Orris - Capabilities, Information & Accuracy

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  new Paragraph({
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// ─── SECTION 1: Introduction ─────────────────────────────────────────────────

const sec1 = [
  heading1('1. Introduction to the Musculoskeletal System'),
  para('The musculoskeletal system is an organ system composed of bones, cartilage, joints, tendons, ligaments, and skeletal muscles. Together these structures provide the body with form, support, stability, and the capacity for movement. The musculoskeletal system accounts for approximately 75% of body weight and represents the single largest organ system in the human body.'),
  para('Clinically, disorders of the musculoskeletal system are among the most prevalent causes of disability worldwide. Conditions such as osteoporosis, rheumatoid arthritis, osteoarthritis, and muscular dystrophies impose significant burdens on patients and healthcare systems. Understanding the normal anatomy, histology, and physiology of these tissues is therefore a prerequisite for effective clinical practice.'),

  heading2('1.1 Embryological Origins'),
  para('The musculoskeletal system derives predominantly from mesoderm. Somites, which are segmented blocks of paraxial mesoderm, differentiate into sclerotome (giving rise to the vertebral column and ribs), myotome (skeletal muscles of the trunk), and dermatome (dermis of the back). Limb bones and muscles arise from lateral plate mesoderm and migrating myotomal cells respectively. Neural crest cells contribute to the craniofacial skeleton.'),
  para('During embryonic development, most long bones form via endochondral ossification - a process whereby a cartilage template (anlage) is progressively replaced by bone tissue. Flat bones of the skull, on the other hand, form through intramembranous ossification, in which mesenchymal stem cells differentiate directly into osteoblasts.'),

  heading2('1.2 Functional Roles'),
  bullet('Mechanical support and maintenance of posture'),
  bullet('Protection of internal organs (e.g., thoracic cage, skull)'),
  bullet('Force generation and locomotion via skeletal muscle contraction'),
  bullet('Mineral homeostasis - bone serves as the primary reservoir of calcium (99%) and phosphate (85%) in the body'),
  bullet('Hematopoiesis - the red marrow within cancellous bone produces all formed elements of blood'),
  bullet('Endocrine function - osteocalcin released by osteoblasts influences insulin secretion and energy metabolism'),
  pageBreak(),
];

// ─── SECTION 2: Bone ────────────────────────────────────────────────────────

const sec2 = [
  heading1('2. Bone'),
  para('Bone is a specialized connective tissue characterized by a mineralized extracellular matrix. It provides mechanical support for the body, transmits forces generated by muscles, protects viscera, provides a niche for blood cell progenitors, and is intimately involved in calcium and phosphate homeostasis.'),

  heading2('2.1 Bone Matrix Composition'),
  para('Bone matrix consists of an organic component called osteoid (35%) and an inorganic mineral component (65%).'),
  boldPara('Organic component (osteoid):', 'Predominantly type I collagen (approximately 90%), which provides tensile strength, with smaller amounts of glycosaminoglycans, osteocalcin, osteopontin, osteonectin, and bone sialoprotein.'),
  boldPara('Inorganic component:', 'Primarily hydroxyapatite [Ca10(PO4)6(OH)2], which is responsible for the compressive hardness of bone. The tightly integrated collagen-hydroxyapatite composite gives bone its remarkable combination of strength and resistance to fracture.'),
  para('Bone matrix is synthesized in two structural forms:'),
  boldPara('Woven bone:', 'Produced rapidly during fetal development and in fracture repair. It has a haphazard arrangement of collagen fibers that imparts less structural stability than lamellar bone. In adults, the presence of woven bone is always abnormal, indicating ongoing repair or pathological processes such as Paget disease or osteosarcoma.'),
  boldPara('Lamellar bone:', 'The mature, organized form found in adult skeletons. Collagen fibers are arranged in parallel lamellae, conferring superior mechanical properties. Cortical (compact) bone and cancellous (trabecular/spongy) bone are both composed of lamellar bone.'),

