FSHN 552: Part 3: Structural Support

Module 9: Calcium and Phosphorus

Session 1: Distribution, Bone Structure, and Hydroxyapatite

Part 3: Structural Support

Overview

• Module 09: Calcium & Phosphorus
• Week 9: Module 10 - Vitamin D
• Week 10: Module 11 - Vitamin K & Integration
• Week 11: Cross-theme connections include:
  • Vitamin C (collagen hydroxylation, see Module 06)
  • Copper (lysyl oxidase, see Module 08)
  • Magnesium (hydroxyapatite stabilization, see Module 05)

Calcium: Distribution in the Body

• 99% of calcium is found in bones and teeth, specifically as hydroxyapatite:

$Ca_{10}(PO_4)_6(OH)_2$

• 1% in:
  • Blood
  • Extracellular Fluid (ECF)
  • Soft Tissue
• Key physiological roles include:
  • Muscle contraction
  • Calcium (Ca²⁺) release from sarcoplasmic reticulum (SR) binds to troponin C, exposing actin binding sites for myosin to facilitate cross-bridge cycling.
  • Nerve transmission
  • Calcium (Ca²⁺) influx at nerve terminals triggers neurotransmitter vesicle fusion and release (e.g., acetylcholine).
  • Blood clotting
  • Calcium (Factor IV) is involved at multiple steps during intrinsic and extrinsic coagulation cascades.
  • Intracellular signaling
  • Calcium (Ca²⁺) acts as a second messenger and interacts with calmodulin, activating kinases, phosphatases, and synthases.
• Serum calcium levels are critical, maintained at 8.5–10.5 mg/dL; ionized calcium levels are 4.6–5.2 mg/dL.

The Non-Skeletal 1%: Importance of Calcium

• Muscle Contraction:
  • Involves calcium release, which binds to troponin, facilitating muscle movement.
• Nerve Transmission:
  • Calcium influx at terminals is essential for neurotransmitter release, crucial for communication in the nervous system.
• Blood Clotting:
  • Calcium is indispensable at numerous steps in coagulation pathways, contributing to hemostasis.
• Intracellular Signaling:
  • Calcium serves as a mediator in signal transduction pathways, modulating various cellular processes.

Phosphorus: Distribution in the Body

• 85% found in bone and teeth as part of hydroxyapatite.
• 14% in soft tissue, playing critical roles in:
  • ATP Formation: Integral for energy transfer.
  • Genetic Information: Essential for DNA and RNA structure.
  • Membrane Structure: Part of phospholipids, underlying cell membrane integrity.
  • Enzyme Regulation: Through phosphorylation events, influencing signaling pathways.
  • Acid-Base Buffering: Utilizes HPO₄²⁻ and H₂PO₄⁻ to help maintain pH homeostasis.
• 1% in extracellular fluid exists as free phosphate ions: HPO₄²⁻, H₂PO₄⁻, along with protein-bound and complexed forms.
• The broad functional scope of phosphorus contrasts with calcium's focus on bone and serum regulation, underscoring its diverse roles in metabolism.
• Approximately 700 g of phosphorus in an adult, making it the second most abundant mineral in the body.

Phosphorus: Functions Beyond Bone

• Energy Transfer:
  • Components of ATP, GTP, and creatine phosphate, facilitating all energy-dependent reactions.
• Genetic Information:
  • Phosphodiester bonds are crucial for constructing DNA and RNA backbones.
• Membrane Structure:
  • Phospholipids' phosphate head groups impart essential amphipathic characteristics for membrane formation.
• Enzyme Regulation:
  • Phosphorylation/dephosphorylation acts as a switch for numerous cellular signaling pathways.
• Acid-Base Buffering:
  • Between HPO₄²⁻ and H₂PO₄⁻, this system buffers intracellular pH, acting as a titratable acid in renal excretion.
• Additional role: 2,3-Diphosphoglycerate (2,3-DPG) in red blood cells regulates oxygen delivery from hemoglobin. Depletion of phosphorus leads to reduced 2,3-DPG and impaired oxygen transport.

Bone Composition: The Three Components

1. Inorganic Mineral
   • Constitutes 50–70% of bone.
   • Hydroxyapatite: $Ca_{10}(PO_4)_6(OH)_2$ is central to bone rigidity and compressive strength.
2. Organic Matrix
   • Accounts for 20–40% of bone.
   • Composed of approximately 90% type I collagen, which provides tensile strength and flexibility.
3. Water
   • Comprises 5–10% of bone.
4. Lipid
   • Represents less than 3%.
5. Analogy:
   • Bone can be likened to reinforced concrete:
     • Collagen fibers (organic) mimic rebar, imparting tensile strength.
     • Hydroxyapatite crystals (inorganic) resemble cement, providing compressive strength.
     • Alone, neither component suffices for robust bone structure.

