Human Anatomy & Physiology - Bones and Skeletal Tissues

Bones and Skeletal Tissues

• Major components of the skeletal system are crucial for support, movement, and protection of vital organs.

Skeletal Cartilage

• Definition: Made of specialized cartilage tissue, primarily composed of water, devoid of blood vessels and nerves.

• Perichondrium: A dense irregular connective tissue that surrounds cartilage, resisting outward expansion.

• Types of Cartilage:

  • Hyaline Cartilage

  • Features:

    • Provides support, flexibility, and resilience.

    • Most abundant type of skeletal cartilage.

  • Locations:

    • Articular: Covers ends of long bones.

    • Costal: Connects ribs to the sternum.

    • Respiratory: Forms larynx and reinforces air passages.

    • Nasal: Supports the nose.

  • Elastic Cartilage

  • Similar to hyaline but contains elastic fibers.

  • Locations:

    • External ear.

    • Epiglottis.

  • Fibrocartilage

  • Highly compressed with great tensile strength; contains collagen fibers.

  • Locations:

    • Menisci of the knee.

    • Intervertebral discs.

Growth of Cartilage

• **Types of Growth: **

  • Appositional Growth: Cells in the perichondrium secrete a matrix against the external face of existing cartilage.

  • Interstitial Growth: Chondrocytes inside lacunae divide and secrete new matrix, expanding cartilage from within.

• Calcification: Occurs during normal bone growth and in old age.

Classification of Bones

• Types of Skeleton:

  • Axial Skeleton: Bones of the skull, vertebral column, and rib cage.

  • Appendicular Skeleton: Bones of upper/lower limbs, shoulder, and hip.

Classification by Shape

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

• Short Bones: Cube-shaped (e.g., wrist, ankle) and include bones that form within tendons (e.g., patella).

• Flat Bones: Thin, flattened, often curved (e.g., sternum, most skull bones).

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

Function of Bones

• Support: Form framework that supports body and cradles soft organs.

• Protection: Provide protective case for the brain, spinal cord, and vital organs.

• Movement: Serve as levers for muscle action.

• Mineral Storage: Reservoir for minerals (calcium, phosphorus).

• Blood Cell Formation: Hematopoiesis occurs in marrow cavities of bones.

Gross Anatomy of Bones: Bone Markings

• Bone Markings: Bulges, depressions, and holes that serve various functions:

  • Sites for muscle, ligaments, and tendon attachment.

  • Joint surfaces.

  • Conduits for blood vessels and nerves.

Bone Markings (Detailed)

• Projection Types (for Muscle & Ligament Attachment):

  • Tuberosity: Large rounded projection; may be roughened.

  • Crest: Narrow ridge of bone, usually prominent.

  • Trochanter: Very large, blunt, irregularly shaped process (e.g., femur).

  • Line: Narrow ridge of bone, less prominent than a crest.

  • Tubercle: Small rounded projection or process.

• Joint Formation:

  • Head: Bony expansion carried on a narrow neck.

  • Facet: Smooth, nearly flat articular surface.

  • Condyle: Rounded articular projection.

  • Ramus: Armlike bar of bone.

• Depressions & Openings:

  • Meatus: Canal-like passageway.

  • Sinus: Cavity within a bone, lined with mucous membrane.

  • Fossa: Shallow depression, often serving as an articular surface.

  • Groove: Furrow (narrow slit-like opening).

  • Foramen: Round or oval opening through a bone.

Bone Textures

• Compact Bone: Dense outer layer of bones.

• Spongy Bone: Honeycomb structure of trabeculae filled with yellow bone marrow.

Structure of Long Bone

• Diaphysis:

  • Tubular shaft forming the axis of long bones.

  • Composed of compact bone surrounding the medullary cavity which contains yellow bone marrow (fat).

• Epiphysis:

  • Expanded ends of long bones.

  • Comprised of compact bone exterior and spongy interior.

  • Joint surface covered with articular (hyaline) cartilage.

  • Epiphyseal line separates diaphysis from epiphyses.

