Skeletal System: Bones and Bone Tissue Notes

Components of the Skeletal System

• Bone
• Cartilage
• Tendons
• Ligaments

Functions of the Skeletal System

• Support: Bone (rigid), cartilage (flexible).
• Protection: Skull, ribs, sternum, vertebrae.
• Movement: Muscles attach to bones via tendons; ligaments allow/prevent movement.
• Storage: Calcium, phosphate, adipose tissue.
• Blood cell production: Bone marrow.

Cartilage

• Types: Hyaline, fibrocartilage, elastic.
• Chondroblasts: form matrix.
• Chondrocytes: within lacunae, surrounded by matrix.
• Matrix: Collagen (strength), proteoglycans (resiliency).
• Perichondrium: C.T. sheath (except at articulations); inner layer (chondroblasts), outer layer (blood vessels, nerves). No blood vessels in cartilage itself.
• Articular cartilage: Covers joints; no perichondrium.
• Growth:
  • Appositional: New chondrocytes/matrix at periphery.
  • Interstitial: Chondrocytes divide within tissue.

Bone Histology

• Bone matrix:
  • Organic: collagen and proteoglycans.
  • Inorganic: hydroxyapatite ($CaPO_4$ crystals).
  • Mineral removed = bendable bone; collagen removed = brittle bone

Bone Cells

• Osteoblasts:
  • Bone formation (ossification/osteogenesis).
  • Collagen produced by E.R. and Golgi.
  • Precursors of hydroxyapatite are stored in vesicles.
  • Communicate through gap junctions.
• Osteocytes:
  • Mature bone cells.
  • Stellate.
  • Maintain matrix.
  • Lacunae (house cell body), canaliculi (house cell processes).
  • Nutrients diffuse through liquid; transfer via gap junctions.
• Osteoclasts:
  • Bone resorption.
  • Ruffled border.
  • $H^+$ ions create acid to dissolve bone; release enzymes.
  • Derived from monocytes.
  • Multinucleated.
• Stem Cells:
  • Mesenchyme (osteochondral progenitor cells) become chondroblasts/osteoblasts.

Woven and Lamellar Bone

• Woven bone:
  • Collagen fibers randomly oriented.
  • Formed during fetal development, fracture repair.
  • Remodeled into lamellar bone.
• Lamellar bone:
  • Mature bone in sheets (lamellae).
  • Fibers oriented in one direction per layer.

Spongy Bone

• Trabeculae:
  • Interconnecting rods/plates.
  • Spaces filled with marrow.
  • Covered with endosteum.
  • Oriented along stress lines.

Compact Bone

• Osteon (Haversian system):
  • Central canal.
  • Concentric lamellae.
  • Osteocytes.
• Lamellae: concentric, circumferential, interstitial.
• Perforating (Volkmann’s) canal: perpendicular to long axis.
• Canals contain blood vessels.

Compact Bone Details

• Osteons:
  • Blood vessel-filled central canal.
  • Concentric lamellae.
  • Lacunae and canaliculi.
  • Circumferential lamellae on periphery.
  • Interstitial lamellae between osteons.
• Perforating canals: Deliver blood to central canals.
• Nutrients/wastes travel via interstitial fluid and gap junctions.

Bone Anatomy

• Long bone structure:
  • Diaphysis: shaft (compact bone).
  • Epiphysis: end (spongy bone).
  • Epiphyseal plate: growth plate; hyaline cartilage until growth stops.
  • Epiphyseal line: bone stops growing.
  • Medullary cavity: red marrow (children), yellow marrow (adults).

Long Bone

• Periosteum:
  • Outer (fibrous), inner (bone cells).
  • Tendon fibers continuous with periosteum.
  • Sharpey’s fibers: attach tendon to bone.
• Endosteum:
  • Similar to periosteum; lines internal spaces.

Bone Structure

• Flat bones:
  • No diaphyses, epiphyses; spongy bone between compact bone.
• Short/Irregular bones:
  • Compact bone surrounds spongy bone.
  • No diaphyses, not elongated.
  • Some skull bones have sinuses.

Bone Development

• Intramembranous ossification:
  • Connective tissue membrane.
• Endochondral ossification:
  • Cartilage.
• Both:
  • Produce woven bone, then remodeled.

