Lang, Florian, Gillian L. Busch, Markus Ritter, Harald Volkl, Siegfried Waldegger, Erich Gulbins, and Dieter Haussinger. Functional Significance of Cell Volume Regulatory Mechanisms. Physiol. Rev. ...

Functional Significance of Cell Volume Regulatory Mechanisms

and Introduction Abstract Study Design: Review and reinterpretation of existing literature. Objective: To suggest how intervertebral disc degeneration might be distinguished from the physiologic processes of growth, aging, healing, and adaptive remodeling. Summary of Background Data: The research literature concerning disc degeneration is particularly diverse, and there are no accepted definitions to guide biomedical research, or medicolegal practice. Definitions: The process of disc degeneration is an aberrant, cell-mediated response to progressive structural failure. A degenerate disc is one with structural failure combined with accelerated or advanced signs of aging. Early degenerative changes should refer to accelerated age-related changes in a structurally intact disc. Degenerative disc disease should be applied to a degenerate disc that is also painful. Justification: Structural defects such as endplate fracture, radial fissures, and herniation are easily detected, unambiguous markers of impaired disc function. They are not inevitable with age and are more closely related to pain than any other feature of aging discs. Structural failure is irreversible because adult discs have limited healing potential. It also progresses by physical and biologic mechanisms, and, therefore, is a suitable marker for a degenerative process. Biologic progression occurs because structural failure uncouples the local mechanical environment of disc cells from the overall loading of the disc, so that disc cell responses can be inappropriate or aberrant. Animal models confirm that cell-mediated changes always follow structural failure caused by trauma. This definition of disc degeneration simplifies the issue of causality: excessive mechanical loading disrupts a disc's structure and precipitates a cascade of cell-mediated responses, leading to further disruption. Underlying causes of disc degeneration include genetic inheritance, age, inadequate metabolite transport, and loading history, all of which can weaken discs to such an extent that structural failure occurs during the activities of daily living. The other closely related definitions help to distinguish between degenerate and injured discs, and between discs that are and are not painful.

What is intervertebral disc degeneration, and what causes it?

The intervertebral disc is a cartilaginous structure that resembles articular cartilage in its biochemistry, but morphologically it is clearly different. It shows degenerative and ageing changes earlier than does any other connective tissue in the body. It is believed to be important clinically because there is an association of disc degeneration with back pain. Current treatments are predominantly conservative or, less commonly, surgical; in many cases there is no clear diagnosis and therapy is considered inadequate. New developments, such as genetic and biological approaches, may allow better diagnosis and treatments in the future.

/pdf/degeneration-of-the-intervertebral-disc-297lygwdik.pdf

Degeneration of the intervertebral disc

STUDY DESIGN A histologic study on age-related changes of the human lumbar intervertebral disc was conducted. OBJECTIVES To investigate comprehensively age-related temporospatial histologic changes in human lumbar intervertebral disc, and to develop a practicable and reliable classification system for age-related histologic disc alteration. SUMMARY OF THE BACKGROUND DATA No comprehensive microscopic analysis of age-related disc changes is available. There is no conceptual morphologic framework for classifying age-related disc changes as a reference basis for more sophisticated molecular biologic analyses of the causative factors of disc aging or premature aging (degeneration). METHODS A total of 180 complete sagittal lumbar motion segment slices obtained from 44 deceased individuals (fetal to 88 years of age) were analyzed with regard to 11 histologic variables for the intervertebral disc and endplate, respectively. In addition, 30 surgical specimens (3 regions each) were investigated with regard to five histologic variables. Based on the semiquantitative analyses of 20,250 histologic variable assessments, a classification system was developed and tested in terms of validity, practicability, and reliability. The classification system was applied to cadaveric and surgical disc specimens not included in the development of the classification system, and the scores were assessed by two additional independent raters. RESULTS A semiquantitative analyses provided clear histologic evidence for the detrimental effect of a diminished blood supply on the endplate, resulting in the tissue breakdown beginning in the nucleus pulposus and starting in the second life decade. Significant temporospatial variations in the presence and abundance of histologic disc alterations were observed across levels, regions, macroscopic degeneration grades, and age groups. A practicable classification system for age-related histologic disc alterations was developed, resulting in moderate to excellent reliability (kappa values, 0.49-0.98) depending on the histologic variable. Application of the classification system to cadaveric and surgical specimens demonstrated a significant correlation with age ( < 0.0001) and macroscopic grade of degeneration ( < 0001). However, substantial data scatter caution against reliance on traditional macroscopic disc grading and favor a histology-based classification system as a reference standard. CONCLUSIONS Histologic disc alterations can reliably be graded based on the proposed classification system providing a morphologic framework for more sophisticated molecular biologic analyses of factors leading to age-related disc changes. Diminished blood supply to the intervertebral disc in the first half of the second life decade appears to initiate tissue breakdown.

Classification of age-related changes in lumbar intervertebral discs: 2002 Volvo Award in basic science.

