Skeletal System Overview

Skeletal System Divisions

• Axial skeleton: Forms the central axis of the body. Includes the skull, vertebral column, and rib cage. Protects vital organs and provides support for the body.
• Appendicular skeleton: Includes the bones of the limbs and the girdles that attach them to the axial skeleton. Enables movement and manipulation of the environment.

Functions of Bone

• Support: Provides framework for the body and supports soft tissues.
• Protection: Protects vital organs like the brain, heart, and lungs.
• Movement: Provides attachment sites for muscles, allowing for movement.
• Mineral Storage: Serves as a reservoir for calcium and phosphorus, essential minerals for many bodily functions.

Compact vs Spongy Bone Structure

• Compact bone: Dense and solid, found in the shafts of long bones and the outer layer of other bones. Provides strength and support.
• Spongy bone: Porous and lightweight, found in the ends of long bones and within flat bones. Provides strength with less weight.

Long Bone Structure

• Diaphysis: The shaft of a long bone, providing the main support.
• Epiphysis: The ends of a long bone, responsible for growth and articulation with other bones.
• Metaphysis: The region between the diaphysis and epiphysis, where bone growth occurs.
• Trabeculae: Interconnecting bony spicules that form the framework of spongy bone.
• Periosteum: A tough, fibrous membrane that surrounds the outer surface of bone.
• Endosteum: Thin membrane lining the medullary cavity and trabeculae, contains osteogenic cells.
• Medullary cavity: The hollow space within the diaphysis, filled with bone marrow.
• Epiphyseal plate: Growth plate composed of hyaline cartilage, responsible for bone lengthening.
• Epiphyseal line: Remnant of the epiphyseal plate after growth has ceased.

Compact Bone Histology

• Central canal: A hollow channel running lengthwise through the center of osteons, containing blood vessels, nerves, and lymphatic vessels.
• Lacunae: Small spaces within the lamellae that house osteocytes.
• Canaliculi: Tiny channels that connect lacunae to each other and to the central canal, allowing for nutrient exchange and communication between osteocytes.
• Lamellae: Concentric rings of bone matrix that surround the central canal.
• Circumferential lamellae: Lamellae that run along the outer and inner surfaces of compact bone.
• Interstitial lamellae: Remnants of old osteons that have been partially resorbed.
• Concentric lamellae: Lamellae arranged in rings around the central canal.
• Perforating canals (Volkmann's canals): Channels that run perpendicular to the central canal, connecting the central canals of adjacent osteons, and providing access for blood vessels and nerves.
• Osteons (Haversian systems): Basic structural unit of compact bone, consisting of a central canal and its surrounding lamellae, lacunae, and canaliculi.

Bone Cell Types

• Osteogenic (osteoprogenitor) cell: Undifferentiated stem cells found in the periosteum and endosteum, capable of dividing into osteoblasts.
• Osteoblast: Bone-forming cells, responsible for synthesizing and secreting the organic components of bone matrix.
  • Derived from osteogenic cells.
  • Synthesize and secrete collagen and other proteins, forming the organic matrix of bone.
  • Release alkaline phosphatase, an enzyme that plays a crucial role in bone mineralization.
  • Alkaline phosphatase: An enzyme that promotes the deposition of calcium phosphate crystals onto the collagen fibers of the bone matrix, leading to mineralization.
• Osteocyte: Mature bone cells, found within lacunae, responsible for maintaining bone matrix and sensing mechanical stress.
  • Derived from osteoblasts.
  • Play a crucial role in bone remodeling, responding to mechanical stress and regulating bone deposition and resorption.
  • Interact with neighboring osteocytes through gap junctions within canaliculi, forming a network for communication and nutrient exchange.
• Osteoclast: Large, multinucleated cells that resorb (break down) bone tissue, responsible for bone remodeling.
  • Specialized phagocytic cells responsible for bone resorption.
  • Secrete lysosomal enzymes and acids that dissolve bone matrix, releasing calcium and other minerals.
  • Important for bone repair and remodeling.

Extracellular Matrix Components

• Organic component: Composed primarily of collagen fibers and ground substances (proteoglycans and glycoproteins). Provides tensile strength and flexibility.
• Inorganic component: Composed mainly of hydroxyapatite crystals (calcium phosphate), giving bone its hardness and rigidity.

Intramembranous vs Endochondral Ossification

• Intramembranous ossification: Bone formation directly within a mesenchymal membrane, forming flat bones such as the skull bones and clavicle.
• Endochondral ossification: Bone formation by replacing a hyaline cartilage model, forming most bones of the skeleton, including long bones.

Bone Growth

• Interstitial growth: Lengthening of bones, occurring at the epiphyseal plate (growth plate) of long bones.
  • Resting zone: Contains relatively inactive chondrocytes.
  • Proliferation zone: Chondrocytes undergo rapid mitosis, producing new cartilage.
  • Hypertrophic zone: Mature chondrocytes enlarge and become hypertrophic.
  • Calcification zone: The matrix surrounding hypertrophic chondrocytes calcifies, and chondrocytes die.
  • Ossification zone: Osteoblasts invade the calcified cartilage and begin forming bone.
• Appositional growth: Increase in bone width, occurring at the periosteal surface of bones.
  • Osteoblasts on the periosteal surface lay down new bone tissue, while osteoclasts on the endosteal surface resorb existing bone, leading to a net increase in bone thickness.

