6.1 Hyaline, Elastic, and Fibrocartilage Help Form the Skeleton

Types and Functions of Skeletal Cartilages

Cartilage is a specialized connective tissue that provides support, flexibility, and resilience to various parts of the skeleton. The three main types of cartilage found in the human body are hyaline cartilage, elastic cartilage, and fibrocartilage.

• Hyaline cartilage: Most abundant type; found in articular, costal, respiratory, and nasal cartilages. Provides smooth surfaces for joint movement and supports respiratory structures.

• Elastic cartilage: More flexible than hyaline; found in the external ear and epiglottis. Maintains shape while allowing flexibility.

• Fibrocartilage: Withstands pressure and stretch; found in intervertebral discs and knee menisci. Provides strong support and shock absorption.

Perichondrium is a dense layer of irregular connective tissue that surrounds cartilage, providing nutrients and aiding in growth and repair.

Growth of cartilage occurs by:

• Appositional growth: Cartilage grows outward due to production of new matrix on the outer surface.

• Interstitial growth: Cartilage expands from within as chondrocytes divide and secrete new matrix.

6.2 Bones Perform Several Important Functions

Functions of Bones

Bones are dynamic organs that serve multiple vital functions in the body.

• Support: Provide structural framework for the body.

• Protection: Shield vital organs (e.g., skull protects brain, rib cage protects heart and lungs).

• Movement: Serve as levers for muscles to act upon.

• Mineral storage: Reservoir for calcium and phosphate.

• Blood cell formation: Hematopoiesis occurs in red bone marrow.

• Triglyceride storage: Yellow bone marrow stores fat.

6.3 How Are Bones Classified?

Classification by Location and Shape

Bones are classified based on their location in the skeleton and their shape.

• Axial skeleton: Includes skull, vertebral column, and rib cage.

• Appendicular skeleton: Includes bones of the limbs and girdles.

Bones are also classified by shape:

• Long bones: Longer than wide, with a shaft and two ends (e.g., femur, humerus).

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

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

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

6.4 Bone Structure

Gross Anatomy of Bones

The structure of all bones consists of compact bone sandwiching spongy bone. This arrangement provides strength and lightness.

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

• Spongy bone (trabecular bone): Internal honeycomb of small needle-like or flat pieces called trabeculae.

Structure of a typical long bone:

• Diaphysis: Shaft, surrounds a medullary cavity filled with yellow marrow.

• Epiphyses: Ends of the bone, covered by articular cartilage.

• Membranes: Periosteum (outer) and endosteum (inner) cover bone surfaces.

Microscopic anatomy includes:

• Osteon (Haversian system): Structural unit of compact bone, consisting of concentric lamellae around a central canal.

• Osteocytes: Mature bone cells housed in lacunae, connected by canaliculi.

• Bone cells: Osteogenic cells, osteoblasts, osteocytes, and osteoclasts.

6.5 How Do Bones Develop?

Bone Development: Intramembranous vs. Endochondral Ossification

Bones develop either by intramembranous ossification or endochondral ossification.

• Intramembranous ossification: Forms membrane bone from fibrous connective tissue (e.g., cranial bones, clavicles).

• Endochondral ossification: Forms bone by replacing hyaline cartilage (e.g., all bones below the skull except clavicles).

Key steps in endochondral ossification:

1. Primary ossification center forms in the diaphysis.

2. Bone collar forms around the diaphysis.

3. Cartilage in the center calcifies and develops cavities.

4. Periosteal bud invades cavities, forming spongy bone.

5. Diaphysis elongates, medullary cavity forms.

6. Epiphyses ossify.

Key steps in intramembranous ossification:

1. Ossification centers appear in fibrous connective tissue membrane.

2. Osteoid is secreted and calcifies.

3. Woven bone and periosteum form.

Postnatal bone growth occurs at the epiphyseal plate, with zones for resting, proliferation, hypertrophy, and calcification.

6.6 How Are Bones Remodeled?

Bone Remodeling: Deposit and Removal

Bone remodeling is a continuous process involving bone deposit by osteoblasts and bone resorption by osteoclasts.

• Bone deposit: New bone matrix is laid down; osteoid seam marks the area of new bone.

• Bone resorption: Osteoclasts break down bone matrix, releasing minerals into the blood.

Remodeling is controlled by hormonal and mechanical factors:

• Hormonal control: Parathyroid hormone (PTH) and calcitonin regulate blood calcium levels.

• Mechanical stress: Bones remodel in response to the forces and stresses placed upon them.

Negative feedback mechanisms maintain calcium homeostasis:

• When blood calcium drops, PTH is released, stimulating osteoclasts to resorb bone and release calcium.

• When blood calcium rises, calcitonin is released, stimulating calcium deposit in bone.

Key equation:

6.7 How Are Bones Repaired?

Bone Repair: Hematoma, Callus Formation, and Remodeling

Bone repair after fracture involves several stages:

1. Hematoma formation: Blood vessels break, forming a clot at the fracture site.

2. Fibrocartilaginous callus formation: Soft callus forms as fibroblasts and chondroblasts invade the area.

3. Bony callus formation: Osteoblasts produce new bone, converting the soft callus to a hard bony callus.

4. Bone remodeling: The bony callus is remodeled to restore the bone's original shape and structure.

Bone repair involves both hematoma and callus formation, followed by remodeling to restore strength and function.

6.8 What Happens When Things Go Wrong?

Abnormal Bone Deposition and Resorption

Disorders of bone remodeling can lead to diseases such as osteoporosis, osteomalacia, and Paget's disease.

• Osteomalacia and rickets: Inadequate mineralization of bone due to vitamin D deficiency; bones are soft and weak.

• Osteoporosis: Bone resorption outpaces deposit, leading to porous, fragile bones and increased fracture risk.

• Paget's disease: Excessive and abnormal bone remodeling, resulting in enlarged and misshapen bones.

Table: Comparison of Bone Disorders

Short Answer Essay Questions (Review)

• Explain why cartilage is more resilient than bone and how its structure relates to its function.

• Describe the events of endochondral and intramembranous ossification.

• Discuss the role of the epiphyseal plate in bone growth.

• Explain the hormonal regulation of bone growth and remodeling.

• Contrast the effects of bone remodeling in osteoporosis, osteomalacia, and Paget’s disease.

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries and equations have been inferred and formatted for study purposes.