Chapter 6 – Bones and Skeletal Tissues (A&P 11e)

Why This Matters

• Grasping bone anatomy & remodeling is critical for diagnosing, treating, and educating patients with bone pathologies (e.g., osteoporosis)

• Clinical skills supported: selecting imaging, prescribing load-bearing exercise, advising on Ca^{2+}/vitamin D, monitoring fracture risk, interpreting bone-density scans

Skeletal Cartilages (6.1)

• Human fetal skeleton starts as cartilage; osseous tissue gradually replaces it except where flexibility is essential

• General traits of cartilage

  • Highly resilient, molded tissue → primarily water → excellent shock absorber

  • Avascular & aneural; nutrients diffuse through matrix

  • Encased by perichondrium (dense irregular CT)

  • Resists outward expansion

  • Houses blood vessels → nutrient diffusion

  • Cells = chondrocytes sitting in lacunae within jelly-like ECM

Types & Locations

• Hyaline cartilage

  • Only collagen fibers; most abundant

  • Functions: support, flexibility, resilience

  • Sites: articular ends of long bones, costal ribs, respiratory structures (larynx, trachea), nasal cartilages

• Elastic cartilage

  • Hyaline + elastic fibers → withstands repeated bending

  • External ear, epiglottis

• Fibrocartilage

  • Thick collagen bundles → greatest tensile strength

  • Menisci of knee, intervertebral discs, pubic symphysis

Growth Modes

• Appositional growth

  • Perichondrial chondroblasts secrete new matrix on external face → width growth

• Interstitial growth

  • Chondrocytes divide inside lacunae → push matrix from within → length expansion

• Calcification

  • Occurs in youth (normal bone formation) & aging; produces hardened cartilage (NOT true bone)

Functions of Bone (6.2)

• Support: structural framework for body & soft organs

• Protection: encase brain, spinal cord, thoracic organs, pelvic organs

• Movement: act as levers for skeletal muscles

• Mineral/Growth-factor reservoir: Ca^{2+}, PO_4^{3-} and GF storage & release

• Hematopoiesis: red marrow produces formed elements

• Triglyceride storage: yellow marrow stores energy-rich fat

• Hormone production: osteocalcin regulates insulin secretion, glucose homeostasis, metabolic rate; emerging therapeutic target

Classification of Bones (6.3)

By Position

• Axial skeleton: skull, vertebral column, rib cage → protective & support axis

• Appendicular skeleton: upper/lower limbs + girdles → locomotion & manipulation

By Shape

• Long bones: longer than wide (humerus, femur, phalanges)

• Short bones: cube-like (carpals, tarsals); sesamoids form in tendon (patella) — vary individually

• Flat bones: thin, slightly curved (sternum, ribs, scapulae, cranial bones)

• Irregular bones: complex shapes (vertebrae, coxal bones)

Bone Structure Overview (6.4)

• Bone = organ: contains osseous tissue, cartilage, nerves, blood vessels, muscle & epithelial elements

• Three structural levels: Gross → Microscopic → Chemical

Gross Anatomy

Compact vs Spongy Bone

• Compact (cortical): dense external layer → strength

• Spongy (cancellous): trabecular network aligned along stress lines; spaces filled with red/yellow marrow

Short, Flat, Irregular Bones

• Thin plates of spongy bone (diploë) sandwiched by compact bone

• Periosteum (outer) & endosteum (inner) cover compact layers

• No dedicated marrow cavity; marrow dispersed in trabeculae

• Articular surfaces capped with hyaline cartilage

Typical Long Bone

• Diaphysis: tubular shaft; compact bone collar surrounding medullary cavity (yellow marrow in adults)

• Epiphyses: proximal & distal ends; compact exterior, spongy interior; articular cartilage covers joint surfaces

• Epiphyseal line (adult remnant of growth plate) between diaphysis & epiphysis

Membranes

• Periosteum (external)

  • Outer fibrous layer: dense irregular CT, Sharpey’s fibers anchor tendons/ligaments

  • Inner osteogenic layer: osteoprogenitor cells → osteoblasts/osteoclasts

  • Richly supplied with nerve fibers & blood vessels entering via nutrient foramina

• Endosteum (internal)

  • Delicate CT lining medullary cavity, trabeculae, and central canals; contains osteogenic cells

Marrow Distribution

• Newborns: medullary cavities + all spongy bone = red marrow

• Adults: red marrow mainly in diploë of flat bones & irregular bones (hip), and heads of femur/humerus; yellow marrow can revert to red if anemic

Bone Markings

• Provide attachment, leverage, passages; 3 categories:

