Bone Development and Histology

Key questions

1. Q1How is bone similar to and different from cartilage?

   Similar: hard tissues, living cells in lacunae, common mesenchymal progenitor. Different: bone is heavily vascularized and calcified, and its cells reach vessels through canaliculi; cartilage is avascular, uncalcified and relies on long-range diffusion.
2. Q2What makes up the organic matrix of bone?

   About 25% of bone mass, mostly fibrous: type I collagen (~90%, gives elasticity and tensile strength) plus ground substance — GAGs (chondroitin and keratan sulfate) and glycoproteins.
3. Q3Which glycoproteins matter and what do they do?

   Osteonectin and osteopontin anchor mineral to collagen and start mineralization/crystal formation. Osteocalcin and bone sialoprotein are calcium-binding proteins.
4. Q4What is osteoid?

   Newly secreted organic matrix that is not yet mineralized, sitting between the osteoblast layer and existing bone.
5. Q5What makes up the inorganic matrix?

   About 65–70% of bone mass: calcium phosphate as hydroxyapatite crystals Ca10(PO4)6(OH)2, plus citrate, bicarbonate and fluoride. Water is ~10–15%; the hydration shell around crystals allows calcium exchange. Mineral gives compressive strength.
6. Q6Where do osteoprogenitor cells come from and what commits them?

   Mesenchymal stem cells lining inner and outer bone surfaces. BMPs (and osteogenin) commit them; PDGF, TGF-β and IGF drive self-renewal; BMP and vitamin D3 drive differentiation.
7. Q7Describe the osteoblast.

   Bone-forming, non-dividing cell on the bone surface; abundant RER and Golgi (secretory); secretes osteoid; activated by GH and sex steroids; also secretes factors (RANKL) that promote osteoclast activity.
8. Q8Describe the osteocyte.

   Terminally differentiated osteoblast trapped in its own mineralized matrix, sitting in a lacuna; extends filopodia through canaliculi joined by gap junctions to exchange nutrients and waste; can do limited osteocytic osteolysis.
9. Q9Describe the osteoclast.

   Large, multinucleated, mobile cell of monocyte/macrophage lineage (fusion promoted by vitamin D) sitting in a Howship lacuna; has a ruffled border (resorption zone) surrounded by a clear (sealing) zone.
10. Q10How does the osteoclast resorb bone?

    Acidification decalcifies (citric acid, carbonic anhydrase makes H+), then hydrolytic enzymes digest the matrix (collagenase, acid phosphatase, sulfatase).

Key questions

1. Q11Which hormone increases bone resorption?

   Parathyroid hormone (from parathyroid chief cells) — it increases both osteoclastic and osteocytic osteolysis.
2. Q12Does PTH act directly on osteoclasts?

   No — indirectly. PTH binds receptors on osteoblasts, which release factors (RANKL; less OPG) that activate osteoclasts.
3. Q13Which hormones decrease resorption?

   Calcitonin (thyroid parafollicular C cells) and estrogens.
4. Q14Which hormones stimulate osteoblast secretion/deposition?

   Growth hormone (somatotropin) and sex steroids.
5. Q15What is osteocytic vs osteoclastic osteolysis?

   Osteocytic = limited calcium release by osteocytes inside their lacunae. Osteoclastic = true matrix resorption by osteoclasts. Both are increased by PTH and decreased by calcitonin.
6. Q16What happens with hyperparathyroidism?

   Excess PTH → excess resorption and bone loss, fragile bones, high blood calcium, and calcium deposits in soft tissue (kidney stones).

Key questions

1. Q17Woven vs lamellar bone?

   Woven (primary, immature): random collagen fibrils and amorphous calcium phosphate; seen in embryos and fracture repair. Lamellar (secondary, mature): collagen organized in lamellae with crystalline mineral; comes from remodeling woven bone; adult bone.
2. Q18What are the two forms of lamellar bone?

   Cancellous (spongy) and compact (cortical).
3. Q19Describe spongy bone.

   A network of irregular lamellae forming trabeculae (spicules) with large spaces filled with marrow (hematopoietic tissue). Found in epiphyses, short bones, flat-bone diploë. Anastomosing trabeculae trap vessels; it can be converted to compact bone.
4. Q20Where is compact bone?

   Diaphysis of long bones, a thin shell around epiphyses, and the inner and outer "tables" of flat skull bones.
5. Q21What lamellar systems make up compact bone?

   Outer and inner circumferential lamellae, Haversian systems (osteons) and interstitial lamellae (remnants of old osteons between the current ones).
6. Q22What is an osteon (Haversian system)?

   A cylinder of 4–15 concentric lamellae around a central Haversian canal carrying blood vessels, lymphatics and nerves; osteocytes in lacunae connected by canaliculi.
7. Q23Haversian vs Volkmann canals?

   Haversian canals run longitudinally in the center of osteons. Volkmann canals run at right angles, connecting Haversian canals to each other and to the periosteum and marrow.
8. Q24Describe the periosteum.