• Bone Membranes:

  • Periosteum: Double-layered protective membrane.

  • Outer fibrous layer made of dense regular connective tissue.

  • Inner osteogenic layer consisting of osteoblasts and osteoclasts.

  • Rich in nerve fibers, blood, and lymphatic vessels; secured to underlying bone via Sharpey's fibers.

  • Endosteum: Covers internal surfaces of bone.

Structure of Short, Irregular, and Flat Bones

• Comprised of thin plates of compact bone (periosteum-covered) on the outside, with endosteum-covered spongy bone (diploë) on the inside.

• No diaphysis or epiphyses.

• Contain bone marrow between trabeculae.

Hematopoietic Tissue Location

• In Infants:

  • Located in the medullary cavity and all areas of spongy bone.

• In Adults:

  • Found in diploë of flat bones and head of femur and humerus.

Microscopic Structure of Bone: Compact Bone

• Haversian System (Osteon): Structural unit of compact bone.

• Lamella: Column-like matrix tubes composed mainly of collagen for weight-bearing.

• Haversian Canal: Central channel containing blood vessels and nerves.

• Volkmann’s Canals: Channels lying at right angles to the central canal, connecting blood and nerve supply of periosteum to the Haversian canal.

• Osteocytes: Mature bone cells that reside in lacunae.

• Lacunae: Small cavities in bone containing osteocytes.

• Canaliculi: Hairlike canals connecting lacunae to each other and the central canal.

Chemical Composition of Bone

• Organic Components:

  • Osteoblasts: Bone-forming cells.

  • Osteocytes: Mature bone cells.

  • Osteoclasts: Large cells that resorb or break down bone matrix.

  • Osteoid: Unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen.

• Inorganic Components:

  • Hydroxyapatites (Mineral Salts): 65% of bone by mass, primarily calcium phosphates, responsible for bone hardness and resistance to compression.

Bone Development

• Osteogenesis and Ossification: Process of bone tissue formation that leads to:

  • Formation of the bony skeleton in embryos.

  • Bone growth until early adulthood.

  • Bone thickness, remodeling, and repair.

Formation of the Bony Skeleton

• Begins at week 8 of embryonic development.

• Intramembranous Ossification: Bone develops from a fibrous membrane.

• Endochondral Ossification: Bone forms by replacing hyaline cartilage.

Postnatal Bone Growth

• Growth in length of long bones involves the activity of the epiphyseal plate:

  • Cartilage adjacent to the epiphyseal plate remains relatively inactive.

  • Cartilage near the bone shaft organizes for efficient growth.

• Functional Zones in Long Bone Growth:

  • Growth Zone: Cartilage cells undergo mitosis, increasing the distance between the diaphysis and epiphysis.

  • Transformation Zone: Older cells enlarge, the matrix calcifies, cartilage cells die, and matrix deteriorates.

  • Osteogenic Zone: New bone formation occurs.

Long Bone Growth and Remodeling

• Continual growth in length through the replacement of cartilage with bone.

• Remodeling involves the resorption and addition of bone through appositional growth.

Hormonal Regulation of Bone Growth During Youth

• Growth Hormone: Stimulates epiphyseal plate activity during infancy and childhood.

• Puberty Hormones: Testosterone and estrogens cause:

  • Initial promotion of adolescent growth spurts.

  • Masculinization and feminization of specific skeletal regions.

  • Induction of epiphyseal plate closure, thus ending longitudinal growth.

Bone Remodeling

• Remodeling Units: Adjacent osteoblasts and osteoclasts depositing and resorbing bone at periosteal and endosteal surfaces.

• Bone Deposition: Occurs in response to injury or in areas needing additional strength.

  • Requires a diet rich in protein, calcium, vitamins C, D, A, phosphorus, magnesium, and manganese.

  • Alkaline phosphatase is critical for the mineralization process.

  • Osteoid Seam: Unmineralized band of bone matrix; Calcification Front: Transition zone between unmineralized and mineralized bone.

