Quantifying Baseline Fixed Charge Density in Healthy Human Cartilage Endplate

Abstract

Study Design Regional measurements of fixed charge densities (FCDs) of healthy human cartilage endplate (CEP) using a two-point electrical conductivity approach. Objective To determine the FCDs at four different regions (central, lateral, anterior, and posterior) of human CEP, and correlate the FCDs with tissue biochemical composition. Summary of Background Data The CEP, a thin layer of hyaline cartilage on the cranial and caudal surfaces of the intervertebral disc, plays an irreplaceable role in maintaining the unique physiological mechano-electrochemical environment inside the disc. FCD, arising from the carboxyl and sulfate groups of the glycosaminoglycans (GAG) in the extracellular matrix of the disc, is a key regulator of the disc ionic and osmotic environment through physicochemical and electrokinetic effects. While FCD in the annulus fibrosus (AF) and nucleus pulposus (NP) have been reported, quantitative baseline FCD in healthy human CEP has not been reported. Methods CEP specimens were regionally isolated from human lumbar spines. FCD and ion diffusivity were concurrently investigated using a two-point electrical conductivity method. Biochemical assays were used to quantify regional glycosaminoglycan (GAG) and water content. Results FCD in healthy human CEP was region-dependent, with FCD lowest in the lateral region (p=0.044). Cross-region FCD was 30–60% smaller than FCD in NP, but similar to the AF and articular cartilage (AC). CEP FCD (average: 0.12±0.03 mEq/g wet tissue) was correlated with GAG content (average: 31.24±5.06 μg/mg wet tissue) (p=0.005). Additionally, the cross-region ion diffusivity in healthy CEP (2.97±1.00×10−6 cm2/s) was much smaller compared to the AF and NP. Conclusions Healthy human CEP acts as a biomechanical interface, distributing loads between the bony vertebral body and soft disc tissues and as a gateway impeding rapid solute diffusion through the disc.

Electrical Conductivity Method to Determine Sexual Dimorphisms in Human Temporomandibular Disc Fixed Charge Density

“…The measurement of FCD from tissue electrical conductivity has been shown to be a simple, robust and sensitive method 14, 35 , compared to traditional tracer cation methods, magnetic resonance imaging and mechanical indentation 2, 20, 22 . Other studies have established linear relationships between FCD and GAG contents 14, 35 , likely due to differences in tissue type, which may contain higher GAG content. Regional variations in GAG composition have been reported for porcine TMJ articular discs, with the most abundant GAG types being chondroitin sulfate, dermatan sulfate and keratan sulfate 6 .…”

“…FCD was determined by a two-point electrical conductivity method 14, 35 . Following preparation, TMJ articular disc specimens were immersed in 0.15 M KCl solution (isotonic) for 12 hours at 4 °C while axially confined between two hydrophilic polyethylene porous platens (50–90 µm, Small Parts, Inc., Miami Lakes, FL) and radially confined within a 5 mm diameter acrylic chamber, minimizing swelling and proteoglycan leaching.…”

“…Following each conductivity experiment, specimen height was measured by current sensing micrometer, to determine no specimen swelling occurred. Specimen FCD ( c F ) was determined from the measured electrical conductivities in isotonic (0.15 M KCl) and hypotonic (0.03 M KCl) conditions, calculated by 35 : $$c^F=2\sqrt{\frac{{\chi }_{\text{italicIso}}^2[{(c_{\text{italicIso}}^{\ast })}^2-{(c_{\text{italicHypo}}^{\ast })}^2]}{{\chi }_{\text{italicIso}}^2-{\chi }_{\text{italicHypo}}^2}-{(c_{\text{italicIso}}^{\ast })}^2}$$where χ i and $c_i^{\ast }$ (i = Iso or Hypo) are the measured specimen electrical conductivities and concentrations of the KCl solution under isotonic and hypotonic conditions, respectively.…”

“…MRI), and mechanical indentation testing 2, 20, 22 . Recently, an electrical conductivity approach has proved to be a reliable FCD measurement in bovine intervertebral disc tissues and human cartilage endplate 14, 35 . This two-point electrical conductivity method determines fixed charge density from measurements of electrical conductivity on tissue samples equilibrated at two concentrations of bathing solution.…”

“…FCD has been studied in other cartilaginous tissues including the spinal intervertebral disc and articular cartilages 11, 13, 14, 19, 21, 22, 29, 32, 35 . However, FCD of the human TMJ disc has not been investigated, or correlated with GAG content and porosity.…”