Skeletal System: Cartilage, Bone Structure, and Ossification Study Guide

Chapter 6: Skeletal System Overview

Hyaline, Elastic, and Fibrocartilage Help Form the Skeleton

The skeletal system is composed of various types of cartilage and bone, each with distinct functions and properties. Understanding the structure and function of these tissues is essential for comprehending how the skeleton supports and protects the body.

Types of Cartilage and Their Functions

• Hyaline Cartilage: Maintains the shape of a structure while allowing great flexibility. It is the most abundant cartilage type and provides support with some pliability.

• Elastic Cartilage: Provides support with flexibility and resilience. Found in structures such as 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

Type of cartilage	Location in the body
Hyaline cartilage	Covers the ends of most bones at movable joints
Elastic cartilage	The cartilage of the external ear; forms the epiglottis
Fibrocartilage	Menisci of the knee joint; discs between vertebrae; pubic symphysis
Costal cartilage	Connects ribs to sternum (breastbone)
Cartilage of respiratory structures	Reinforces walls of the trachea

Cartilage Growth

• Appositional growth: Cartilage tissue is added at the periphery of existing cartilage tissue.

• Interstitial growth: New cartilage grows from within existing cartilage tissue.

Bones Perform Several Important Functions

Functions of the Skeleton and Bone Tissue

Bones serve multiple vital functions in the human body, contributing to support, movement, and homeostasis.

Function	Description
Support	Provides a framework that supports the body and cradles soft organs.
Protection	Protects vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).
Anchorage	Serves as attachment points for muscles, enabling movement.
Mineral storage	Stores minerals, especially calcium and phosphorus, which can be released into the bloodstream as needed.
Blood cell formation	Hematopoiesis occurs in red bone marrow.
Triglyceride (fat) storage	Fat is stored in yellow bone marrow.
Hormone production	Produces osteocalcin, which helps regulate bone formation and protects against obesity and diabetes.

Major Regions and Classes of Bones

Axial and Appendicular Skeleton

• Axial skeleton: Consists of the bones of the skull, vertebral column, and rib cage. Functions mainly in protection and support.

• Appendicular skeleton: Includes bones of the upper and lower limbs and the girdles (shoulder and pelvic) that attach the limbs to the trunk. Functions primarily in movement.

Bone Classes by Shape

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

• Short bones: Cube-shaped (e.g., carpals, tarsals).

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

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

Bone Structure: Compact and Spongy Bone

Gross Anatomy of Flat and Long Bones

Bones consist of compact bone (dense outer layer) and spongy bone (internal honeycomb structure). Key anatomical features include:

• Diaphysis: Shaft of a long bone.

• Epiphysis: Ends of a long bone.

• Periosteum: Outer fibrous membrane covering the bone.

• Endosteum: Membrane lining the internal bone surfaces.

• Articular cartilage: Covers joint surfaces.

• Red bone marrow: Site of blood cell formation; found in trabecular cavities of spongy bone in flat bones and epiphyses of long bones.

• Yellow bone marrow: Stores fat; found in medullary cavities of long bones.

Functional Importance of Bone Markings

Bone Marking	Projections that are	Surfaces that help to	Depressions and openings
Meatus			X
Condyle	X
Tubercle	X
Foramen			X
Spine	X
Trochanter	X
Facet		X
Fossa		X
Epicondyle	X
Tuberosity	X
Head	X

Histology of Compact and Spongy Bone

• Osteons: Structural units of compact bone.

• Trabeculae: Network of spongy bone.

• Osteocytes: Mature bone cells that monitor and maintain bone matrix.

• Canaliculi: Tiny canals that connect osteocytes and facilitate nutrient exchange.

Chemical Composition of Bone

Organic and Inorganic Components

• Organic components: Include cells (osteoblasts, osteocytes, osteoclasts) and osteoid (collagen fibers and ground substance). Provide tensile strength and flexibility.

• Inorganic components: Mainly hydroxyapatite (mineral salts, primarily calcium phosphate). Provide hardness and resistance to compression.

Analogy: The organic components are like the rebar in concrete, providing flexibility and tensile strength, while the inorganic components are like the concrete, providing hardness and compressive strength.

Bone Development: Intramembranous vs. Endochondral Ossification

Intramembranous Ossification

• Bone develops from a fibrous membrane.

• Forms flat bones such as the cranial bones and clavicles.

Endochondral Ossification

• Bone forms by replacing hyaline cartilage.

• Forms most bones of the skeleton, including long bones.

• Sequence: Cartilage model develops → bone collar forms → cavities form → secondary ossification centers appear → epiphyses ossify.

Epiphyseal Plates and Bone Growth

How Epiphyseal Plates Allow Long Bones to Grow

• Growth in length occurs at the epiphyseal plate (growth plate).

• Growth in width (thickness) occurs by appositional growth.

• The epiphyseal plate maintains a constant thickness by balancing cartilage growth and bone replacement.

Zones of the Epiphyseal Plate

Zone	Description
Resting zone	Inactive cartilage cells
Proliferation zone	Rapidly dividing cartilage cells
Hypertrophic zone	Older cartilage cells enlarge
Calcification zone	Matrix becomes calcified; cartilage cells die
Ossification zone	New bone formation occurs

Example: During childhood and adolescence, long bones lengthen as new cartilage is produced at the epiphyseal plate and then replaced by bone tissue.