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The Skeletal System: Bone Tissue (Chapter 6)

Bone as an Organ and the Skeletal Framework

  • Bone = an organ composed of multiple tissues
    • Osseous tissue (compact & spongy)
    • Cartilage
    • Dense connective tissue (periosteum, ligaments)
    • Adipose tissue (yellow marrow)
    • Nervous tissue (sensory fibers accompanying blood vessels)
  • Skeleton = bones + cartilages ➜ complete supportive framework
  • Division of skeleton
    • Axial (skull, vertebral column, ribs, sternum, hyoid)
    • Appendicular (upper & lower limbs, pectoral & pelvic girdles)

Functions of the Skeletal System

  • Support: structural scaffolding for body
  • Protection: encases vital organs (brain, heart, lungs, spinal cord, reproductive organs)
  • Movement assistance: levers for muscle contraction
  • Mineral homeostasis
    • Major reservoir for Ca and P
    • Releases minerals on demand
  • Hemopoiesis: red marrow forms RBCs, WBCs, platelets
  • Triglyceride storage: yellow marrow (medullary cavities) stores energy

Classification of Bones

  • Long: length > width; mostly compact bone
    • Humerus, radius, ulna, femur, tibia, fibula, metacarpals/metatarsals, phalanges
  • Short: nearly equal dimensions; mostly spongy bone
    • Carpals, tarsals
  • Flat: thin, two parallel plates of compact bone enclosing spongy
    • Cranial bones, sternum, ribs, scapulae
  • Irregular: complex shapes
    • Vertebrae, many facial bones
  • Sesamoid: seed-like, develop in tendons under friction/ stress
    • Patellae (largest), variable small sesamoids in hands/feet

Structure of a Typical Long Bone

  • Diaphysis: shaft; thick compact bone surrounding medullary cavity
  • Epiphyses (2): proximal & distal ends; mostly spongy bone with red marrow; articular cartilage (hyaline) covers joint surfaces
  • Metaphyses (2): transition regions; in growing bone house epiphyseal (growth) plate ➜ becomes epiphyseal line after fusion
  • Periosteum: double-layered membrane around diaphysis
    • Outer fibrous dense irregular CT ➜ tendon/ligament anchor via Sharpey’s fibers
    • Inner osteogenic layer ➜ osteoprogenitors/osteoblasts for growth & repair
  • Medullary cavity: hollow space; yellow marrow in adults
  • Endosteum: thin membrane lining cavity & trabeculae; contains bone-forming cells

Histology of Bone

  • Extracellular matrix (ECM)
    • \approx 15\% water
    • 30\% collagen fibers ➜ flexibility & tensile strength
    • 55\% crystallized mineral salts (hydroxyapatite = Ca3(PO4)2 · Ca(OH)2) ➜ hardness
  • Why “inorganic”? ➜ mineral crystals lack carbon; responsible for rigidity

Cellular components

  • Osteoprogenitor cells: mitotic stem cells in periosteum/endosteum; derive from mesenchyme
  • Osteoblasts (“blast = build”): synthesize & secrete bone matrix (collagen, organic components); initiate calcification
  • Osteocytes: mature cells trapped in lacunae; maintain metabolism, communicate via canaliculi (“cytes maintain the site”)
  • Osteoclasts (“clast = crack”): large multinucleated cells from WBC lineage; ruffled border secretes acids & lysosomal enzymes for resorption

Tissue Types

  • Compact (cortical) bone
    • Strong, dense outer layer of all bones
    • Predominates diaphyses of long bones
    • Resists stresses of weight & movement
  • Spongy (cancellous, trabecular) bone
    • Lattice of trabeculae; spaces filled with red marrow
    • Forms interiors of short, flat, irregular, sesamoid bones and epiphyses of long bones
    • Lighter weight ➜ reduces overall mass; supports hemopoiesis

Compact Bone Micro-architecture

  • Osteon (Haversian system) = repeating unit aligned along stress lines
    • Central (Haversian) canal: blood/lymph vessels, nerves
    • Concentric lamellae around canal
    • Lacunae between lamellae house osteocytes
    • Canaliculi interconnect lacunae & central canal for nutrient/waste diffusion
  • Interosteonic (Volkmann’s) canals: transverse channels connecting periosteum, medullary cavity, and central canals
  • Circumferential lamellae: external & internal layers encircling entire bone
  • Interstitial lamellae: remnants of old osteons between new ones

Spongy Bone Micro-architecture

  • Trabeculae arranged along lines of stress, containing concentric lamellae, lacunae, canaliculi
  • No osteons; nutrients diffuse from vessels in marrow spaces
  • Osteoblasts & osteoclasts line trabecular surfaces under endosteum
  • Marrow spaces ➜ red marrow in adults (site of hemopoiesis)

Vascular & Neural Supply

  • Bone = highly vascular & innervated
    • Periosteal arteries/veins: enter/exit diaphysis via perforating (Volkmann’s) canals
    • Nutrient artery/vein: large vessel through nutrient foramen into medullary cavity; branches proximally & distally
    • Metaphyseal & epiphyseal vessels: supply respective regions
  • Sensory nerves accompany vessels; periosteum richly innervated ➜ pain with damage

Ossification (Bone Formation)

  • Occurs in four situations
    1. Embryonic/fetal development
    2. Childhood/adolescent growth
    3. Remodeling throughout life
    4. Fracture repair

