Bones and Cartilage: Structure, Function, and Growth (Lecture 7 Study Notes)

Bones and Cartilage: Structure, Function, and Growth

Overview of Skeletal Tissues

The human skeleton is composed primarily of bone and cartilage, each serving distinct structural and functional roles. Understanding the types and properties of these tissues is essential for studying anatomy and physiology.

• Bones: Rigid organs forming the skeleton, providing support, protection, and facilitating movement.

• Cartilage: Flexible connective tissue found in joints, respiratory structures, and other areas requiring pliability.

• Axial and Appendicular Skeletons: The skeleton is divided into axial (skull, vertebral column, rib cage) and appendicular (limbs, girdles) components.

• Body Mass: Bones constitute approximately 20% of total body mass.

Cartilage: Structure and Function

Cartilage is a specialized connective tissue that provides flexibility and resilience in areas where bone would be too rigid. It is composed of cells called chondrocytes housed in spaces called lacunae, embedded within an extracellular matrix.

• Chondrocytes: Mature cartilage cells located in lacunae; responsible for maintaining the cartilage matrix.

• Lacunae: Small cavities within the matrix that house chondrocytes. The term derives from Latin, meaning 'pit' or 'hollow.'

• Matrix: Composed of water, collagen fibers, and proteoglycans, providing both strength and flexibility.

• Perichondrium: A dense layer of connective tissue surrounding most cartilage types, supplying nutrients via diffusion (since cartilage is avascular).

Types of Cartilage

There are three main types of cartilage, each with unique structural and functional properties:

• Hyaline Cartilage: Most abundant type; provides support with flexibility. Found in articular surfaces of joints, costal cartilage (ribs), larynx, respiratory tract, and nasal cartilage.

• Elastic Cartilage: Contains both collagen and elastic fibers, allowing for greater flexibility. Located in the external ear and epiglottis.

• Fibrocartilage: Rich in thick collagen fibers, providing high tensile strength. Found in intervertebral discs, menisci of the knee, and pubic symphysis.

Type	Main Fibers	Locations	Function
Hyaline	Collagen	Joints, ribs, nose, larynx, trachea	Support, flexibility
Elastic	Collagen & Elastic	Ear, epiglottis	Flexibility, shape retention
Fibrocartilage	Thick Collagen	Intervertebral discs, menisci, pubic symphysis	Shock absorption, tensile strength

Growth and Development of Cartilage

Cartilage grows and develops through two main processes, which are essential during embryonic development and adolescence:

• Appositional Growth: New cartilage is added to the surface by cells in the perichondrium.

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

• Cartilage growth continues until the end of adolescence, after which most cartilage is replaced by bone in a process called ossification.

Transition from Cartilage to Bone

During development, much of the skeleton starts as cartilage and is gradually replaced by bone through ossification. This process is crucial for forming the mature skeleton.

• Endochondral Ossification: Cartilage is replaced by bone, forming most bones of the body (especially long bones).

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue without a cartilage precursor, forming flat bones like those of the skull.

• Epiphyseal Plate: A region of hyaline cartilage at the ends of long bones, responsible for longitudinal growth during childhood and adolescence.

Key Terms and Definitions

• Chondrocyte: Cartilage cell responsible for matrix production and maintenance.

• Lacuna: Small cavity within cartilage or bone matrix housing a cell.

• Perichondrium: Dense connective tissue layer surrounding cartilage.

• Ossification: The process of bone formation from cartilage or membrane.

Example: Epiglottis Histology

The epiglottis is composed of elastic cartilage, visible under the microscope as chondrocytes within lacunae surrounded by a flexible matrix. This structure allows the epiglottis to bend and return to its original shape, essential for its function in the airway.

Additional info:

• Cartilage is avascular, meaning it lacks blood vessels; nutrients diffuse through the matrix from surrounding tissues.

• Bone replaces cartilage in most regions during development, except where flexibility is required (e.g., articular surfaces, ear, nose).