The Organic Matrix: Collagen and Beyond

• Type I Collagen (~90%):
  • Structurally a triple helix formed by three polypeptide chains (α chains).
  • Every third residue is glycine, represented as a Gly-X-Y repeat, where X and Y are often proline and hydroxyproline.
  • Requirement of vitamin C as a cofactor for hydroxylation of proline and lysine, critical for stability of the collagen triple helix.
  • Cross-linking of collagen fibers needs copper, facilitated by enzyme lysyl oxidase.
• Other Organic Components (~10%):
  • Osteocalcin: A vitamin K-dependent protein that binds calcium in bones.
  • Osteopontin: Regulates mineralization and mediates cell adhesion.
  • Bone Sialoprotein: Initiates hydroxyapatite crystal formation.
  • Proteoglycans: Organize the matrix and modulate mineralization.
• Cross-theme connection:
  • Vitamin C functions as an antioxidant (Module 06) and is also crucial for collagen hydroxylation; without it, collagen's stability is compromised, leading to connective tissue disorders akin to scurvy.

Hydroxyapatite: The Mineral Phase

• Composition:
  • A unit cell comprises 10 calcium ions, 6 phosphate groups, and 2 hydroxyl groups:
    $Ca_{10}(PO_4)_6(OH)_2$
• Calcium to Phosphorus Molar Ratio:
  • By weight, approximately 1.67:1 (range: 1.3:1 to 2:1).
• Crystal Structure:
  • Formed as hexagonal crystals deposited along and within collagen fibrils.
  • Crystals are typically small (20–80 nm), yielding substantial surface area for ion exchange.
• Substitutions in Hydroxyapatite:
  • Molecules such as Mg²⁺, Na⁺, F⁻, carbonate (CO₃²⁻), and citrate can integrate into the hydroxyapatite lattice, adjusting its physical properties.

Bone Cells: The Construction Crew

Builders

• Osteoblasts:
  • Responsible for synthesizing and secreting the organic matrix (osteoid).
  • Facilitate mineralization by releasing matrix vesicles loaded with alkaline phosphatase, concentrating Ca²⁺ and PO₄³⁻ to initiate hydroxyapatite formation.
  • Originates from mesenchymal stem cells.

Sensors

• Osteocytes:
  • Mature osteoblasts that become embedded in mineralized bone.
  • Utilize canaliculi (tiny channels) for communication.
  • Sense mechanical load and coordinate bone remodeling signals.
  • Produce FGF23, a factor that regulates phosphorus levels.
  • Develop from osteoblasts.

Demolition

• Osteoclasts:
  • Large, multinucleated cells involved in bone resorption.
  • Release hydrochloric acid (HCl) to dissolve mineral content and cathepsin K to break down collagen.
  • Release Ca²⁺ and PO₄³⁻ into extracellular fluid (ECF).
  • Activation prompted by RANKL secreted from osteoblasts.
  • Originates from hematopoietic lineage (monocyte/macrophage).

Cortical vs. Trabecular Bone

Cortical (Compact) Bone

• Accounts for approximately 80% of skeletal mass.
• Structurally dense outer shell of bone.
• Organized into osteons (Haversian systems).
• Main function is to provide mechanical strength.
• Exhibits slower turnover rate (~3% per year).
• Typically found in long bone shafts (diaphysis) and outer bone layers.

Trabecular (Spongy) Bone

• Comprises about 20% of skeletal mass.
• Features a porous, honeycomb-like structure.
• Composed of a network of thin plates and rods known as trabeculae.
• Has a higher surface area, thus being more metabolically active.
• Demonstrates a faster turnover rate (~25% per year).
• Located in vertebrae, pelvis, and ends of long bones (epiphyses).
• Clinical Relevance:
  • Trabecular bone is often the first to decline in osteoporosis due to its higher turnover rate, resulting in an increased likelihood of vertebral compression fractures and distal radius fractures (Colles' fractures) occurring before long bone fractures.

