Bone Formation and Remodeling

Bone Formation and Remodeling – Study Notes

Types of Bone Formation

  • Intramembranous ossification: bone formation within primitive connective tissue (mesenchyme).

  • Endochondral ossification: pre-existing cartilage is turned into bone.

  • Ectopic bone formation: pathologic ossification of connective tissues that don’t normally form bone.

Intramembranous Ossification

  • Occurs often in skull regions as animal grows.

  • Mesenchyme: tissue of primordial cells with abundant extracellular matrix (loose connective tissue).

    • Embryonic development: areas intended to become flat bones condense and become vascularized.

  • Woven bone: first-formed bone with randomly interwoven collagen fibrils.

    • Quick construction but weak; higher osteocyte density; faster turnover; weaker and more flexible.

  • Osteoid: unmineralized organic bone matrix; present in woven and lamellar bone.

  • Process (4 steps):
    1) Formation of ossification spicules: mesenchymal cells form collagen fibrils → matrix forms → converts to osteoblasts.
    2) Formation of extracellular matrix and trabeculae: osteoblasts lay down fibrils with Ca^{2+} → calcification → trabeculae form.
    3) Osteoid vascularization development: fibrils organize around blood vessels → lamellar layers form around vessels.
    4) Osteons and compact bone formation: outer connective tissue condenses into periosteum → osteoblasts produce periosteal surface; spongy bone expands; red marrow forms as spaces fill.

  • Summary: mesenchymal cells -> osteoblasts -> osteoid -> calcifies into bony spicules -> osteocytes develop -> periosteum forms -> compact bone forms; spongy bone surrounds vessels and can generate red marrow.

Endochondral Ossification

  • Starts from hyaline cartilage model (matrix of type II collagen) and forms most bones of the axial skeleton and limbs.

  • Occurs in: bones of extremities, base of skull, vertebral column, pelvis.

  • Process:

    • Mesenchyme differentiates into chondrocytes to form cartilage model; perichondrium forms around model.

    • Chondrocytes secrete hyaline matrix; osteoprogenitor cells in perichondrium are activated to deposit a thin collar of woven bone on the outside of the cartilage model.

Centers of Ossification (General Concept)

  • Within hyaline cartilage, centers enlarge where chondrocytes hypertrophy and lacunae expand.

  • Calcification occurs as matrix around enlarging chondrocytes accumulates Ca^{2+} phosphate; hypertrophied chondrocytes undergo apoptosis.

  • Blood vessels invade from the newly formed perichondrium; osteoprogenitor cells follow and differentiate into osteoblasts on calcified cartilage spicules to form bone matrix.

Specifics in Horses and Cows

  • Ossification starts in the diaphysis of long bones by around the 3rd month of fetal development.

  • Secondary centers appear later in the epiphyses.

  • Epiphyseal vs. diaphyseal ossification:

    • Epiphyseal ossification expands but cartilage in articular surfaces remains.

    • Transverse disk of epiphyseal cartilage (epiphyseal plate) remains between sections; chondrocytes align in columns to allow longitudinal growth.

Endochondral Ossification – Detailed Steps (4–5 Stages)

1) Mesenchymal cells differentiate into chondrocytes; cartilage model with perichondrium forms.
2) Cartilage model and perichondrium form; a bone collar begins to develop around the diaphysis (periosteal collar).
3) Capillaries invade cartilage; periosteum converts and osteoblasts appear on calcified cartilage to form primary ossification center; perichondrium becomes periosteum.
4) Cartilage and chondrocytes continue to grow at the ends of the bone; secondary ossification centers develop in the epiphyses.
5) Ossification of epiphyses; cartilage remains at the epiphyseal growth plate and at joint surfaces as articular cartilage.

Growing Bones – Length (Longitudinal Growth)

  • Growth plate (physes) typically located between epiphysis and diaphysis; some bones retain additional growth sites.

  • Zones of chondrocyte organization (from near the diaphysis toward the epiphysis) reflect stages of bone formation:

    • Zone of proliferation: chondrocytes closer to diaphysis divide and organize into columns.

    • Zone of maturation: chondrocytes stop dividing and enlarge.

    • Zone of hypertrophy: hypertrophied and vacuolated chondrocytes.

    • Zone of provisional calcification: cartilaginous matrix around cells begins to calcify.

  • Diaphyseal side: apoptosis of hypertrophic chondrocytes; lacunae invaded by blood vessels and osteoprogenitor cells from marrow → osteoblasts form calcified cartilage bars; collagen matrix laid down on calcified cartilage; osteoclasts resorb woven cartilage and rest of calcified cartilage; osteoblasts replace with lamellar bone.

