Chapter 6: Cartilage and Bone Structure, Function, and Classification

6.1 Hyaline, Elastic, and Fibrocartilage Help Form the Skeleton

Types of Cartilage Tissue

Cartilage is a specialized connective tissue that provides support, flexibility, and resilience to various parts of the skeleton. There are three main types of cartilage, each with distinct properties and functions.

• Hyaline cartilage: Most abundant type, providing support with flexibility and resilience.

  • Articular cartilages: Cover the ends of most bones at movable joints.

  • Costal cartilages: Connect ribs to the sternum (breastbone).

  • Respiratory cartilages: Form the skeleton of the larynx (voice box) and reinforce other respiratory passageways.

  • Nasal cartilages: Support the external nose.

• Elastic cartilage: Contains more elastic fibers than hyaline cartilage, allowing it to withstand repeated bending.

  • Found in the external ear and the epiglottis.

• Fibrocartilage: Resistant to compression and has great tensile strength.

  • Consists of parallel rows of chondrocytes with thick collagen fibers.

  • Found in areas subjected to pressure and stretch, such as the menisci of the knee and intervertebral discs.

Cells of Cartilage

All cartilage types share basic cellular components that contribute to their structure and function.

• Chondrocytes: Mature cartilage cells found within small cavities called lacunae in the cartilage matrix.

• Chondroblasts: Immature, actively mitotic cells responsible for producing the cartilage matrix.

• Lacunae: Small cavities within bone and cartilage tissues that house chondrocytes.

• Extracellular matrix: Substance found outside of cells, consisting of flexible, gel-like ground substance and fibers, primarily collagen, which give cartilage its strength and resilience. Produced by chondroblasts and maintained by chondrocytes.

Growth of Cartilage

Cartilage grows by two main mechanisms, allowing it to expand and adapt during development and repair.

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

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

Cartilage vs. Bone: Cartilage has a flexible matrix, while bone has a hard matrix. Cartilage is ideal for the embryonic skeleton and new skeletal growth.

6.2 Bones Perform Several Important Functions

Functions of the Bones

Bones are dynamic organs that perform a variety of essential functions for the body.

• Support: Provide a framework that supports the body and cradles soft organs.

• Protection: Fused bones of the skull protect the brain, vertebrae surround the spinal cord, and the rib cage protects vital thoracic organs.

• Anchorage (movement): Skeletal muscles attach to bones by tendons and use bones as levers to move the body and its parts.

• Mineral storage: Bones store minerals, especially calcium and phosphate, which are released into the blood as needed.

• Blood cell formation: Hematopoiesis, or blood cell formation, occurs in the red bone marrow of certain bones.

• Triglyceride (fat) storage: Fat, serving as an energy source, is stored as yellow bone marrow in the cavities of long bones.

• Hormone production: Bones produce osteocalcin, a hormone that regulates insulin secretion, glucose homeostasis, and energy expenditure.

6.3 Bones Are Classified by Their Location and Shape

Classification by Location

The human skeleton has 206 named bones (in the adult), divided into two groups: axial and appendicular.

• Axial skeleton: Forms the long axis of the body, including the skull, vertebral column, and rib cage. These bones protect, support, or carry other body parts.

• Appendicular skeleton: Includes the bones of the upper and lower limbs and the girdles (shoulder and hip bones) that attach the limbs to the axial skeleton. These bones facilitate movement and environmental manipulation.

Classification by Shape

Bones are classified into four shapes, each with specific examples and functions.

• Long bones: Longer than they are wide, with a cylindrical shaft and two distinct ends. Examples: limb bones (except the patella, wrist, and ankle bones).

• Short bones: Roughly cube-shaped. Examples: bones of the wrist and ankle.

• Sesamoid bones: A special type of short bone that forms in tendons. Example: patella.

• Flat bones: Thin, flattened, and usually curved. Examples: bones of the skull, sternum, ribs, and scapulae.