Physiology of Osseous Tissue (Chapter 7)

Mineralization, remodeling, and calcium/phosphate balance

  • Mineralization: deposition of calcium and phosphate into bone

    • Osteoblasts first produce collagen fibers; fibers become encrusted with minerals.

    • Seed crystals act as nucleation points that attract more calcium phosphate.

    • Both calcium and phosphate must be present for bone formation.

    • Abnormal calcification is called ectopic ossification; can form a calculus in abnormal areas (lung, eye, arteries).

  • Bone dissolution (resorption): osteoclasts dissolve bone minerals and matrix

    • Osteoclasts pump hydrogen ions into the extracellular fluid, driving the formation of hydrochloric acid (low pH) to dissolve minerals.

    • They produce enzymes that digest collagen; amino acids enter the blood.

  • Primary minerals in bone: calcium and phosphate

    • Phosphate: component of DNA, RNA, ATP, phospholipids.

    • Calcium: critical for neuron signaling, muscle contraction, blood clotting, and other systems.

  • Calcium vs. phosphate balance

    • Calcium is the limiting factor in many disorders; phosphate disorders are less common.

    • Minerals stored in the skeleton serve as a bank to withdraw when needed.

  • Calcium homeostasis concept

    • Balance between intake (from food) and losses (urinary and fecal).

    • Balance must be maintained between bones and blood.

Hormonal regulation of calcium homeostasis

  • Three key hormones regulate blood calcium: Calcitriol (active vitamin D), Calcitonin, Parathyroid hormone (PTH).

  • Calcitriol (Vitamin D, active form)

    • Hormone that raises blood calcium levels.

    • Increases intestinal calcium absorption in the small intestine.

    • Increases calcium resorption from the skeleton (bone breakdown) and aids a little in renal calcium reabsorption (reducing urinary calcium loss).

    • Vitamin D synthesis pathway: skin (UV light) → liver → kidneys; UV light is required to start the process.

    • Calcitriol is necessary for bone deposition; without it bones become soft (deficiency leads to rickets in children and osteomalacia in adults).

  • Calcitonin

    • Produced by the thyroid gland; released when blood calcium is very high.

    • Lowers blood calcium by two mechanisms:

    • Inhibits osteoclasts (slows bone resorption).

    • Stimulates osteoblasts (promotes bone formation).

    • Appears to be more important in children (skeleton growth) and has a weak effect in adults except during pregnancy/lactation.

  • Parathyroid hormone (PTH)

    • Produced by parathyroid glands (embedded in the posterior thyroid gland).

    • Released when blood calcium is low.

    • PTH increases blood calcium via four pathways:
      1) Stimulates osteoclasts → increased bone resorption → more Ca enters blood.
      2) Promotes renal calcium reabsorption → reduces urinary calcium loss.
      3) Promotes vitamin D synthesis → increases intestinal calcium absorption (via calcitriol).
      4) Slows down osteoblast activity (reduces bone formation), and promotes phosphate excretion by the kidneys.

    • Phosphate management: PTH helps excrete phosphate from the blood in the urine; phosphate is essential for bone formation, so its regulation is important.

Negative feedback and integration of calcium homeostasis

  • Blood calcium remains in a narrow range; if Ca2+ rises, calcitonin is released to suppress osteoclasts and encourage osteoblasts (bone formation), bringing Ca2+ back down.

  • If Ca2+ falls, PTH is released to boost osteoclast activity (bone resorption), reduce urinary calcium loss, increase vitamin D activation (calcitriol) to raise intestinal absorption, and limit osteoblast formation to return Ca2+ to normal.

  • This system acts like a balance/steering mechanism to keep calcium in a critical physiological range.

Negative feedback loops, clinical correlations, and signs

  • Hypocalcemia (low blood calcium)

    • Leads to hyperexcitability of the nervous system and spontaneous muscle contractions/spasms.

    • Risk of laryngeal spasm and suffocation if unrelieved.

    • Common causes: vitamin D deficiency or inadequate PTH.

    • Pregnancy and lactation increase risk in women.

    • Trousseau's sign: inflation of a blood pressure cuff induces involuntary carpal spasm; demonstration of low calcium.

  • Hypercalcemia (high blood calcium)

    • Decreased excitability of nerves and muscles → muscle weakness, cardiac issues, and possible emotional disturbances.

    • Rare because blood calcium is maintained as a slowly draining system; long-term high calcium is difficult to sustain.

  • Visual aids from exam scenarios

    • A patient with hypocalcemia may show a positive Trousseau’s sign during a cuff inflation test (hand/finger spasm).

Growth and development of bones

  • Epiphyseal plate (growth plate) activity in infancy and childhood

    • Stimulated by several hormones; the main one is human growth hormone (GH).

    • Puberty: testosterone (in males) and estrogen (in females) drive the growth spurt and differentiation of male vs. female skeleton.

  • Growth patterns and endpoints

    • Growth ends anywhere between 18 to 20 years (typical).

    • Girls reach full height earlier and typically grow faster because estrogen has a stronger effect on bone growth.

    • If anabolic steroids are used, growth can stop early due to premature epiphyseal plate closure, resulting in shortened stature.

Real-world relevance and connections

  • Bone acts as a mineral reservoir; maintaining Ca and phosphate homeostasis is essential for neuromuscular function, vascular stability, and bone integrity.

  • Vitamin D status directly affects bone deposition and overall calcium balance; deficiency yields soft bones and deformities in children, and bone pain in adults.

  • Endocrine control of bone links dermatology (skin synthesis of vitamin D), nutrition (dietary calcium/phosphate), renal physiology (calcium/phosphate handling), and endocrinology (PTH, calcitonin, calcitriol).

Quick reference: key terms and concepts

  • Ectopic ossification: abnormal calcification in soft tissues; formation of a calculus in non-bone sites.

  • Rickets: vitamin D deficiency in children leading to softened bones and deformities.

  • Osteomalacia: vitamin D deficiency in adults causing bone softening and pain.

  • Trousseau's sign: a clinical sign of hypocalcemia (carpal spasm when a BP cuff is inflated).

  • Calcitonin: thyroid hormone; lowers blood Ca; predominant role in growing children.

  • Parathyroid hormone (PTH): increases blood Ca via osteoclast activation, renal reabsorption, and vitamin D activation; promotes phosphate excretion; slows bone formation.

  • Calcitriol: active vitamin D; increases Ca2+ absorption from the gut; supports bone deposition; requires UV exposure for synthesis.

  • Epiphyseal plate: growth plates; closure signals end of growth; estrogen and GH regulate activity; premature closure reduces final height.

  • Bone as a calcium/phosphate bank: dynamic balance between deposition and resorption maintains systemic Ca2+ levels.

ext{Calcium balance equation (conceptual):} \ rac{d[ ext{Ca}^{2+}]_{ ext{blood}}}{dt} \,\approx \, ext{Intake} - ext{UrinaryLoss} - ext{Deposition} + ext{Resorption}

ext{Steady-state condition:} \ ext{Intake} + ext{Resorption} = ext{UrinaryLoss} + ext{Deposition}