   Tough connective tissue on the outer surface (except articular surfaces and tendon insertions): outer fibrous layer (vascular) and inner osteogenic layer (osteoprogenitors, osteoblasts). Anchored by Sharpey fibers (type I collagen); Volkmann canals originate here.
9. Q25Describe the endosteum.

   A thin single layer of progenitor cells, osteoblasts and osteoclasts lining internal surfaces (trabeculae, marrow cavity, Haversian canals); important for nutrition and maintenance.
10. Q26Which structure is NOT mineralized: Haversian system, circumferential lamellae, interstitial lamellae or endosteum?

    The endosteum — it is a cell layer, not mineralized bone.
11. Q27How do periosteal/endosteal progenitors decide their fate?

    With blood supply (oxygen) → osteoprogenitor → osteoblast. Without blood supply → chondroprogenitor → chondroblast. This drives cartilage-first fracture repair.
12. Q28How does spongy bone become compact bone?

    The spaces between trabeculae fill in and the irregular lamellae are remodeled into organized concentric cylinders — osteons (Haversian systems).

Key questions

1. Q29What are the four steps of calcification?

   1) Osteoblasts secrete osteoid. 2) Osteoblasts secrete factors that allow mineralization (alkaline phosphatase, osteonectin, osteocalcin, calcium salts). 3) Osteoblasts become trapped in mineralized matrix as osteocytes in lacunae. 4) Osteocytes keep a limited ability to secrete and resorb bone.
2. Q30How does alkaline phosphatase help mineralization?

   It raises local inorganic phosphate in the osteoid until calcium phosphate reaches saturation and precipitates.
3. Q31What role do matrix vesicles play?

   They contain pyrophosphate and enzymes; the enzymes break down the calcification inhibitor pyrophosphate and release phosphate, allowing crystals to form.
4. Q32What is bone remodeling?

   Constant balance of deposition and resorption: deposition traps new vessels (keeping bone vascular); resorption creates canals and the marrow cavity; it maintains shape in growing bones, adapts to mechanical stress and helps calcium homeostasis. Osteons are constantly remodeled.
5. Q33How does mechanical stress affect bone?

   Compression and movement are required for proper remodeling (bone lost in space flight or immobilization; orthodontists move teeth using this plasticity). Piezoelectric potential: negative → deposition; positive → resorption.
6. Q34Which diseases come from an imbalance?

   Osteoporosis: resorption > deposition → bone loss (elderly). Osteopetrosis: deposition > resorption from defective osteoclasts → dense "marble" bone that is thick but brittle.

Key questions

1. Q35What does vitamin C deficiency do to bone?

   Scurvy: poor hydroxylation of procollagen → unstable collagen → fragile bone and fractures (plus bleeding gums, poor wound healing).
2. Q36What does vitamin D deficiency do?

   Impaired intestinal calcium absorption → poor mineralization: rickets in children (bowed legs, knock-knee) and osteomalacia in adults (weak bones, fractures).
3. Q37What does excess vitamin D do?

   Abnormal calcification of soft tissues; toxic.
4. Q38What does excess vitamin A do?

   Slows cartilage growth and accelerates ossification (premature growth-plate closure).
5. Q39What happens with GH deficiency or excess?

   Deficiency in childhood → pituitary dwarfism. Excess in childhood → gigantism. Excess in adults (pituitary tumor) → acromegaly (enlarged hands, feet, jaw).
6. Q40How do sex steroids affect bone growth?

   They set the timing of ossification centers and epiphyseal closure. Early puberty or sex-steroid tumor → early closure → short stature. Delayed puberty or sex-hormone deficiency → late closure → tall stature.
7. Q41What does hyperparathyroidism do to bone?

   Excess resorption and bone loss, fragile bones, hypercalcemia and soft-tissue (kidney) calcium deposits.

Key questions

1. Q42What are the two ways primary (woven) bone forms?

   Intramembranous ossification (direct: mesenchyme → osteoblasts) and endochondral ossification (indirect: a hyaline cartilage model is replaced by bone).
2. Q43Which bones form by intramembranous ossification?

   Flat bones of the skull vault and face (and the clavicle, in part); also the subperiosteal bone collar that widens long bones.
3. Q44List the steps of intramembranous ossification.

   1) Mesenchymal condensation. 2) Blood vessels appear. 3) Mesenchymal cells differentiate into osteoblasts (primary ossification center). 4) Mineralization; osteoblasts become osteocytes. 5) Spicules/trabeculae form. 6) Trabeculae interweave → spongy bone and marrow cavities.
4. Q45Which bones form by endochondral ossification?

   Long and short bones of the limbs and the axial skeleton (vertebrae, ribs, sternum, skull base).
5. Q46List the steps of endochondral ossification.