  heading2('2.2 Bone Cells'),
  para('Four distinct cell populations maintain bone homeostasis:'),
  boldPara('Osteoblasts:', 'Mononucleated cells located on bone surfaces that synthesize and secrete osteoid and regulate its mineralization. They are derived from mesenchymal stem cells located under the periosteum and in the medullary space. Osteoblasts that become embedded within the matrix they have secreted differentiate into osteocytes.'),
  boldPara('Osteocytes:', 'Terminally differentiated osteoblasts within lacunae in the mineralized matrix, interconnected by an intricate network of cytoplasmic processes running through canaliculi. They act as mechanosensors, responding to mechanical loading by regulating osteoblast and osteoclast activity via signaling molecules including sclerostin, FGF-23, and RANKL.'),
  boldPara('Osteoclasts:', 'Large, multinucleated cells (macrophage lineage, derived from circulating monocytes) responsible for bone resorption. They attach to bone via integrins, create a sealed resorption pit (Howship lacuna), and secrete hydrochloric acid and matrix metalloproteinases (MMPs) to dissolve mineral and digest organic matrix. Osteoclast activity is regulated by the RANK/RANKL/OPG axis: RANKL (on osteoblasts/stromal cells) activates RANK (on osteoclast precursors), while osteoprotegerin (OPG) acts as a decoy receptor to inhibit this pathway.'),
  boldPara('Bone lining cells:', 'Quiescent osteoblasts that cover inactive bone surfaces; they can be reactivated to resume bone formation.'),

  heading2('2.3 Gross Structure of Bone'),
  para('Macroscopically, bones are classified by shape: long bones (femur, humerus), short bones (carpals, tarsals), flat bones (sternum, skull), irregular bones (vertebrae), and sesamoid bones (patella). A long bone provides an instructive anatomical model:'),
  bullet('Diaphysis: The shaft, composed predominantly of compact cortical bone surrounding a medullary canal containing yellow (adipose) marrow in adults.'),
  bullet('Epiphyses: The expanded ends of long bones, covered by articular cartilage and composed mainly of trabecular bone containing red (hematopoietic) marrow.'),
  bullet('Metaphysis: The flared region between diaphysis and epiphysis; the site of the growth plate (physis) in skeletally immature individuals.'),
  bullet('Periosteum: A fibrous connective tissue sheath covering all outer bone surfaces except articular cartilage. It contains osteoprogenitor cells and is the site of appositional bone growth.'),
  bullet('Endosteum: A thin cellular layer lining the medullary cavity and trabecular surfaces, also containing osteoprogenitor cells.'),

  heading2('2.4 Bone Development'),
  heading3('2.4.1 Endochondral Ossification'),
  para('Long bones, the vertebral column, and the pelvis form via endochondral ossification. The process involves the following stages:'),
  bullet('Mesenchymal condensation and chondroblast differentiation - a hyaline cartilage anlage is established.'),
  bullet('At approximately 8 weeks of gestation, the central portion (diaphysis) of the cartilage template is vascularized and periosteal osteoblasts begin depositing bone collar.'),
  bullet('Chondrocytes in the center hypertrophy, calcify the surrounding matrix, then undergo apoptosis, creating the primary center of ossification.'),
  bullet('Osteoprogenitor cells invade with blood vessels, deposit osteoid on calcified cartilage remnants (primary spongiosa).'),
  bullet('Secondary centers of ossification develop in the epiphyses after birth (mostly).'),
  bullet('The growth plate (physis) persists between epiphysis and metaphysis until skeletal maturity, driving longitudinal bone growth by chondrocyte proliferation and hypertrophy.'),

  heading3('2.4.2 Intramembranous Ossification'),
  para('Flat bones (frontal, parietal, parts of occipital and temporal bones, mandible, clavicle) form directly from mesenchymal condensations without a cartilage intermediate. Mesenchymal stem cells differentiate directly into osteoblasts that deposit woven bone, which is subsequently remodeled to lamellar bone.'),

  heading2('2.5 Bone Remodeling'),
  para('Throughout life, bone undergoes continuous remodeling through sequential cycles of resorption (osteoclasts) and formation (osteoblasts). This process maintains mineral homeostasis, repairs microdamage, and adapts bone architecture to mechanical demands (Wolff\'s law). The basic multicellular unit (BMU) of remodeling consists of a leading front of osteoclasts followed by osteoblasts that refill the resorption cavity.'),