  • Projections (e.g., trochanter, tubercle) — muscle/ligament pull, joint formation

  • Depressions (fossa, groove) — conduits, articulation sites

  • Openings (foramen, canal) — vessel & nerve passage

Microscopic Anatomy

Cell Types

• Osteogenic (osteoprogenitor) cells: mitotic stem cells in periosteum/endosteum → differentiate or remain dormant

• Osteoblasts: bone-forming; secrete osteoid (90 % collagen + Ca-binding proteins); mitotic

• Osteocytes: mature cells in lacunae; maintain matrix, act as mechanosensors; relay info to remodeling cells

• Bone-lining cells: flat periosteal (external) & endosteal (internal) cells; matrix maintenance

• Osteoclasts: multinucleate macrophage lineage; resorb bone; ruffled border ↑ SA; reside in resorption bays

Compact (Lamellar) Bone Microstructure

• Osteon (Haversian system): concentric lamellae (collagen fibers alternating orientation) around central canal; resist twisting & compressive forces

• Central canal: blood vessels, nerves

• Perforating (Volkmann’s) canals: transverse connections linking periosteum, medullary cavity, and central canals

• Lacunae: small cavities housing osteocytes

• Canaliculi: tiny channels connecting lacunae & central canal; allow nutrient/waste diffusion & cell communication via gap junctions

• Interstitial lamellae: remnants filling gaps or cut-through osteons

• Circumferential lamellae: extend around diaphysis just deep to periosteum/endosteum; reinforce bone against torsion

Spongy Bone

• No osteons; trabeculae aligned along stress → strength with minimal weight

• Trabeculae contain irregular lamellae & osteocytes; nutrients via capillaries in endosteum

Chemical Composition

• Organic (~1/3 by mass)

  • Cells (osteogenic, osteoblasts, osteocytes, lining cells, osteoclasts)

  • Osteoid: ground substance (proteoglycans, glycoproteins) + collagen → tensile strength & flexibility

  • Sacrificial bonds between collagen molecules dissipate energy, re-form if trauma sub-fracture

• Inorganic (~2/3 by mass)

  • Hydroxyapatites: tiny Ca{10}(PO4)6(OH)2 crystals around collagen

  • Provide hardness & compressive strength; bone ≈ half as strong as steel in compression, equal in tension

  • Mineral stability allows fossilization & archeologic analysis

Cartilage & Bone Growth Processes (Integration)

• Normal development: embryonic cartilage → endochondral ossification; intramembranous ossification for flat bones (covered in later sections)

• Childhood/Adolescence: epiphyseal plate activity governs longitudinal bone growth; appositional growth widens bones

• Remodeling: lifelong balance of osteoblast (formation) vs osteoclast (resorption); vital for calcium homeostasis, fracture repair, and adapting to mechanical load

Clinical / Real-World Connections

• Osteoporosis: imbalance favoring resorption; understanding cell biology aids pharmacological targeting (bisphosphonates ↓ osteoclast activity; PTH analogs ↑ osteoblast)

• Fracture healing: periosteum & endosteum’s osteogenic layers crucial for callus formation

• Exercise: mechanical stress sensed by osteocytes stimulates remodeling → ↑ bone density (Wolff’s Law)

• Aging: cartilage calcification (not ossification) contributes to stiffened costal cartilage; red → yellow marrow conversion impacts hematopoiesis

Key Numbers & Equations

• Total named bones: (206)

• Bone mass composition: organic ( rac13), inorganic ( rac23) → hydroxyapatite ext{≈}65\% of mass

• Collagen ≈ 90\% of bone protein

• Comparative strength: bone ≈ rac12 steel (compression), ≈ steel (tension)

Example / Metaphor Highlights

• Trabeculae likened to cables on a suspension bridge → distribute loads efficiently

• Sacrificial bonds function like Velcro® strips: detach under stress to prevent catastrophic failure, then re-attach when stress removed

Ethical & Practical Implications

• Knowledge of bone biochemistry guides safe prescription of supplements; excessive Ca^{2+} may calcify soft tissues

• Understanding remodeling informs ethical athletic training loads to minimize stress fractures

• Forensic anthropology & archeology leverage mineral persistence to study ancient remains, raising considerations of cultural heritage and repatriation

Connections to Previous / Future Content

• Links with Chapter on Connective Tissues: cartilage & bone are specialized CTs sharing ECM emphasis

• Sets foundation for future chapters on joints (arthrology), muscle leverage, endocrine regulation (PTH, calcitonin), and pathophysiology (rickets, Paget’s disease)