Intramembranous Ossification

• Connective tissue membrane formed from embryonic mesenchyme.
• Forms skull bones, part of mandible, diaphyses of clavicles.
• Centers of ossification: where ossification begins.
• Fontanels: unossified spaces between skull bones.

Endochondral Ossification

• Bones of the base of skull, part of mandible, epiphyses of clavicles, and most skeletal system bones.
• Cartilage formation begins at end of fourth week of development.
• Some ossification beginning at about week eight; some does not begin until 18-20 years of age.

Bone Growth

• Growth in length:
  • Epiphyseal plate (five zones).
  • Interstitial cartilage growth.
  • Appositional growth on cartilage surface.
  • Closure of epiphyseal plate.
  • Articular cartilage does not ossify; appositional growth only.

Zones of the Epiphyseal Plate

1. Zone of resting cartilage: Slowly dividing chondrocytes.
2. Zone of proliferation: New cartilage is produced as chondrocytes divide and form stacks of cells.
3. Zone of hypertrophy: Chondrocytes mature and enlarge.
4. Zone of calcification: Matrix is calcified, and chondrocytes die.
5. Ossified bone: Cartilage replaced by bone.

Growth at Articular Cartilage

• Increases size of bones with no epiphyses (short bones).
• Chondrocytes near surface similar to those in zone of resting cartilage.

Bone Growth in Width

1. Osteoblasts lay down bone to form ridges.
2. Groove transforms into tunnel; periosteum becomes endosteum.
3. Appositional growth forms concentric lamella.
4. Additional lamellae complete osteon formation.

Factors Affecting Bone Growth

• Genetics, nutrition, hormones.
• Nutrition:
  • Lack of calcium, protein = small bones.
  • Vitamin D: calcium absorption.
  • Rickets (childhood), osteomalacia (adulthood).
  • Vitamin C: collagen synthesis.
  • Scurvy: wounds don't heal, teeth fall out.
• Hormones:
  • Growth hormone: interstitial cartilage/appositional bone growth.
  • Thyroid hormone: growth of all tissues.
  • Reproductive hormones: growth at puberty, closure of epiphyseal plates.

Bone Remodeling

• Converts woven bone into lamellar bone.
• Basic multicellular units (osteoclasts/osteoblasts).
• Involved in growth, shape changes, stress adjustments, repair, $Ca^{2+}$ regulation.
• Bone constantly removed/formed.
• Osteoclasts remove bone; osteoblasts replace bone.

Mechanical Stress and Bone Strength

• Increased stress: increased strength (remodeling, bone formation, trabeculae alignment).
• Reduced stress: increased osteoclast activity.

Bone Fracture Classification and Repair

• Classification based on mechanism, soft-tissue damage, displacement, pattern, fragments, age.

Bone Repair

1. Hematoma formation: blood clot.
2. Callus formation:
   • Internal: blood vessels grow into clot; macrophages clean, osteoclasts break down dead tissue, fibroblasts produce collagen.
   • Chondroblasts produce cartilage.
   • Osteoblasts invade: new bone formed.
   • External: collar around ends.
   • Periosteal osteochondral progenitor cells -> osteoblasts and chondroblasts.
3. Callus ossification: woven, spongy bone.
4. Bone remodeling: spongy bone replaced by compact bone; sculpting by osteoclasts.

Calcium Homeostasis

• Bone is major calcium storage site.
• Calcium levels depend on movement into/out of bone.
• Osteoblasts create new bone; osteoclasts break down bone.
• Hormones: parathyroid hormone (PTH), calcitriol, calcitonin.

Parathyroid Hormone (PTH)

• Secreted by parathyroid gland when blood calcium is low.
• Activates osteoclasts, increases osteoclast number, prevents inhibition of osteoclast formation.
• Stimulates calcium reabsorption in kidney tubules.
• Promotes activation of calcitriol.

Calcitriol and Calcitonin

• Calcitriol:
  • Increases blood calcium by stimulating intestinal absorption (derived from vitamin D3).
• Calcitonin:
  • Lowers blood calcium by inhibiting osteoclast activity.

Effects of Aging on Skeletal System

• Bone matrix decreases (more brittle).
• Bone mass decreases (highest around 30).
• Increased bone fractures.
• Bone loss causes deformity, loss of height, pain, stiffness, stooped posture, loss of teeth.