  makeTable(
    ['Phase', 'Duration', 'Cell Type', 'Process'],
    [
      ['Activation', '1-3 days', 'Osteoclast precursors', 'RANKL signaling recruits and fuses osteoclast precursors'],
      ['Resorption', '2-4 weeks', 'Osteoclasts', 'Acid + MMPs dissolve mineral and matrix'],
      ['Reversal', '~2 weeks', 'Mononuclear cells', 'Surfaces prepared for osteoblasts; coupling signals released'],
      ['Formation', '3-4 months', 'Osteoblasts', 'Osteoid synthesis and mineralization'],
      ['Quiescence', 'Years', 'Bone lining cells', 'Inactive surface maintenance'],
    ]
  ),
  pageBreak(),
];

// ─── SECTION 3: Cartilage ─────────────────────────────────────────────────

const sec3 = [
  heading1('3. Cartilage'),
  para('Cartilage is an avascular, aneural specialized connective tissue composed of chondrocytes embedded within a firm but resilient extracellular matrix rich in proteoglycans and collagen. Because it lacks blood vessels, cartilage receives nutrients by diffusion from synovial fluid or adjacent tissues. This avascularity accounts for its limited regenerative capacity after injury.'),

  heading2('3.1 Types of Cartilage'),

  heading3('3.1.1 Hyaline Cartilage'),
  para('The most abundant type, hyaline cartilage has a glass-like (Gr. hyalos = glass) pale blue appearance in histological sections. It contains type II collagen (not visible as distinct fibers by light microscopy), aggrecan proteoglycan, and water (60-80% by weight). Found at:'),
  bullet('Articular surfaces of synovial joints'),
  bullet('Costal cartilages of ribs'),
  bullet('Tracheal rings and bronchi'),
  bullet('Laryngeal cartilages (thyroid, cricoid, arytenoid)'),
  bullet('Epiphyseal growth plates'),

  heading3('3.1.2 Fibrocartilage'),
  para('Fibrocartilage contains abundant type I collagen bundles visible by light microscopy, making it the strongest cartilage type. It lacks a perichondrium. Found at sites requiring both tensile strength and compressive resistance:'),
  bullet('Intervertebral discs (anulus fibrosus)'),
  bullet('Pubic symphysis'),
  bullet('Menisci of the knee'),
  bullet('Temporomandibular joint disc'),
  bullet('Insertion zones of tendons and ligaments into bone (entheses)'),

  heading3('3.1.3 Elastic Cartilage'),
  para('Contains abundant elastic fibers in addition to type II collagen, giving it flexibility. Found in the external ear (auricle), epiglottis, and Eustachian tube. It stains distinctively with orcein or Weigert\'s elastin stain.'),

  heading2('3.2 Articular Cartilage'),
  para('Articular (hyaline) cartilage is a highly specialized tissue covering the ends of bones in synovial joints. It is organized into four zones:'),
  bullet('Zone I (Superficial/Tangential zone): Thin, dense layer. Collagen fibers are oriented parallel to the surface, resisting shear forces. Chondrocytes are flat and disc-shaped.'),
  bullet('Zone II (Transitional zone): Collagen fibers adopt an oblique orientation. Chondrocytes become more rounded.'),
  bullet('Zone III (Radial/Deep zone): Collagen fibers are perpendicular to the surface, resisting compressive loads. Chondrocytes are arranged in columns. Highest proteoglycan content.'),
  bullet('Zone IV (Calcified zone): Mineralized cartilage anchoring the tissue to the subchondral bone. The "tidemark" (a basophilic line visible by light microscopy) separates zones III and IV.'),
  pageBreak(),
];

