Chapter 6
Cartilage
Location and Basic Structure of Cartilage
Found throughout the adult body.
Cartilage in the External Ear
Cartilage in the Nose
Articular Cartilages and Costal Cartilage
Cartilages in the Larynx and Trachea
Intervertebral Discs, Pubic Symphysis, and Articular Discs
Types of Cartilage
Types of Cartilage
All cartilages share some similarities:
Cell Type: Chondrocyte, located within lacunae.
Matrix Contains:
Fibers
Jellylike ground substance
Hyaline Cartilage
Most abundant type of cartilage.
Chondrocytes: Appear spherical.
Matrix Composition:
Collagen unit fibril is the only fiber type.
Ground substance holds a large amount of water.
Provides support through flexibility and resilience.
Elastic Cartilage
Contains many elastic fibers.
Tolerates repeated bending.
Locations:
Epiglottis
Cartilage of the external ear
Fibrocartilage
Resists strong compression and tension.
Intermediate between hyaline and elastic cartilage.
Locations:
Pubic symphysis
Menisci of the knee
Anulus fibrosus
Growth of Cartilage
Appositional Growth
Chondroblasts in the surrounding perichondrium produce new cartilage.
Interstitial Growth
Chondrocytes within cartilage divide and secrete a new matrix.
Cartilage stops growing when the skeleton stops growing.
Tissues in Bone
Components of Bones
Dominated by bone connective tissue.
Contains:
Nervous tissue
Blood connective tissue
Cartilage in articular areas
Epithelial tissue lining blood vessels
Function of Bones
Functions of Bones
Support: Provides hard framework.
Movement: Skeletal muscles use bones as levers.
Protection: Shields underlying organs.
Mineral Storage: Reservoir for important minerals.
Blood Cell Formation: Contains red marrow.
Energy Metabolism: Osteoblasts secrete osteocalcin.
Bone Tissue
Bone Tissue Composition
Organic Components:
Cells, fibers, and ground substance.
Inorganic Components:
Mineral salts that invade the bony matrix.
Extracellular Matrix of Bone
Unique Composition of Matrix
Gives bone exceptional properties:
35% Organic Components: Contributes to flexibility and tensile strength.
65% Inorganic Components: Provide hardness and resist compression.
Bone Cells
Types of Cells in Bone
Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.
Osteoblasts:
Actively produce and secrete bone matrix.
Bone matrix is referred to as osteoid.
Osteocytes:
Maintain health of bone matrix.
Osteoclasts:
Found within bone tissue.
Responsible for bone resorption.
Derived from a line of white blood cells.
Secrete hydrochloric acid and lysosomal enzymes.
Classification of Bones
Categories of Bones
Long Bones: Longer than wide; includes a shaft plus ends.
Short Bones: Roughly cube-shaped.
Flat Bones: Thin and flattened, usually curved.
Irregular Bones: Various shapes, do not fit into other categories.
Gross Anatomy of Bones
Compact Bone:
Dense outer layer of bone.
Spongy Bone:
Internal network of bone.
Trabeculae: Little “beams” of bone.
Open spaces between trabeculae filled with marrow.
Structure of a Typical Long Bone
Diaphysis: Shaft of the bone.
Epiphyses: Ends of the bone.
Blood Vessels: Well vascularized.
Medullary Cavity: Hollow cavity filled with yellow marrow.
Membranes:
Periosteum and endosteum.
Structure of Short, Irregular, and Flat Bones
Flat Bones, Short Bones, and Irregular Bones:
Contain bone marrow but no marrow cavity.
Diploë: Internal spongy bone of flat bones.
Bone Markings
Functions of Bone Markings
Superficial surfaces of bones reflect stresses on them.
Three Broad Categories of Bone Markings:
Projections for muscle attachment.
Surfaces that form joints.
Depressions and openings.
Microscopic Structure of Compact Bones
Compact Bone:
Contains passageways for blood vessels, lymph vessels, and nerves.
Osteons:
Function in support.
Long cylindrical structures.