Hormone Regulation of Blood-Calcium Levels

• Parathyroid hormone (PTH): Produced by the parathyroid glands, increases blood calcium levels by stimulating osteoclast activity and bone resorption.
• Calcitonin: Produced by the thyroid gland, decreases blood calcium levels by inhibiting osteoclast activity and stimulating calcium uptake in bone.
• Calcitriol (vitamin D3): Produced by the kidneys, promotes calcium absorption from the intestines and increases blood calcium levels.
• Importance of calcium levels: Essential for muscle contraction, nerve impulse transmission, blood clotting, and bone health.
  • Hypocalcemia: Low blood calcium levels, can cause weakness, muscle spasms, and seizures.
  • Hypercalcemia: High blood calcium levels, can cause fatigue, constipation, and kidney stones.

Hormones Involved in Bone Growth

• Estrogen: Promotes epiphyseal plate closure, reducing bone growth, particularly in females.
• Testosterone: Promotes bone growth initially, but later promotes epiphyseal plate closure.
• Growth hormone: Stimulates both osteoblast and chondrocyte activity, promoting longitudinal bone growth.
• Calcitriol: Promotes calcium absorption and retention, supporting bone growth.
• Effect on osteoblast and osteoclast activity: Hormones regulate bone growth by influencing osteoblast and osteoclast activity.
  • Osteoblast activity: Promoted by growth hormone, testosterone, and calcitriol, leading to bone formation.
  • Osteoclast activity: Promoted by parathyroid hormone, inhibited by calcitonin, leading to bone resorption.

Fracture Types and Bone Repair

• Fracture: A break in a bone.
  • Compound fracture: Broken bone protrudes through the skin.
  • Simple fracture: Broken bone does not pierce the skin.
  • Complete fracture: Bone is broken all the way through.
  • Incomplete fracture: Bone is cracked, but not broken all the way through.
  • Displaced fracture: Bone ends are not aligned.
  • Nondisplaced fracture: Bone ends remain aligned.
• Bone remodeling and repair:
• Hematoma formation: Blood clot forms at the fracture site.
• Soft callus formation: Fibrocartilage forms a soft callus bridging the broken bone.
• Hard callus formation: Osteoblasts replace fibrocartilage with bony callus.
• Bone remodeling: The bony callus is remodeled to the original shape and size of the bone.

Skeletal System Divisions

• Axial Skeleton: Includes the skull, vertebral column, ribs, and sternum. Provides support and protection for vital organs
• Appendicular Skeleton: Includes the bones of the limbs, pectoral girdle, and pelvic girdle. Enables movement and locomotion.

Functions of Bones

• Support: Bones provide a framework for the body, holding organs and tissues in place.
• Protection: Bones shield vital organs from injury, such as the skull protecting the brain or the rib cage protecting the heart and lungs.
• Movement: Bones act as levers for muscles to pull on, allowing us to move.
• Mineral Storage: Bones store minerals like calcium and phosphorus, which can be released into the bloodstream when needed.

Compact vs Spongy Bone

• Compact Bone: Dense and solid, found in the outer layer of bones. Provides strength and rigidity. Contains osteons, which are structural units composed of concentric lamellae and a central canal.
• Spongy Bone: Lighter and less dense than compact bone, found in the interior of bones. Provides strength and flexibility. Contains trabeculae, which are thin plates of bone that interlace to form a network.

Long Bone Structure

• Diaphysis: Long, cylindrical shaft of a bone; composed primarily of compact bone.
• Epiphysis: Rounded ends of a bone; composed primarily of spongy bone.
• Metaphysis: Region between the diaphysis and epiphysis; contains the epiphyseal plate or line.
• Trabeculae: Lattice-like network of bony plates found in spongy bone, providing strength and reducing weight.
• Periosteum: Tough, fibrous membrane covering the outer surface of bone; contains osteoblasts and osteoclasts.
• Endosteum: Thin membrane lining the medullary cavity; contains osteoblasts, osteoclasts, and bone marrow.
• Medullary Cavity: Hollow space within the diaphysis; contains bone marrow.
• Epiphyseal Plate: Cartilaginous growth plate found in young growing bones; responsible for increasing bone length.
• Epiphyseal Line: Line of bone tissue that marks the former location of the epiphyseal plate in adults, indicating that bone growth has ceased.

Compact Bone Histology

• Central Canal: Hollow space found in the center of each osteon, containing blood vessels and nerves.
• Lacunae: Small cavities found within bony matrix, housing osteocytes.
• Canaliculi: Tiny canals that connect lacunae and the central canal, allowing for nutrient and waste exchange between osteocytes.
• Lamellae: Concentric rings of mineralized matrix found in osteons.
• Circumferential Lamellae: Lamellae located at the outer and inner surfaces of compact bone.
• Interstitial Lamellae: Remnants of old osteons, located between newer osteons.
• Concentric Lamellae: Layers of mineralized matrix that surround the central canal in osteons.
• Perforating Canals (Volkmann's Canals): Channels that run perpendicular to the central canals, connecting blood vessels and nerves from the periosteum to the central canals.
• Osteons (Haversian Systems): Structural units of compact bone, comprised of concentric lamellae, central canal, lacunae, canaliculi, and osteocytes.