Bone Remodeling: A Dynamic Process

• Approximately 10% of the adult skeleton is replaced annually.
• Full remodeling cycle spans 4–6 months (resorption takes about 2 weeks, followed by formation lasting 4–6 months).
• Remodeling processes are coupled: new bone formation typically matches resorption, resulting in balanced turnover.
• When resorption exceeds formation, it leads to net bone loss, progressing from osteopenia to osteoporosis.
• Peak bone mass is generally achieved by age 25–30; maintaining balance post-peak is crucial.

Phases of Bone Remodeling

1. Activation: Osteocytes detect microdamage or respond to hormonal signals.
2. Resorption: Osteoclasts dissolve mineral (via HCl) and degrade matrix (via cathepsin K).
3. Formation: Osteoblasts deposit new osteoid and initiate mineralization.
4. Reversal: Osteoclasts leave; mononuclear cells prepare the bone surface for new formation.

Bone as a Mineral Reservoir

• The body's regulatory priority focuses on serum calcium levels over bone density.
• Bone may be compromised to preserve blood calcium concentrations within optimal limits.

Short-term Response

• When serum Ca²⁺ levels decrease, parathyroid hormone (PTH) is released within minutes.
• PTH acts on both bone and kidney to elevate serum calcium back to target levels without necessitating full osteoclast-mediated resorption from hydroxyapatite.

Chronic Adaptation

• With persistent low dietary calcium intake, PTH levels remain chronically elevated.
• This triggers osteoclast-driven bone resorption, maintaining serum calcium homeostasis over the long term, ultimately leading to significant bone loss while maintaining normal serum calcium values.
• Clinical Implication: Individuals with chronically low calcium intake can demonstrate normal serum calcium levels despite considerable bone density loss.
• Regular assessment requires different markers (such as DEXA scans, bone turnover markers), as serum calcium merely reflects hormonal regulation rather than overall calcium status.

Calcium and Phosphorus: Partners in Bone

Calcium in Bone

• 99% of the body's calcium is located in bone.
• Approximately 200 mg of calcium is exchanged with ECF on a daily basis.
• Provides essential Ca²⁺ ions required for hydroxyapatite formation.
• Serum calcium level maintained at 8.5–10.5 mg/dL.

Phosphorus in Bone

• 85% of phosphorus found in bone.
• Daily exchange of approximately 200 mg with ECF occurs.
• Contributes PO₄³⁻ ions necessary for hydroxyapatite structure.
• Serum phosphorus levels register at 2.5–4.5 mg/dL.

The Regulatory Tension (Preview for Upcoming Session)

• PTH elevates serum Ca²⁺ while reducing serum phosphorus (actuated at the kidney).
• Calcitriol (1,25(OH)₂D) increases both serum calcium and phosphorus absorption in the intestine.
• Fibroblast Growth Factor 23 (FGF23) reduces serum phosphorus and suppresses calcitriol production.
• The product of calcium and phosphorus ($Ca imes P$) must remain within specific limits to avert calcification in soft tissues.
• This regulatory synergy illustrates why calcium and phosphorus are interconnected in physiology.

Key Figures for This Module

• Figure 11.3: Calcium absorption routes - transcellular (TRPV6 → calbindin D₉ₖ → PMCA1b) and paracellular pathways.
• Figure 11.4: Mechanisms of blood calcium regulation by PTH and calcitriol at bone, kidney, and intestine.
• Figure 11.5: Bone composition breakdown: organic matrix (20–40%), minerals (50–70%), water (5–10%), lipids (<3%).
• Figure 11.7: Calcium's intracellular actions and mechanisms for maintaining cytosolic [Ca²⁺].
• Figure 11.9: Pathways for phosphorus digestion, absorption, and transport.
• Table 11.4: Hormonal effects of PTH, calcitriol, and calcitonin on calcium homeostasis.
• Table 11.7: Hormonal actions of FGF23, PTH, and calcitriol on phosphorus homeostasis.
• Focus on the regulatory figures and tables prior to the next session. Understanding the directional arrows (↑ or ↓) for each hormone at each target organ is critical to mastering content.

Upcoming Session Overview

• Absorption & Regulation:
  • Distinctions between transcellular vs. paracellular calcium absorption.
  • Factors enhancing and inhibiting calcium absorption.
  • Phosphorus absorption metrics and efficiencies compared to calcium.
  • Overview of calcium homeostasis: roles of PTH, calcitonin, and 1,25(OH)₂D.
  • Regulation of phosphorus: involvement of PTH and FGF23.
  • Concept of calcium-phosphorus regulatory axis.
• Review: Gropper Chapter 11, including Figures 11.3, 11.4, 11.9, and Tables 11.4, 11.7.