// ─── SECTION 4: Joints ──────────────────────────────────────────────────────

const sec4 = [
  heading1('4. Joints (Articulations)'),
  para('Joints are the anatomical connections between two or more bones. They are classified by the type of tissue connecting the bones (structural classification) and by the range of motion they permit (functional classification).'),

  heading2('4.1 Structural Classification'),
  makeTable(
    ['Joint Type', 'Connecting Tissue', 'Mobility', 'Examples'],
    [
      ['Fibrous', 'Dense fibrous CT', 'Little or none', 'Skull sutures, distal tibiofibular joint'],
      ['Cartilaginous - Synchondrosis', 'Hyaline cartilage', 'Little', 'Epiphyseal plates, 1st sternocostal joint'],
      ['Cartilaginous - Symphysis', 'Fibrocartilage', 'Slight', 'Pubic symphysis, intervertebral discs'],
      ['Synovial', 'Synovial capsule + fluid', 'Free movement', 'Knee, hip, shoulder, elbow, wrist'],
    ]
  ),

  heading2('4.2 Synovial Joints'),
  para('Synovial joints (diarthroses) are the most mobile and structurally complex joints. Key structural components include:'),
  boldPara('Articular cartilage:', 'Hyaline cartilage covering the bone ends, reducing friction and distributing compressive loads.'),
  boldPara('Joint capsule:', 'A fibrous sleeve connecting the two bones. The outer fibrous layer provides mechanical strength; the inner synovial membrane (stratum synoviale) lines the non-cartilaginous intra-articular surfaces.'),
  boldPara('Synovial membrane:', 'A specialized connective tissue lining composed of two cell types - type A synoviocytes (macrophage lineage, phagocytic) and type B synoviocytes (fibroblast lineage, producing hyaluronan and lubricin). It is responsible for producing synovial fluid and removing waste products from the joint cavity.'),
  boldPara('Synovial fluid:', 'A filtrate of plasma supplemented with hyaluronic acid and lubricin. It provides nutrition to articular cartilage, lubricates joint surfaces, and reduces friction. Normal volume in the knee is 1-4 mL.'),
  boldPara('Ligaments:', 'Dense regular connective tissue bands that stabilize the joint and limit excessive movement. Composed primarily of type I collagen, with a smaller amount of elastin.'),
  boldPara('Intra-articular structures:', 'Some synovial joints contain additional structures such as menisci (knee), labra (hip, shoulder), and fat pads (knee, elbow) that improve congruence and distribute loads.'),

  heading2('4.3 Subtypes of Synovial Joints'),
  makeTable(
    ['Subtype', 'Movement', 'Example'],
    [
      ['Plane (gliding)', 'Gliding only', 'Intercarpal, intertarsal joints'],
      ['Hinge (ginglymus)', 'Flexion / Extension', 'Elbow (humeroulnar), ankle (talocrural)'],
      ['Pivot (trochoid)', 'Rotation only', 'Atlantoaxial (C1-C2), proximal radioulnar'],
      ['Condyloid (ellipsoid)', 'F/E + Ab/Add', 'Wrist (radiocarpal), MCP joints'],
      ['Saddle (sellar)', 'F/E + Ab/Add (biaxial)', '1st carpometacarpal (thumb CMC)'],
      ['Ball and socket (spheroidal)', 'Multi-axial', 'Hip (coxofemoral), glenohumeral'],
    ]
  ),
  pageBreak(),
];

// ─── SECTION 5: Skeletal Muscle ──────────────────────────────────────────────

const sec5 = [
  heading1('5. Skeletal Muscle'),
  para('Skeletal (striated) muscle produces quick, forceful contractions, usually under voluntary control via the somatic nervous system. It accounts for approximately 40% of total body weight in a healthy adult and is the primary effector of locomotion, posture, respiration, and facial expression.'),

  heading2('5.1 Histological Organization'),
  para('Skeletal muscle consists of muscle fibers - long, cylindrical, multinucleated cells with diameters of 10-100 micrometers and lengths up to several centimeters. Three concentric layers of connective tissue organize these fibers:'),
  boldPara('Epimysium:', 'A thick layer of dense connective tissue enclosing the entire muscle, continuous with fascia and the tendon.'),
  boldPara('Perimysium:', 'Connective tissue surrounding bundles (fascicles) of muscle fibers. Contains blood vessels and nerves supplying the fascicle.'),
  boldPara('Endomysium:', 'A delicate layer of reticular fibers (type III collagen) surrounding each individual muscle fiber. Contains capillaries and the nerve terminals of motor neurons.'),
  para('Nuclei in skeletal muscle fibers are peripherally located (subsarcolemmal), a feature that distinguishes skeletal from cardiac muscle (which has centrally placed nuclei).'),

