Bones and Skeletal Tissues

Cartilages Help Form the Skeleton

Cartilage is a resilient connective tissue that provides support and flexibility to parts of the skeleton. It is surrounded by a layer called the perichondrium, which contains blood vessels for nutrient delivery and aids in growth and repair.

• Growth of Cartilage: Cartilage grows in two ways:

  • Appositional growth: New matrix is secreted on the external surface of the cartilage by chondroblasts in the perichondrium.

  • Interstitial growth: Chondrocytes within the cartilage divide and secrete new matrix, expanding the cartilage from within.

Functions of Bones

Bones perform several essential functions in the body:

• Support: Provide a framework for the body and support soft tissues.

• Protection: Encase and protect vital organs (e.g., skull protects the brain).

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

• Mineral Storage: Store minerals, especially calcium and phosphorus.

• Blood Cell Formation: Hematopoiesis occurs in red marrow cavities.

• Triglyceride Storage: Fat is stored in yellow marrow.

• Hormone Production: Osteocalcin is produced by bones to regulate bone formation and energy metabolism.

Classification of Bones by Shape

Bones are classified according to their shapes:

• 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).

• Sesamoid bones: Special type of short bone that forms within tendons (e.g., patella).

Classification of Bones: Axial vs. Appendicular Skeleton

• Axial skeleton: Forms the long axis of the body (skull, vertebral column, rib cage).

• Appendicular skeleton: Bones of the limbs and girdles (shoulder and hip bones).

Gross Anatomy of Compact and Spongy Bone

Bones consist of two types of osseous tissue:

• Compact bone: Dense outer layer; smooth and solid.

• Spongy bone (trabecular bone): Honeycomb of small needle-like or flat pieces called trabeculae. Spaces between trabeculae are filled with red or yellow marrow.

Structure of Short, Irregular, and Flat Bones

• Consist of thin plates of spongy bone (diploë) covered by compact bone.

• No shaft or epiphyses.

• Bone marrow is present between trabeculae but no marrow cavity.

Structure of a Long Bone

• Diaphysis: Shaft; compact bone surrounding medullary cavity.

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

• Epiphyseal line/plate: Remnant of growth plate; site of bone lengthening during childhood.

• Periosteum: Double-layered membrane covering the external surface except at joints.

• Endosteum: Delicate membrane lining internal bone surfaces.

• Medullary cavity: Central cavity containing marrow.

Periosteum vs. Endosteum

• Periosteum: Outer fibrous layer (dense irregular connective tissue) and inner osteogenic layer (osteoblasts and osteoclasts).

• Endosteum: Thin membrane lining internal bone surfaces; contains osteogenic cells.

Red vs. Yellow Bone Marrow

• Red marrow: Site of hematopoiesis; found in trabecular cavities of spongy bone in adults (e.g., hip bones, sternum).

• Yellow marrow: Fat storage; can convert to red marrow if necessary (e.g., severe anemia).

Bone Markings

Bone markings are projections, depressions, and openings that serve as sites of muscle, ligament, and tendon attachment, or as passages for nerves and blood vessels.

Microscopic Anatomy of Bones

• Bone Cells:

  • Osteogenic cells: Stem cells that differentiate into osteoblasts.

  • Osteoblasts: Bone-forming cells; secrete bone matrix.

  • Osteocytes: Mature bone cells; maintain bone matrix.

  • Bone lining cells: Flat cells on bone surfaces; help maintain matrix.

  • Osteoclasts: Multinucleated cells that resorb bone; have a ruffled border to increase surface area for enzyme release.

• Osteons (Haversian systems): Structural unit of compact bone; concentric lamellae surround a central canal.

• Lacunae: Small cavities containing osteocytes.

• Canals and Canaliculi:

  • Central (Haversian) canal: Contains blood vessels and nerves.

  • Perforating (Volkmann's) canals: Connect blood vessels and nerves of periosteum to central canals.

  • Canaliculi: Tiny canals connecting lacunae; allow communication and nutrient/waste exchange.