Two embryonic mechanisms

  1. Intramembranous ossification
    • Bone develops directly within mesenchyme sheets ➜ flat bones of skull, mandible, clavicle
  2. Endochondral ossification
    • Hyaline cartilage model first, replaced by bone
    • Forms most of skeleton; continues at epiphyseal plates for length growth (interstitial)

Growth in Length (Interstitial)

  • Epiphyseal plate zones (from epiphysis → diaphysis)
    1. Resting cartilage
    2. Proliferating cartilage
    3. Hypertrophic cartilage
    4. Calcified cartilage (osteoclasts dissolve; osteoblasts lay bone)
  • Balance of cartilage growth on epiphyseal side & replacement on diaphyseal side ➜ elongation
  • Closure ➜ epiphyseal line; length growth ceases \approx ages 18–21

Growth in Diameter (Appositional)

  • Periosteal osteoblasts secrete new circumferential lamellae ➜ external growth
  • Endosteal osteoclasts enlarge medullary cavity maintaining cortical thickness
  • Cooperative cycle continues through life (thickening/widening)

Bone Remodeling & Wolff’s Law

  • Remodeling: coordinated osteoclast resorption + osteoblast deposition
    • Renews tissue before deterioration
    • Heals micro-damage
    • Adapts to mechanical stress; redistributes matrix along stress lines
  • Wolff’s Law: bone architecture reflects demands
    • ↑ mechanical load (bodybuilder) ➜ ↑ bone mass
    • ↓ load (astronaut, bedridden) ➜ bone loss

Fractures & Repair

  • Fracture = break in bone; types: open (compound), comminuted, greenstick, impacted, Pott, Colles, etc.
  • Healing phases (3 phases, 4 steps)
    1. Reactive: fracture hematoma (inflammatory) forms
    2. Reparative: fibrocartilaginous (soft) callus ➜ bony (hard) callus
    3. Remodeling: callus remodeled; original shape restored

Bone & Calcium Homeostasis

  • Bones store \approx 99\% of body Ca$^{2+}$
  • Hypocalcemia stimulus ➜ parathyroid glands release PTH
    • PTH actions
    • Stimulate osteoclasts ➜ ↑ resorption, Ca$^{2+}$ release
    • Kidneys retain Ca$^{2+}$, excrete phosphate, produce calcitriol (active Vitamin D)
    • Calcitriol + dietary Vitamin D ➜ ↑ intestinal Ca$^{2+}$ absorption
  • Hypercalcemia stimulus ➜ thyroid C-cells release calcitonin
    • Calcitonin actions: inhibit osteoclasts & stimulate osteoblasts ➜ ↓ blood Ca$^{2+}$

Exercise & Aging Effects

  • Weight-bearing exercise ➜ osteoblast stimulation, ↑ bone density, retard age-related loss
  • Aging
    • Until adolescence: deposition > resorption
    • Early–middle adulthood: equilibrium
    • Older age / post-menopause: resorption > deposition ➜ osteoporosis risk (esp. females due to ↓ estrogen)

Factors Affecting Bone Growth

Minerals

  • Ca & P: hardness of ECM
  • Mg: cofactor for ECM formation
  • F: strengthens ECM
  • Mn: enzyme co-factor for ECM synthesis

Vitamins

  • Vitamin A: osteoblast activity; deficiency stunts growth
  • Vitamin C: collagen synthesis; deficiency ⇒ scurvy-like bone issues
  • Vitamin D (calcitriol): ↑ Ca absorption; deficiency ⇒ rickets (children), osteomalacia (adults); toxicity possible
  • Vitamins K & B12: bone protein synthesis; deficiency ⇒ low density

Hormones

  • GH (anterior pituitary): via IGFs ➜ general growth, osteoblast stimulation
  • IGFs (liver & tissues): protein synthesis, osteoblast activity
  • Thyroid hormones (T3, T4): stimulate osteoblasts, normal growth
  • Insulin: ↑ bone protein synthesis
  • Sex hormones (estrogen, testosterone)
    • Teenage growth spurt
    • Epiphyseal plate closure \approx 18–21
    • Adult remodeling: ↓ osteoclast, ↑ osteoblast activity
  • PTH: ↑ resorption, ↑ calcitriol
  • Calcitonin: ↓ resorption

Lifestyle & Aging

  • Exercise: weight-bearing maintains mass
  • Aging/menopause: ↓ sex hormones ➜ bone loss & osteoporosis

Homeostatic Imbalances & Diagnostics

  • Bone Scan: nuclear medicine to detect areas of altered metabolism (dark “hot spots” ⇒ ↑ turnover; light “cold spots” ⇒ ↓ blood flow)
  • Osteoporosis: porous bone; post-menopausal women, Caucasians, small frame, low Ca/Vit D, sedentary, smoking/alcohol, family history ➜ preventive weight-bearing exercise, Ca & Vit D intake, meds (bisphosphonates, SERMs)
  • Rickets: Vitamin D deficiency in children ➜ improper calcification, bowed legs
  • Osteomalacia: adult counterpart; soft bones

Ethical & Practical Implications

  • Importance of nutritional education (Vit D, Ca) across life stages
  • Public health policies for osteoporosis screening (DEXA) & fall-prevention in elderly
  • Ethical duty in sports/space travel to mitigate bone loss via training protocols