Bone Cell Types

• Osteogenic (Osteoprogenitor) Cells: Undifferentiated stem cells found in the periosteum and endosteum; able to differentiate into osteoblasts.
• Osteoblasts: Bone-forming cells; responsible for depositing new bone matrix. Derived from osteogenic cells. Secrete osteoid (unmineralized bone matrix).
  • Alkaline Phosphatase: An enzyme secreted by osteoblasts that plays a key role in mineralization of the bone matrix.
• Osteocytes: Mature bone cells that maintain bone tissue; found within lacunae. Derived from osteoblasts. Act as sensors to detect stress and signals.
• Osteoclasts: Large, multinucleated cells responsible for bone resorption (breakdown). Derived from hematopoietic stem cells.
  • Bone Resorption: Osteoclasts secrete enzymes and acids that break down the mineralized bone matrix, releasing calcium and other minerals into the bloodstream.

Bone Extracellular Matrix

• Organic Components: Collagen fibers, proteoglycans, and glycoproteins.
• Inorganic Components: Calcium phosphate crystals (hydroxyapatite) and other minerals.
  • Collagen Fibers: Provide flexibility and tensile strength.
  • Hydroxyapatite: Provides rigidity and hardness.

Intramembranous Ossification

• Bone Formation Within a Membrane: Occurs directly within a mesenchymal membrane, without a cartilage precursor.
• Formation of Flat Bones: Example - Skull bones, clavicle

Endochondral Ossification

• Bone Formation Within a Cartilage Template: Occurs with a cartilage model as a precursor, replaced by bone.
• Formation of Long Bones: Example - Femur, humerus.

Bone Growth

• Interstitial Growth: Increase in bone length; occurs at the epiphyseal plate.
  • Zones of Epiphyseal Plate:
    • Resting Zone: Contains inactive chondrocytes.
    • Proliferation Zone: Chondrocytes divide rapidly and undergo mitosis.
    • Hypertrophic Zone: Mature chondrocytes enlarge and begin to deteriorate.
    • Calcification Zone: Matrix around chondrocytes calcifies, leaving behind spaces for blood vessels and osteoblasts.
    • Ossification Zone: Osteoblasts deposit new bone matrix on the calcified cartilage remnants.
• Appositional Growth: Increase in bone width, occurs along the outer surface of the bone. Osteoblasts deposit new bone matrix on the periosteal surface, while osteoclasts resorb bone on the endosteal surface, creating a wider medullary cavity.

Hormones Regulating Blood Calcium

• Parathyroid Hormone (PTH): Released from the parathyroid glands when blood calcium levels are low. Stimulates osteoclasts to resorb bone and release calcium into the blood, increasing calcium levels. Also acts on the kidneys to increase reabsorption of calcium from urine and increases production of calcitriol.
• Calcitonin: Released from the thyroid gland when blood calcium levels are high. Inhibits osteoclast activity, reducing bone resorption and lowering calcium levels.
• Calcitriol (Vitamin D3): Stimulates the absorption of calcium from the small intestine. Production is stimulated by PTH.
  • Hypocalcemia: Low blood calcium levels; can lead to muscle cramps, tetany, and weakened bones.
  • Hypercalcemia: High blood calcium levels; can lead to kidney stones and heart problems.

Hormones and Bone Growth

• Estrogen: Inhibits osteoclast activity, promotes osteoblast activity; responsible for the growth spurt during puberty and eventually stops bone growth.
• Testosterone: Inhibits osteoclast activity, promotes osteoblast activity; responsible for the growth spurt during puberty and eventually stops bone growth.
• Growth Hormone: Stimulates bone growth by stimulating cartilage proliferation and bone formation.
• Calcitriol: Promotes bone growth by increasing calcium absorption in the gut, leading to increased mineral deposition in bone matrix.

Fractures and Repair

• Fracture: A break in a bone.

  • Compound (Open) Fracture: Bone breaks through the skin.
  • Simple (Closed) Fracture: Bone does not break through the skin.
  • Complete Fracture: Bone breaks completely through.
  • Incomplete Fracture: Bone does not break completely through.
  • Displaced Fracture: Bone ends are out of alignment.
  • Non-displaced Fracture: Bone ends remain aligned.

• Bone Remodeling and Repair:

  • Hematoma Formation: Blood clots form at the fracture site.
  • Soft Callus Formation: Fibrocartilage forms at the fracture site, bridging the gap between bone ends.
  • Hard Callus Formation: New bony tissue replaces the fibrocartilage, forming a hard callus.
  • Remodeling: The hard callus is remodeled, and the fracture site is gradually replaced with normal bone tissue.

Explore the divisions of the skeletal system, including the axial and appendicular skeletons. Understand the essential functions of bones such as support, protection, and movement. Learn the differences between compact and spongy bone structures, and their roles in the body.