  heading2('5.2 Ultrastructure: The Sarcomere'),
  para('The sarcomere is the functional and structural unit of skeletal muscle contraction, bounded by Z discs and measuring approximately 2.2 micrometers at resting length. Each sarcomere contains:'),
  boldPara('Thick filaments (A band):', 'Composed of approximately 200-500 myosin II molecules. Each myosin heavy chain has a rodlike tail (for filament packing) and a globular head (S1 fragment) that contains the actin-binding site and ATPase activity. The heads project outward to form crossbridges.'),
  boldPara('Thin filaments (I band and overlap zone):', 'Composed of F-actin (a double helix of G-actin monomers, each with a myosin-binding site), tropomyosin (coiled-coil dimers lying in the actin groove), and troponin (a complex of three subunits: TnT, TnI, and TnC).'),
  boldPara('Titin:', 'The largest protein in the body (~3700 kDa), spanning from Z disc to M line. Provides scaffolding and elastic recoil to the sarcomere.'),
  boldPara('Nebulin:', 'A giant protein associated with thin filaments, acting as a molecular ruler to specify actin filament length.'),
  boldPara('Z disc:', 'The boundary of the sarcomere, anchoring thin filament barbed ends via alpha-actinin.'),
  boldPara('M line:', 'The midline of the A band, containing myomesin and M-protein, anchoring the thick filament tails.'),

  heading2('5.3 Mechanism of Contraction: The Sliding Filament Theory'),
  para('The sliding filament theory (Huxley & Hanson, 1954; Huxley & Niedergerke, 1954) states that muscle contraction occurs without shortening of thick or thin filaments; instead, they slide past one another, bringing Z discs closer together and shortening the sarcomere.'),
  para('The crossbridge cycle proceeds as follows:'),
  bullet('1. In the resting state, tropomyosin sterically blocks myosin-binding sites on actin. Myosin heads are in an energized (cocked) conformation with ADP + Pi bound.'),
  bullet('2. Depolarization of the sarcolemma propagates along T-tubules, triggering Ca2+ release from the sarcoplasmic reticulum (SR) via ryanodine receptor 1 (RYR1).'),
  bullet('3. Ca2+ binds TnC of troponin, causing a conformational change that shifts tropomyosin away from actin-binding sites.'),
  bullet('4. Myosin heads bind actin (weak binding, then strong rigor binding).'),
  bullet('5. Power stroke: Pi release triggers rotation of the myosin head (~60 degrees), generating ~1-5 pN of force and sliding thin filaments toward the M line.'),
  bullet('6. ADP is released; a new ATP molecule binds the myosin head, causing dissociation from actin.'),
  bullet('7. ATP hydrolysis re-cocks the myosin head, ready for the next cycle.'),
  bullet('8. When Ca2+ is re-sequestered by SERCA (SR Ca2+-ATPase), TnI inhibits actin-myosin interaction and the muscle relaxes.'),

  heading2('5.4 Neuromuscular Junction'),
  para('The motor unit consists of a single alpha motor neuron and all the muscle fibers it innervates. The neuromuscular junction (NMJ) is the synapse between the motor neuron terminal and the motor end plate of the muscle fiber.'),
  para('Transmission at the NMJ: An action potential in the motor neuron triggers exocytosis of acetylcholine (ACh) into the synaptic cleft. ACh binds nicotinic ACh receptors (nAChRs, ligand-gated Na+/K+ channels) on the end plate, generating an end-plate potential that initiates the muscle action potential. Acetylcholinesterase in the cleft rapidly hydrolyzes ACh to terminate the signal.'),

  heading2('5.5 Fiber Types'),
  makeTable(
    ['Property', 'Type I (Slow-Twitch)', 'Type IIa (Fast Oxidative)', 'Type IIx/IIb (Fast Glycolytic)'],
    [
      ['Contraction speed', 'Slow', 'Fast', 'Very fast'],
      ['Fatigue resistance', 'High', 'Intermediate', 'Low'],
      ['Primary metabolism', 'Oxidative (aerobic)', 'Mixed oxidative/glycolytic', 'Glycolytic (anaerobic)'],
      ['Mitochondria', 'Abundant', 'Moderate', 'Sparse'],
      ['Myosin heavy chain isoform', 'MHC-I (beta)', 'MHC-IIa', 'MHC-IIx'],
      ['Color', 'Red (myoglobin-rich)', 'Red/Pink', 'White (pale)'],
      ['Example function', 'Postural muscles', 'Walking, moderate activity', 'Sprinting, power movements'],
    ]
  ),