Study design A review of the literature on disc nutrition. Objectives To summarize the information on disc nutrition in relation to disc degeneration. Summary of the background data The disc is avascular, and the disc cells depend on diffusion from blood vessels at the disc's margins to supply the nutrients essential for cellular activity and viability and to remove metabolic wastes such as lactic acid. The nutrient supply can fail due to changes in blood supply, sclerosis of the subchondral bone or endplate calcification, all of which can block transport from blood supply to the disc or due to changes in cellular demand. Methods A review of the studies on disc blood supply, solute transport, studies of solute transport in animal and human disc in vitro, and of theoretical modeling studies that have examined factors affecting disc nutrition. Results Small nutrients such as oxygen and glucose are supplied to the disc's cells virtually entirely by diffusion; convective transport, arising from load-induced fluid movement in and out of the disc, has virtually no direct influence on transport of these nutrients. Consequently, there are steep concentration gradients of oxygen, glucose, and lactic acid across the disc; oxygen and glucose concentrations are lowest in the center of the nucleus where lactic acid concentrations are greatest. The actual levels of concentration depend on the balance between diffusive transport and cellular demand and can fall to critical levels if the endplate calcifies or nutritional demand increases. Conclusions Loss of nutrient supply can lead to cell death, loss of matrix production, and increase in matrix degradation and hence to disc degeneration.

Nutrition of the intervertebral disc.

Study design Disc cell viability was analyzed in relation to nutrient supply and cellular demand in vitro in a diffusion chamber. Objective To determine relations among nutrient supply, nutrient concentrations. and cell viability. Summary of background data Although a fall in nutrient supply has long been thought the cause of disc degeneration in vivo, little information exists about the effects of nutrient levels or supply on cell viability and metabolism. Methods Isolated bovine nucleus cells were cultured in agarose gels in a diffusion chamber up to 13 days. Nutrients were supplied to the open sides of the chamber and diffused through the gel to the center, 12.5 mm away from the nutrient supply, in a configuration analogous to that of the disc in vivo. Profiles of cell viability and concentration of glycosaminoglycans across the chamber were measured in relation to cell density and medium composition. Results Cells remained viable across the chamber at low cell densities. However, at higher densities, cells in the center of the chamber died. The viable distance from the nutrient supply fell with an increase in cell density. Glucose was a critical nutrient. Survival was also poor at acidic pH (6.0). At 0% oxygen, disc cells survived up to 13 days with no loss of viability, but produced very little proteoglycan. Conclusions The results support the idea that maximum cell density in the disc is regulated by nutritional constraints, and that a fall in nutrient supply reduces the number of viable cells in the disc and thus leads to degeneration.

2001 Volvo Award Winner in Basic Science Studies: Effect of nutrient supply on the viability of cells from the nucleus pulposus of the intervertebral disc.

Transport of diverse substrates into malaria-infected erythrocytes via a pathway showing functional characteristics of a chloride channel.

Transport and Metabolism of the Essential Vitamin Pantothenic Acid in Human Erythrocytes Infected with the Malaria Parasite Plasmodium falciparum

Study design This study examined how the culture system and region of cellular origin affect disc cell morphology and extracellular matrix production. Objective To determine the role of the cell populations in the different regions of the adult intervertebral disc in maintaining gradients in composition across the disc. Summary of background data It is not known whether the steep profiles in composition across the intervertebral disc are maintained by distinct cell populations or whether differences in cell metabolism are determined by changes in the physical environment across the disc. Very little information exists on the matrix produced by cells from the mature, non-notochordal nucleus pulposus. Methods Cells were extracted from articular cartilage, nucleus pulposus, and the inner and outer anulus fibrosus of caudal discs from 18- to 24-month-old steers cultured in alginate or collagen gels or in monolayer. The effect of culture system and cell origin on cell morphology and matrix synthesis was measured using 35S-sulphate labeling and indirect immunolocalization. Results Distinct morphologic differences between cells from different regions cultured in monolayer were retained through two passages. The rate of sulfate incorporation varied with cell type. Immediately after isolation, it was two- to threefold greater for nucleus cells than for cells from the disc inner anulus or articular cartilage. The rate was lowest for outer anulus cells. It also varied with culture system. For all cell types, the incorporation rate was highest in alginate and lowest in monolayer. Immunolocalization showed that nucleus cells stained strongly for all proteoglycan epitopes, whereas outer anulus cells stained least and in monolayer produced little proteoglycan. Conclusions The disc has at least three distinct cell populations, which differ in morphology and in amount and type of matrix they produce. Cells from mature nucleus pulposus produced sulfated glycosaminoglycans at a high rate in contrast to reported results for notochordal nucleus cells. Alginate, although an appropriate culture system for inner anulus and nucleus cells, may not be a suitable medium for outer anulus cells.

Heather A. Horner

Papers

2001 Volvo Award Winner in Basic Science Studies: Effect of nutrient supply on the viability of cells from the nucleus pulposus of the intervertebral disc.

Transport of diverse substrates into malaria-infected erythrocytes via a pathway showing functional characteristics of a chloride channel.

Transport and Metabolism of the Essential Vitamin Pantothenic Acid in Human Erythrocytes Infected with the Malaria Parasite Plasmodium falciparum

Cells from different regions of the intervertebral disc: effect of culture system on matrix expression and cell phenotype.

Glucose uptake in Plasmodium falciparum-infected erythrocytes is an equilibrative not an active process