   Hyaline cartilage model → subperiosteal bone collar forms around the shaft (intramembranous) → central chondrocytes hypertrophy, calcify the matrix and die → vessels invade bringing osteoprogenitors → primary ossification center (diaphysis) → secondary ossification centers in the epiphyses → growth plates remain until adulthood.
6. Q47What determines bone width vs length?

   Width: the intramembranous subperiosteal bone collar (appositional). Length: endochondral cartilage replacement at the growth plate.
7. Q48When do ossification centers appear?

   Primary centers around week 12 of gestation; secondary centers around birth; long-bone growth continues into early adulthood.
8. Q49What decides whether a mesenchymal precursor becomes bone or cartilage?

   The vascular environment: with blood/oxygen → osteoprogenitor; without → chondroprogenitor. The differentiation is reversible.
9. Q50What does a forming trabecula look like on a slide?

   Mineralized matrix with osteocytes in lacunae, edged by a pale band of osteoid, then a continuous row of osteoblasts (with occasional osteoclasts). That osteoblast row separates the trabecula from the surrounding mesenchyme, whose cells keep differentiating into new osteoblasts that join the row.

Key questions

1. Q51What provides growth in length?

   The epiphyseal growth plate (hyaline cartilage without perichondrium) growing by interstitial growth.
2. Q52List the zones of the growth plate from epiphysis to diaphysis.

   Resting (reserve) → proliferative → hypertrophic (maturation) → calcified → ossification zone.
3. Q53What happens in each zone?

   Resting: small quiescent chondrocytes. Proliferative: chondrocytes divide in stacked columns (interstitial mitosis). Hypertrophic: cells enlarge and store glycogen. Calcified: matrix calcifies and chondrocytes die. Ossification: vessels and osteoprogenitors invade; osteoblasts lay bone on calcified cartilage spicules; chondroclasts/osteoclasts remodel.
4. Q54Which zone determines height and is the GH target?

   The proliferative zone — GH (via somatomedin C/IGF-1) drives chondroblast and chondrocyte mitosis there.
5. Q55How does the bone get longer?

   Chondrocytes keep proliferating on the epiphyseal side while cartilage is replaced by bone on the diaphyseal side, so the plate moves away from the shaft and the diaphysis lengthens; epiphyseal spicules are incorporated into the diaphysis.
6. Q56What controls growth-plate proliferation long term?

   FGF-dependent proliferation of chondrocytes (FGFR3 signaling restrains it) plus GH/IGF-1; sex steroids close the plate at puberty.

Key questions

1. Q57How are unstable (mobile) fractures repaired?

   By endochondral ossification: a cartilage callus forms first and is later replaced by bone.
2. Q58Walk through the steps of endochondral fracture repair.

   Fracture → blood supply interrupted → necrosis and blood clot (hematoma) → low oxygen; macrophages and fibroblasts form fibrous tissue → periosteal osteoprogenitors become chondrogenic → cartilage bridge (callus) as early as 4 days → blood vessels reappear → osteogenic potential rises; cartilage replaced by bone (primary woven bone by ~2 weeks) → remodeled to secondary lamellar bone; mechanical stress and muscle activity restore shape.
3. Q59Why does cartilage form first?

   The fracture site is avascular/hypoxic, and progenitors without oxygen become chondroprogenitors. When vessels return, they become osteogenic.
4. Q60What changes when blood vessels reappear during fracture repair?

   Osteogenic potential increases — progenitors become osteoblasts and replace the cartilage callus with bone (endochondral).
5. Q61How are hairline or non-weight-bearing fractures repaired?

   Directly by intramembranous ossification (no cartilage stage).
6. Q62What are the clinical stages of fracture healing?

   Hematoma (0–1 day: platelets, fibrin, inflammatory cells) → soft callus (fibrocartilage, ~2 weeks) → hard callus (woven bone) → remodeling to lamellar bone (months).

Key questions

1. Q63What is a diarthrosis?

   A freely movable synovial joint between long bones.
2. Q64What are its components?

   Articular cartilage (hyaline, no perichondrium), synovial fluid, a two-layer capsule (outer fibrous layer, inner synovial membrane), reinforcing ligaments, and stabilizing muscles and tendons.
3. Q65What cells line the synovial membrane?

   Type A cells (macrophage-like, phagocytic) and type B cells (fibroblast-like, secrete synovial fluid — hyaluronic acid and lubricin).
4. Q66What anchors ligaments and tendons to bone?

   Sharpey fibers (type I collagen). Tendon: muscle to bone. Ligament: bone to bone.
5. Q67Describe articular cartilage.

   Hyaline cartilage with type II collagen fibers arched parallel to the surface, no perichondrium, grows interstitially, nourished by synovial fluid.
6. Q68How do RA and OA differ at the joint?

   Rheumatoid arthritis: autoimmune inflammation of the synovial membrane (aggregates of inflammatory cells, pannus) that secondarily destroys cartilage. Osteoarthritis: degeneration of articular cartilage with aging and wear; fragments break off into the joint.