  heading2('5.6 Muscle Regeneration'),
  para('Satellite cells are a population of resident muscle stem cells located beneath the basal lamina of muscle fibers. In mature resting muscle they are mitotically quiescent. Following muscle injury, satellite cells are activated, proliferate, and either fuse with damaged fibers for repair or fuse with each other to form new fibers. The regenerative capacity of skeletal muscle declines with aging due to reduced satellite cell number and responsiveness to activating signals.'),
  pageBreak(),
];

// ─── SECTION 6: Connective Tissues (Tendons, Ligaments, Fascia) ─────────────

const sec6 = [
  heading1('6. Tendons, Ligaments, and Fascia'),

  heading2('6.1 Tendons'),
  para('Tendons are dense regular connective tissue structures that transmit forces generated by muscles to bones. They are composed primarily of type I collagen fibers (approximately 70% of dry weight) arranged in parallel fascicles along the long axis of the tendon, providing exceptional tensile strength in one direction. Fibroblasts (tenocytes) are the primary cellular component, located in rows between collagen bundles.'),
  para('The tendon is organized hierarchically: collagen molecules -> fibrils -> fibers -> primary bundles (fascicles) -> secondary and tertiary bundles -> whole tendon, with each level enclosed by a connective tissue sheath (endotenon, peritenon, epitenon).'),
  para('Blood supply to tendons is sparse, delivered via the peritenon and intrinsic vessels. This avascularity, combined with the dense collagen packing, explains the relatively slow healing of tendon injuries.'),

  heading2('6.2 Ligaments'),
  para('Ligaments connect bone to bone and provide passive stability to joints. Like tendons, they consist primarily of type I collagen, but the fiber arrangement is less strictly parallel (somewhat wavy/crimp pattern) to accommodate multi-directional loading. They also contain a higher proportion of elastin than tendons, allowing some elastic deformation under load.'),
  para('Functionally, ligaments resist joint distraction, limit excessive range of motion, guide joint kinematics, and contain mechanoreceptors (Ruffini endings, Golgi tendon organ-like receptors, Pacinian corpuscles) that contribute to proprioception.'),

  heading2('6.3 Fascia'),
  para('Fascia is a continuous network of fibrous connective tissue enveloping and separating muscles, organs, nerves, and vessels throughout the body. Functionally it transmits mechanical forces, maintains positional relationships of structures, and forms compartments that limit spread of infection.'),
  para('Deep fascia (investing fascia) forms thick, strong sheaths around muscles and muscle groups. Compartment syndrome occurs when elevated pressure within a fascial compartment compromises perfusion, requiring emergency fasciotomy.'),
  pageBreak(),
];

// ─── SECTION 7: Clinical Disorders ───────────────────────────────────────────

const sec7 = [
  heading1('7. Clinical Disorders of the Musculoskeletal System'),

  heading2('7.1 Osteoporosis'),
  para('Osteoporosis is defined radiographically as bone mass 2.5 or more standard deviations below mean peak bone mass (T-score <= -2.5 by DXA). Osteopenia is 1 to 2.5 SD below the mean (T-score -1.0 to -2.5). It is characterized by reduced bone mass and disruption of bone microarchitecture, leading to increased fragility and fracture risk.'),
  para('Pathogenesis: Peak bone mass is achieved in early adulthood. Thereafter, bone turnover leads to a net loss of approximately 0.7% per year. In postmenopausal women, estrogen deficiency dramatically upregulates RANKL expression, leading to enhanced osteoclast activity and high-turnover bone loss (2-3% per year for the first 5-10 years after menopause). Senile osteoporosis (affecting both sexes, >70 years) reflects declining osteoblast function (low-turnover).'),
  boldPara('Risk factors:', 'Age, female sex, estrogen deficiency, low calcium and vitamin D intake, physical inactivity, glucocorticoid use, smoking, excess alcohol, low body weight, family history of fragility fracture.'),
  boldPara('Common fracture sites:', 'Vertebral bodies (most common; often asymptomatic), distal radius (Colles fracture), proximal femur (hip fracture - highest morbidity/mortality).'),
  boldPara('Treatment:', 'Bisphosphonates (alendronate, zoledronate) - inhibit osteoclast activity; Denosumab (anti-RANKL monoclonal antibody); Teriparatide/Abaloparatide (PTH analogues, anabolic); Romosozumab (anti-sclerostin, dual anabolic/anti-resorptive); calcium + vitamin D supplementation.'),