  • Growth plate expansion away from the diaphysis is driven by somatotropin hormone (GH) from the pituitary, which stimulates local production of insulin-like growth factor 1 (IGF-1, also called somatomedin) to promote proliferation.

  • Zones and cellular events summarized in plates:

    • Reserve/resting zone

    • Proliferative zone (mitosis of chondrocytes)

    • Maturation/hypertrophy zone

    • Zone of provisional calcification

    • Metaphysis and primary spongiosa transitioning to secondary spongiosa

  • Growth plate diagram terms include: rapid proliferation/transit-amplifying cells, hypertrophic cells, calcified matrix, and ossification fronts.

Growth Plate Closure

  • Once mature length is reached, cartilage production slows and cartilage is replaced by bone on the diaphyseal side.

  • Metaphysis becomes continuous with the epiphysis; true metaphysis may disappear after complete plate closure.

  • Closure occurs at different rates across bones and even across different plates within the same bone.

  • Example note: Hyena disease describes premature hindlimb growth plate closure due to vitamin A toxicity in calves/heifers.

Growing Bones – Diameter (Appositional Growth)

  • Thickness grows by deposition of bone on the outside surface between old bone and periosteum (intramembranous-like addition) with concurrent resorption on the inner surface.

  • Hyaline model shape remains constant; rates of resorption and new bone formation are balanced.

  • Mechanical stress influences mineral deposition and bone matrix thickness; greater stress yields thicker cortical bone.

  • Spongy bone formation slows in thicker regions, leaving space to occupy with bone marrow.

Bone Remodeling

  • Bone is continuously remodeled in small units (Bone Remodeling Units, BRUs) to replace old/weakened bone.

  • BRUs differ in compact vs. spongy bone structure.

  • Four steps of remodeling:

    • Activation

    • Resorption

    • Reversal

    • Formation

Activation

  • Resting bone surface becomes a remodeling site.

  • Activation could be due to microfracture disrupting collagen orientation, triggering electrical changes in mineral crystals and remodeling signals.

  • Quiet osteoblasts shrink at remodeling sites to expose bone matrix; osteoclasts move in and attach to the bone surface (ruffled border).

Resorption

  • Osteoclasts release acids and proteolytic enzymes to dissolve bone matrix.

    • Acids (carbonic, hydrochloric, citric, lactic) dissolve mineral crystals.

    • Proteolytic enzymes (acid hydrolases) digest organic matrix.

  • Mineral and matrix breakdown products are absorbed and transported back into extracellular fluid.

  • Effects: saucer-shaped depressions in trabeculae; tunnels in compact bone.

Reversal

  • Howship’s lacuna forms at the resorption site.

  • Insulin-like growth factor 2 (IGF-2) and transforming growth factor (TGF-β) released during collagen breakdown activate surface lining cells to revert to osteoblasts.

  • Other factors are hypothesized to be involved.

Formation

  • Osteoid deposition by osteoblasts begins the formation phase.

  • Osteoid consists mainly of type I collagen, proteoglycans, and other bone matrix proteins.

  • Lamellae are laid down with parallel fibrils; compact bone fills from the periphery toward the Haversian canal, while trabecular bone forms curved sheets along hollow shapes.

  • Mineralization follows after a delay; total time for an individual BRU to complete formation is about 150extdays150 ext{ days}.

Osteoblasts, Osteocytes, and Remodeling Phases

  • Osteoblasts generate osteoid and later become embedded as osteocytes.

  • Resting phase: a prolonged resting period before new remodeling begins.

  • Activation of osteoclasts precedes resorption; mononuclear cells and osteoclasts participate in the resorption and reversal phases.

  • Reversal prepares the surface for osteoblast adherence and subsequent bone formation.

Time to Repair After Fracture

  • Osteoprogenitor cells and osteoblasts in periosteum and endosteum migrate to fracture site with invading blood vessels.

  • Blood clot forms; osteocytes die in the fracture area.

  • Proinflammatory cytokines recruit phagocytes to remove debris.

  • If apposition is poor, edge-propagated osteoprogenitor cells form cartilage (endochondral repair) to bridge gaps in hypoxic regions; later replaced with bone.

  • Blood vessels grow back in during calcification to form woven bone by osteoblasts.

  • Fracture repair happens on both sides of the fracture:

    • External callus (periosteum-derived) provides strong early stabilization.

    • Internal callus (endosteum-derived) forms within the marrow and surrounding trabecular spaces.

Fracture Terminology

  • Complete vs incomplete fractures:

    • Complete: through the entire bone.

