Chapter 6: Cartilage and Bone Tissue

6.1 Hyaline, Elastic, and Fibrocartilage Help Form the Skeleton

This section introduces the three main types of cartilage found in the human skeleton, describing their properties and functions.

• Hyaline cartilage: Provides support with flexibility and resilience. It is the most abundant type and forms articular surfaces, costal cartilages, and parts of the respiratory tract.

• Elastic cartilage: Maintains the shape of a structure while allowing great flexibility. Found in the external ear and epiglottis.

• Fibrocartilage: Resists compression and provides great tensile strength. Located in intervertebral discs and menisci of the knee.

Major Cartilages of the Adult Skeleton:

Additional info: Cartilage grows by two mechanisms: interstitial growth (from within existing cartilage tissue) and appositional growth (new cartilage tissue added at the periphery).

6.2 Bones Perform Several Important Functions

Bones are dynamic organs that perform a variety of functions essential for movement, protection, and homeostasis.

6.3a: Major Regions of the Skeleton and Their Functions

The skeleton is divided into two main regions: axial and appendicular.

• Axial skeleton: Includes bones of the skull, vertebral column, and rib cage. Functions mainly in protection, support, and carrying other body parts.

• Appendicular skeleton: Consists of bones of the upper and lower limbs and the girdles that attach them to the axial skeleton. Functions in movement and manipulation of the environment.

6.3b: Bone Classification

Bones are classified into four groups based on their shape:

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

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

• Flat bones: Thin, flattened, and usually curved (e.g., sternum, skull bones).

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

6.4: All Bones Consist of Outer Compact Bone and Inner Spongy Bone

Bones have a complex structure with both compact and spongy regions, as well as specialized tissues.

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

• Spongy bone: Internal layer with a honeycomb structure (trabeculae).

• Red bone marrow: Site of blood cell formation, found in spongy bone of flat bones and some long bones.

• Yellow bone marrow: Fat storage, found in the medullary cavity of long bones.

• Periosteum: Double-layered membrane covering the external surface of bones.

• Endosteum: Membrane lining the internal bone surfaces.

6.4b: Functional Importance of Bone Markings

Bones have surface features called markings that serve various functions.

6.4c: Histology of Compact and Spongy Bone

Bones are composed of different microscopic structures:

• Osteons: Compact bone only

• Trabeculae: Spongy bone only

• Osteocytes: Both compact bone and spongy bone

• Canaliculi: Both compact bone and spongy bone

6.4d: Chemical Composition of Bone

Bones are made of organic and inorganic components, each conferring specific advantages.

• Organic components: Cells and osteoid (ground substance and collagen fibers). Provide flexibility and tensile strength.

• Inorganic components: Mineral salts, mainly hydroxyapatite (calcium phosphates). Provide hardness and resistance to compression.

Analogy: Organic components are like reinforcing bars in concrete, providing flexibility and strength. Inorganic components are like the concrete itself, providing hardness and resistance to compression.

6.5: Bone Development by Intramembranous or Endochondral Ossification

Bones form by two main processes: intramembranous and endochondral ossification.

• Intramembranous ossification: Bone develops from a fibrous membrane. Most flat bones form this way.

• Endochondral ossification: Bone develops by replacing hyaline cartilage. Most bones of the skeleton are formed by this process.

Steps of Endochondral Ossification:

1. Cartilage calcifies in the center of the diaphysis and then develops cavities.

2. The epiphyses ossify.

3. Secondary ossification centers appear in the epiphyses; a medullary cavity forms in the diaphysis.

4. The periosteal bud invades the internal cavities and spongy bone forms.

5. A bone collar forms around the diaphysis of the hyaline cartilage model.

Key Equations:

• Bone growth:

• Bone remodeling:

Additional info: Most bones that form by intramembranous ossification are flat bones. Endochondral ossification is more complex and forms most bones of the body.