  heading2('7.2 Osteoarthritis'),
  para('Osteoarthritis (OA) is the most prevalent joint disease, characterized by progressive degradation of articular cartilage, subchondral bone sclerosis, osteophyte formation, and synovial inflammation. It is the leading cause of pain and disability in adults over 65.'),
  boldPara('Pathogenesis:', 'Mechanical overloading and aging lead to chondrocyte senescence and increased production of catabolic enzymes (MMPs, ADAMTS aggrecanases), inflammatory cytokines (IL-1beta, TNF-alpha), and reactive oxygen species. This shifts the balance toward matrix degradation. Loss of articular cartilage narrows the joint space and increases stress on subchondral bone, leading to sclerosis and cyst formation.'),
  boldPara('Clinical features:', 'Insidious onset; joint pain worsening with activity and relieved by rest (early); morning stiffness <30 minutes; crepitus; bony enlargements (Heberden nodes at DIP joints, Bouchard nodes at PIP joints in hand OA); reduced range of motion.'),
  boldPara('Radiographic features (Kellgren-Lawrence scale):', 'Joint space narrowing, subchondral sclerosis, osteophytes, subchondral cysts. MRI is more sensitive for early cartilage loss.'),

  heading2('7.3 Rheumatoid Arthritis'),
  para('Rheumatoid arthritis (RA) is a chronic, systemic autoimmune disease characterized by symmetric inflammatory polyarthritis, primarily affecting small joints of the hands and feet, with extra-articular manifestations in multiple organ systems.'),
  boldPara('Pathogenesis:', 'Autoimmune-mediated synovitis: CD4+ T cells (Th1 and Th17) activate macrophages and B cells in the synovium. Autoantibodies including rheumatoid factor (RF, anti-IgG Fc) and anti-citrullinated protein antibodies (ACPA/anti-CCP) contribute to inflammation and joint destruction. TNF-alpha, IL-1, and IL-6 are key pathogenic cytokines. The synovium transforms into an invasive pannus, eroding cartilage and bone.'),
  boldPara('Diagnosis:', 'ACR/EULAR 2010 criteria: joint involvement pattern, serology (RF, anti-CCP), acute phase reactants (CRP, ESR), symptom duration (>=6 weeks). Score >=6/10 = definite RA.'),
  boldPara('Treatment:', 'DMARDs: Methotrexate (anchor drug); Biologics: TNF inhibitors (etanercept, adalimumab, infliximab), IL-6 inhibitors (tocilizumab), JAK inhibitors (tofacitinib, baricitinib, upadacitinib), CTLA4-Ig (abatacept), anti-CD20 (rituximab). NSAIDs and glucocorticoids for symptom control.'),

  heading2('7.4 Fracture Healing'),
  para('Bone has a unique capacity for complete regeneration (as opposed to scar repair) following fracture. Healing proceeds through overlapping phases:'),
  bullet('Phase 1 - Hematoma formation (hours to days): Disruption of periosteal and medullary vessels creates a fracture hematoma. Coagulation and platelet degranulation release growth factors (PDGF, TGF-beta, VEGF).'),
  bullet('Phase 2 - Soft callus formation (days to weeks): Fibroblasts and chondrocytes invade the hematoma to form fibrocartilaginous soft callus. This provides mechanical stability while vascularization proceeds.'),
  bullet('Phase 3 - Hard (bony) callus formation (weeks to months): Enchondral ossification of the soft callus produces woven bone hard callus, restoring mechanical continuity.'),
  bullet('Phase 4 - Remodeling (months to years): Osteoclasts and osteoblasts remodel the woven bone callus into lamellar bone, restoring original cortical architecture and medullary canal.'),