Includes:
Lamellae
Central canal
Perforating canals
Canaliculi
Spongy Bone:
Is less complex than compact bone.
Trabeculae: Contain layers of lamellae and osteocytes.
Are too small to contain osteons.
Bone Development
Ossification (Osteogenesis): Process of bone tissue formation.
Membranous Bones: Formed directly from mesenchyme (Intramembranous ossification).
Other Bones: Develop initially from hyaline cartilage (Endochondral ossification).
Endochondral Ossification
Applies to all bones except some skull bones and clavicles.
Bones modeled in hyaline cartilage.
Starts forming late in the second month of embryonic development and continues until early adulthood.
Anatomy of the Epiphyseal Plate
In epiphyseal plates of growing bones:
Cartilage is organized for rapid growth.
Cartilage cells form tall stacks with chondroblasts at the top that divide quickly, pushing the epiphysis away from the diaphysis and lengthening the entire long bone.
Postnatal Growth of Endochondral Bones
During childhood and adolescence:
Bones lengthen entirely by growth of epiphyseal plates.
Cartilage is replaced with bone tissue as quickly as it grows, maintaining constant thickness.
Whole bone lengthens.
As Adolescence Draws to an End:
Chondroblasts divide less often, epiphyseal plates thin, cartilage stops growing and is replaced by bone tissue.
Long bones stop lengthening when diaphysis and epiphysis fuse.
Widening of Growing Bones:
Osteoblasts add bone tissue to the external surface of the diaphysis.
Osteoclasts remove bone from the internal surface of the diaphysis.
This process is referred to as appositional growth.
Hormonal Regulation of Bone Growth
Growth Hormone: Produced by the pituitary gland; stimulates epiphyseal plates.
Thyroid Hormone: Ensures proper proportions of the skeleton.
Sex Hormones (Estrogen and Testosterone): Promote bone growth and later induce closure of epiphyseal plates.
Bone Remodeling
Bone is a dynamic tissue:
500 mg of calcium may enter or leave the adult skeleton each day.
Bone matrix and osteocytes are continually removed and replaced.
Spongy bone of the skeleton is replaced every 3–4 years; compact bone is replaced every 10 years.
Bone Remodeling Components:
Bone deposit occurs at periosteal and endosteal surfaces, accomplished by osteoblasts.
Bone reabsorption is accomplished by osteoclasts.
Osteoclasts
Osteoclasts: A giant cell with many nuclei that crawls along bone surfaces to:
Break down bone tissue.
Secrete concentrated hydrochloric acid and release lysosomal enzymes.
Derived from hematopoietic stem cells.
Repair of Bone Fractures
Fractures classified as simple or compound; treatment by reduction includes:
Closed Reduction: Non-surgical alignment of broken bones.
Open Reduction: Surgical alignment of bones.
Common Types of Fractures
Comminuted Fracture: Bone fragments into three or more pieces; common in aged individuals with brittle bones.
Compression Fracture: Bone is crushed, typical in porous bones under extreme trauma.
Spiral Fracture: Ragged break due to excessive twisting forces; common in sports.
Epiphyseal Fracture: Separation of epiphysis from diaphysis along the epiphyseal plate; common where cartilage cells are dying.
Depressed Fracture: Broken bone portion pressed inward; typical of skull fractures.
Greenstick Fracture: Bone breaks incompletely, similar to a green twig; common in children with more organic matrix in their bones.
Disorders of Bones
Osteoporosis: Bone reabsorption outpaces deposition. More common in women after menopause.
Osteomalacia: Inadequately mineralized bones in adults.
Rickets: Similar to osteomalacia but occurs in children.
Osteosarcoma: A form of bone cancer.
The Skeleton Throughout Life
Cartilage grows quickly in youth.
Skeleton shows fewer chondrocytes in the elderly.
Skeleton grows until ages 18–21 years.
In children and adolescents, bone formation exceeds rate of bone reabsorption.
In young adults, bone formation is balanced with bone reabsorption.
In old age, reabsorption predominates, leading to a decline in bone mass.