiglottis
Fibrocartilage	Thick collagen fibers, great tensile strength	Menisci of knee, vertebral discs

Key Equations and Terms

• Number of bones in adult human skeleton: $206$

• Percentage of body mass as skeleton: $20\%$

Additional info:

• Some context and definitions were expanded for clarity and completeness.

• Scientific names and terms were italicized where appropriate.