    • Incomplete: not through the entire bone.

  • Line types:

    • Transverse: straight across.

    • Oblique: at an angle.

    • Spiral: corkscrew line.

    • Comminuted: more than two fragments.

  • Displacement:

    • Non-displaced vs displaced: misalignment between fracture ends.

  • Other terms: bowing, buckling (concave side), Greenstick (convex side), avulsion (bone fragment pulled off by ligament/tendon).

Fracture Terminology (Continued Images)

  • Common fracture descriptors include traverse, linear, oblique, spiral, comminuted, and greenstick injuries; displaced vs non-displaced states.

Salter-Harris Fractures (Growth Plate Injuries)

  • The most common growth plate injuries.

  • TYPE I: through growth plate only.

  • TYPE II: through growth plate and metaphysis.

  • TYPE III: through growth plate and epiphysis.

  • TYPE IV: through all three elements (growth plate, metaphysis, epiphysis).

  • TYPE V: crush injury of the growth plate.

  • Mnemonic emphasis on the vulnerability of the growth plate in young animals and potential growth disturbances.

Bone Metabolism and Homeostasis

  • Remodeling moves minerals between bone and extracellular fluid (ECF): minerals released by osteoclasts during resorption and re-deposited by osteoblasts during formation.

  • Bone acts as a pH buffer:

    • Releases cations (e.g., Ca^{2+}) during acidosis.

    • Releases anions to counteract alkalosis.

  • Serum Ca^{2+} concentration is tightly regulated; poor dietary intake can lead to bone resorption to maintain Ca^{2+} levels.

  • Osteocytic osteolysis: osteocytes resorb Ca^{2+} from surrounding bone fluid to maintain Ca^{2+} homeostasis.

Osteocytic Osteolysis and Hormonal Regulation

  • Parathyroid hormone (PTH) stimulates osteocytes to pump Ca^{2+} into plasma when Ca^{2+} is low; this helps prevent excessive osteoclast activation and preserves the matrix during short-term deficits.

  • If Ca^{2+} deficit is severe, osteoclastic osteolysis is activated: persistent PTH secretion shrinks osteoblasts to expose bone matrix; osteoblasts release paracrine factors (e.g., prostaglandin E2, IL-1, IL-6) to stimulate osteoclasts.

  • 1,25-dihydroxyvitamin D (active Vitamin D) indirectly affects bone Ca^{2+} by increasing intestinal absorption of Ca^{2+} and phosphorus, aiding mineralization of the bone matrix.

  • Calcitonin: major Ca^{2+}-regulating hormone; secreted by thyroid C-cells when Ca^{2+} is high; inhibits osteoclast-mediated bone resorption.

Hormonal Regulators of Calcium and Bone Formation

  • Parathyroid hormone (PTH): raises blood Ca^{2+} by increasing bone resorption and renal reabsorption of Ca^{2+}; short-term actions support Ca^{2+} homeostasis.

  • Calcitonin: lowers blood Ca^{2+} by inhibiting osteoclasts during hypercalcemia.

  • Vitamin D (1,25-dihydroxyvitamin D): enhances Ca^{2+} and phosphate absorption from the gut, promoting mineralization.

  • Growth hormone (somatotropin) and IGF-1 (somatomedin): stimulate growth plate activity and long-bone elongation.

Practical and Real-World Relevance

  • Growth plate disturbances can lead to disproportionate limb length or angular deformities if not properly regulated.

  • Vitamin A toxicity can prematurely close growth plates (Hyena disease example).

  • Understanding fracture healing informs clinical decisions about immobilization, surgical stabilization, and expectations for rehabilitation.

  • Bone remodeling is essential for adapting to mechanical loads and maintaining mineral homeostasis (Ca^{2+} balance, acid-base buffering).

Key Terms to Remember

  • Osteoid, woven bone, lamellar bone, periosteum, endosteum, Haversian canals, primary and secondary ossification centers, spicules, trabeculae, red marrow, lamellae orientation.

  • BRU: Bone Remodeling Unit.

  • BRU phases: Activation, Resorption, Reversal, Formation.

  • Howship’s lacuna: resorption pit left by osteoclasts.

  • Salter-Harris fracture types I–V and their typical involvement.

  • Osteocytic osteolysis and its hormonal control mechanisms.

  • Growth plate zones and their roles in longitudinal bone growth.

  • Appositional growth and the balance of deposition vs resorption for diameter growth.

  • Calcified cartilage transition to bone during endochondral ossification.

Notes drawn from lecture content (ACBS 400A/500A, Fall 2024) on bone formation, remodeling, growth, repair, and calcium homeostasis.