  • Nutrient foramen: Opening for blood vessels to enter bone.

• Lamellae:

  • Interstitial lamellae: Fill spaces between osteons.

  • Circumferential lamellae: Extend around the entire circumference of the diaphysis.

Chemical Composition of Bones

• Organic components: Cells and osteoid (collagen fibers and ground substance); provide flexibility and tensile strength.

• Inorganic components: Mineral salts (mainly hydroxyapatite, a form of calcium phosphate); provide hardness and resistance to compression.

Bone Development (Ossification)

• Endochondral ossification: Bone forms by replacing hyaline cartilage; forms most bones below the skull except clavicles.

• Intramembranous ossification: Bone develops from fibrous membranes; forms flat bones of the skull and clavicles.

Steps of Endochondral Ossification:

1. Bone collar forms around diaphysis of hyaline cartilage model.

2. Cartilage in the center calcifies and develops cavities.

3. Periosteal bud invades cavities; spongy bone forms.

4. Diaphysis elongates; medullary cavity forms.

5. Epiphyses ossify.

Steps of Intramembranous Ossification:

1. Ossification centers form within fibrous connective tissue membrane.

2. Osteoid is secreted and calcifies.

3. Woven bone and periosteum form.

4. Lamellar bone replaces woven bone; red marrow appears.

Postnatal Bone Growth

• Interstitial growth: Increases length of long bones at the epiphyseal plate.

• Appositional growth: Increases thickness (width) of bones.

Growth of Length in Long Bones

Occurs at the epiphyseal plate, which consists of five zones:

1. Resting (quiescent) zone

2. Proliferation (growth) zone

3. Hypertrophic zone

4. Calcification zone

5. Ossification (osteogenic) zone

As the epiphyseal plates get ready to fuse into an epiphyseal line, they thin. The plates of an 8-year-old are thicker than those of an 18-year-old. Bone lengthening ceases around age 18 in females and 21 in males.

Growth in Width (Thickness)

Appositional growth occurs throughout life as osteoblasts beneath the periosteum secrete bone matrix.

Hormonal Regulation of Bone Growth

• Growth hormone stimulates epiphyseal plate activity.

• Thyroid hormone modulates activity of growth hormone.

• Sex hormones (estrogen and testosterone) promote growth spurts and end growth by inducing epiphyseal plate closure.

Bone Remodeling (Deposit and Resorption)

• Bone deposit is carried out by osteoblasts.

• Bone resorption is carried out by osteoclasts.

Control of Remodeling

• Calcitonin: Released in response to high blood calcium; stimulates calcium deposit in bone.

• Parathyroid hormone (PTH): Released in response to low blood calcium; stimulates osteoclasts to resorb bone, releasing calcium into blood.

• Wolff's Law: Bones grow or remodel in response to the demands placed on them.

Fractures

• Definition: Breaks in bone.

• More common in youth (trauma) and old age (bone thinning).

• Classification:

  • Position of bone ends (nondisplaced/displaced)

  • Completeness of break (complete/incomplete)

  • Whether bone ends penetrate skin (open/compound or closed/simple)

Fracture Treatment and Repair

Involves reduction (realignment) and immobilization.

Stages in Healing of a Bone Fracture

1. Hematoma formation

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

Bone Disorders

• Osteomalacia: Bones are inadequately mineralized; soft and weak.

• Rickets: Osteomalacia in children; causes bone deformities.

• Osteoporosis: Bone resorption outpaces deposit; bones become fragile.

• Paget’s Disease: Excessive and haphazard bone deposit and resorption; high ratio of spongy to compact bone.

Example: The patella is a sesamoid bone formed within the tendon of the quadriceps muscle.

Additional info: The ruffled border of an osteoclast increases surface area for enzyme release during bone resorption. The central canal of an osteon contains blood vessels and nerves. Endochondral ossification forms most bones below the skull, while intramembranous ossification forms flat bones of the skull and clavicles.