  heading2('7.5 Selected Muscle Disorders'),
  boldPara('Duchenne muscular dystrophy (DMD):', 'X-linked recessive disorder caused by mutations in the DMD gene encoding dystrophin. Dystrophin connects intracellular actin to the extracellular matrix via the dystrophin-glycoprotein complex, protecting the sarcolemma from contraction-induced damage. Loss leads to progressive muscle fiber necrosis, fibrosis, and weakness. Serum CK markedly elevated.'),
  boldPara('Myasthenia gravis:', 'Autoimmune disorder in which autoantibodies (most commonly anti-AChR IgG; less commonly anti-MuSK) block or destroy nicotinic acetylcholine receptors at the NMJ. Presents with fatigable weakness, ptosis, diplopia, and potentially respiratory failure (myasthenic crisis). Tensilon (edrophonium) test, repetitive nerve stimulation studies, and AChR antibody titers aid diagnosis.'),
  boldPara('Rhabdomyolysis:', 'Acute necrosis of skeletal muscle releasing myoglobin, CK, potassium, and phosphate into the circulation. Causes include crush injury, intense exertion, ischemia, hyperthermia, drugs (statins, cocaine), and genetic myopathies. Myoglobin is nephrotoxic; acute kidney injury is the principal complication. Treatment: aggressive IV fluid resuscitation to maintain urine output >200 mL/h.'),
  pageBreak(),
];

// ─── SECTION 8: Summary Table ─────────────────────────────────────────────

const sec8 = [
  heading1('8. Summary: Key Musculoskeletal Structures'),
  makeTable(
    ['Structure', 'Primary Composition', 'Primary Function', 'Key Cells'],
    [
      ['Cortical bone', 'Lamellar bone, hydroxyapatite, type I collagen', 'Mechanical support, protection', 'Osteoblasts, osteocytes, osteoclasts'],
      ['Cancellous bone', 'Trabecular lattice, red marrow', 'Hematopoiesis, mineral reservoir', 'Osteoblasts, osteocytes, osteoclasts, hematopoietic stem cells'],
      ['Hyaline cartilage', 'Type II collagen, aggrecan, water', 'Load distribution, reduced friction at joints', 'Chondrocytes'],
      ['Fibrocartilage', 'Type I + II collagen', 'Tensile + compressive resistance', 'Fibro-chondrocytes'],
      ['Synovial membrane', 'Loose CT, type A & B synoviocytes', 'Synovial fluid production, phagocytosis', 'Type A (macrophagic), Type B (fibroblastic) synoviocytes'],
      ['Skeletal muscle fiber', 'Actin, myosin, titin, nebulin', 'Force generation, locomotion', 'Muscle fibers (myocytes), satellite cells'],
      ['Tendon', 'Type I collagen (parallel bundles)', 'Force transmission (muscle to bone)', 'Tenocytes (fibroblasts)'],
      ['Ligament', 'Type I collagen + elastin', 'Joint stability, proprioception', 'Fibroblasts, mechanoreceptors'],
    ]
  ),
  new Paragraph({ spacing: { after: 300 }, children: [] }),
  heading1('9. References'),
  para('1. Mescher AL. Junqueira\'s Basic Histology: Text and Atlas, 17th ed. McGraw Hill; 2024.'),
  para('2. Kumar V, Abbas AK, Aster JC. Robbins & Kumar Basic Pathology, 10th ed. Elsevier; 2023.'),
  para('3. Boron WF, Boulpaep EL. Medical Physiology, 3rd ed. Elsevier; 2017.'),
  para('4. Costanzo LS. Physiology, 7th ed. Elsevier; 2022.'),
  para('5. Standring S (ed). Gray\'s Anatomy, 42nd ed. Elsevier; 2021.'),
  para('6. Longo DL et al. Harrison\'s Principles of Internal Medicine, 22nd ed. McGraw Hill; 2025.'),
];

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