Bone Resorption

• Accomplished by osteoclasts in resorption bays (grooves formed by osteoclast activity).

• Involves osteoclast secretion of:

  • Lysosomal enzymes digesting organic matrix.

  • Acids converting calcium salts to soluble forms.

• Dissolved matrix is transcytosed across osteoclasts into interstitial fluid and blood.

Importance of Ionic Calcium in the Body

• Calcium's Role: Essential for:

  • Nerve impulse transmission.

  • Muscle contraction.

  • Blood coagulation.

  • Gland and nerve cell secretion.

  • Cell division.

Control of Remodeling

• Control Loops: Two primary mechanisms regulate bone remodeling:

  • Hormonal mechanism for maintaining calcium homeostasis in the blood.

  • Mechanical and gravitational forces acting on the skeleton, guiding remodeling processes.

Hormonal Mechanism

• Rising blood Ca2+ levels stimulate thyroid to release calcitonin.

  • Calcitonin promotes calcium salt deposition in bone.

• Falling blood Ca2+ levels trigger parathyroid glands to release PTH (Parathyroid Hormone).

  • PTH signals osteoclasts to degrade bone matrix and release Ca2+ into blood.

Response to Mechanical Stress

• Wolff’s Law: A bone grows or remodels according to the forces or demands placed on it.

  • Evidence:

  • Long bones tend to be thickest midway along the shaft (where bending stress is greatest).

  • Curved bones are thickest at points most likely to buckle.

• Trabeculae form along lines of stress; large bony projections occur where heavy active muscles attach.

Bone Fractures (Breaks)

• Classification: Fractures categorized by:

  • Position of bone ends post-fracture.

  • Completeness of the break.

  • Orientation to the long axis of the bone.

  • Whether ends penetrate the skin.

Types of Bone Fractures

• Nondisplaced: Bone ends retain normal position.

• Displaced: Bone ends are misaligned.

• Complete: Bone is broken through entirely.

• Incomplete: Bone is not fully broken.

• Linear: Fracture is parallel to bone's long axis.

• Transverse: Fracture is perpendicular to long axis of bone.

• Compound (Open): Bone ends penetrate skin.

• Simple (Closed): Bone ends do not penetrate skin.

Common Types of Fractures

• Comminuted: Bone fragments into three or more pieces; common in elderly.

• Spiral: Ragged break from excessive twisting; common sports injury.

• Depressed: Broken bone part pressed inward; typical skull fracture.

• Compression: Bone crushed; common in porous bones.

• Epiphyseal: Separation along epiphyseal line; occurs where cartilage cells are dying.

• Greenstick: Incomplete fracture; one side breaks while the other bends; common in children.

Stages in the Healing of a Bone Fracture

1. Hematoma Formation: Torn blood vessels hemorrhage, leading to clotted blood mass at fracture site; results in swelling and pain.

2. Fibrocartilaginous Callus Formation: Granulation tissue (soft callus) forms; capillaries grow into tissue while phagocytic cells clean debris.

   • Osteoblasts and fibroblasts migrate to the fracture to reconstruct the bone.

3. Bony Callus Formation: New bone trabeculae appear; the fibrocartilaginous callus converts to a hard callus, beginning around 3-4 weeks post-injury.

4. Bone Remodeling: Excess material on bone shaft exterior and within the medullary canal is removed; compact bone is laid down to reconstruct shaft walls.

Homeostatic Imbalances

• Osteomalacia: Inadequately mineralized bones cause softness and weakness; main symptom is pain during weight-bearing.

  • Causes: Insufficient calcium or vitamin D deficiency.

• Rickets: Inadequately mineralized bones in children; leads to bowed legs and deformities.

  • Causes: Insufficient calcium or vitamin D.

• Osteoporosis: Diseases where bone resorption outpaces deposit, leading to fragility, particularly in postmenopausal women.

• Paget’s Disease: Excessive bone formation and breakdown; characterized by local areas of excessive woven bone leading to weakness; unknown cause (possibly viral).