Hydroxyapatite formation in collagen, gelatin, and agarose gels.

Collagen has long been suspected to be a promoter of hydroxyapatite (HA) deposition in bone. This theory was tested by comparison of HA formation in gels composed of collagen, gelatin or agarose. Collagen gels supported substantially more HA precipitation than gelatin gels, but slightly less than agarose gels. Analysis of the relative diffusion rates for calcium in these matrices indicated that, in this system, amount of HA formation is dependent upon the rate of diffusion. Under conditions in which the diffusion rates for collagen and agarose gels were comparable, similar amounts of HA were formed. This suggests that fibrillar collagen is not per se a promoter of HA deposition. Extracellular matrix macromolecules may influence calcification by restricting ionic diffusion through connective tissues.

Calcium pyrophosphate crystal formation in model hydrogels. II. Hyaline articular cartilage as a gel.

We studied calcium pyrophosphate crystal formation in an in vitro cartilage system. Two parallel troughs were excavated in tibial plateau articular cartilage obtained postmortem. One well was filled with solid sodium pyrophosphate, the other with calcium chloride. After incubation for 24 h at either 10 degrees C or 37 degrees C the precipitate band between the troughs was analyzed for the size and nature of crystals present. In subsequent experiments, the cartilage was pretreated by laceration, contusion, trypsin or hyaluronidase denaturation. We found that cartilage denaturation resulted in formation of larger crystals but that the crystal product in all experiments was identical, alpha CaNa2P2O7.4H2O a nonphysiologic crystal.

Calcium pyrophosphate crystal formation in aqueous solutions.

Pseudogout is characterized by the deposition of calcium pyrophosphate dihydrate, triclinic [CPPD(T)] and calcium pyrophosphate dihydrate, monoclinic [CPPD(M)] crystals in articular connective tissues. We studied aqueous solutions over a range of calcium chloride/sodium pyrophosphate concentrations to determine the ionic conditions under which these particular salts form. At 37 degrees C, CPPD(T) forms when [PPi]t greater than or equal to 10(-4), while formation of CPPD(M) occurs at 10(-3) M < [PPi]t less than or equal to 10(-2) M. When [Na+]t > 120 mM, calcium disodium pyrophosphates precipitate. With 1 mM Mg++, CPPD(M) forms at [PPi]t > 10(-3) M, mixed with a calcium magnesium pyrophosphate at [PPi]t greater than or equal to 10(-2) M. We conclude that CPPD(T) and CPPD(M) crystals form in a restricted ratio and range of [Ca++]t and [PPi]t and that other ions, particularly Mg++ and Na+, affect the nature of the crystal products formed.

Calcium pyrophosphate dihydrate crystal formation in model hydrogels.

Using powder x-ray diffraction analysis, we studied calcium pyrophosphate crystal formation in silica and gelatin gels was well as in comparable aqueous solutions. We demonstrated that the physiological salts, monoclinic and triclinic calcium pyrophosphate dihydrate can be formed in gels and that gels do affect the type of crystals formed from ions in solution. These observations support our hypothesis that calcium pyrophosphate dihydrate crystal deposition in joints is regulated by the physical chemical gel state of the connective tissue matrix.

The ultrastructure of urate crystals in gout.

Using correlative scanning and transmission electron microscopic techniques, we studied the ultrastructure of natural and synthetic monosodium urate crystals. The internal architecture of both natural and synthetic urates consists of an interconnecting electron lucent network. This network is different in crystals from tophi, suggesting crystal formation about an electron lucent nidal matrix. Urate crystals from articular cartilage surfaces have smooth faces supporting the hypothesis that acute goat is a two phase process involving first the precipitation of urate crystals in the synovial fluid and second the adherence to the crystals of proteins and cells with the consequent inflammatory response.