Enzymatic cross-linking of bone collagen.
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Aside from cholesterol, cholesterol esters and lyso-lecithin, the de novo lipid synthesic mechanisms which operate in cells grown in the presence of beta-amino-propionitrile are largely depressed and suggest that there may be in operation specific metabolic control mechanisms for regulation of cellular lipid composition.
We have measured the dynamics of extracellular matrix consolidation and strengthening by human dermal fibroblasts in hydrated collagen gels. Constraining matrix consolidation between two porous polyethylene posts held rigidly apart set up the mechanical stress which led to the formation of uniaxially oriented fibroblast-populated collagen matrices with a histology resembling a ligament. We measured the mechanical stiffness and tensile strength of these ligament equivalents (LEs) as a function of age at biweekly intervals up to 12 weeks in culture using a mechanical spectrometer customized for performing experiments under physiologic conditions. The LE load-strain curve changed as a function of LE age, increasing in stiffness and exhibiting less plastic-like behavior. At 12 weeks, LEs had acquired up to 30 times the breaking strength of 1-week-old LEs. Matrix strengthening occurred primarily through the formation of BAPN-sensitive, lysyl oxidase catalyzed crosslinks. Sulfated glycosaminoglycan (GAG) content increased monotonically with LE age, reaching levels that are characteristic of ligaments. Cells in the LEs actively incorporated [3H]proline and [35S]sulfate into the extracellular matrix. Over the first three weeks, DNA content increased rapidly but thereafter remained constant. This data represent the first documentation of strengthening kinetics for cell-assembled biopolymer gels and the results suggest that this LE tissue may be a valuable model for studying the cellular processes responsible for tissue growth, repair, and remodeling.
Bones of 4-week-old Sprague-Dawley rats fed a diet containing 0.25% beta-aminoproprionitrile (beta-APN) for 21 days and injected with 3H-proline, 3H-tetracycline, and cold tetracycline at 5-, 10-, and 20-day intervals were analyzed for incorporation of 3H-proline and 3H-tetracycline. Tibial cross sections were examined by fluorescent microscopy to determine the linear bone apposition between intervals and the distribution and intensity of tetracycline bands. The incorporation of 3H-proline was similar in beta-APN-treated and control rats. Incorporation of 3H-tetracycline, however, was significantly lower in beta-APN-treated rats. Fluorescent microscopy of tibial cross sections showed a comparable amount of linear matrix apposition in control and treated rats. It is suggested that beta-APN does not inhibit the bone matrix formation but does interfere with its mineralization.
Methods are described for screening for teratogenic compounds using differentiating neural crest and prechondrogenic limb bud mesenchyme cells in culture. Substances to be tested are either added directly to the culture medium or are combined in a dialysis bag with the postmitochondrial fraction from rat liver and certain cofactors. In the latter case, the compound and its metabolites are gradually released into the medium from the dialysis bag. The results obtained with 14 compounds demonstrate a positive relationship between teratogenicity in vivo and alterations in the growth or the differentiation of the cultured cells.
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Collagen fibres in the periodontal ligament may have two functions: to resist displacing forces and to cause the tooth to erupt. Their function was examined in the continuously-erupting incisor of the rat using various concentrations and types of lathyrogens. Lathyrogens retarded tooth eruption and increased the quantity of salt-soluble collagen in the ligament, indicating inhibition of the maturation of salt-soluble (young) collagen into salt-insoluble (old) collagen, which would lead to reduction in the tensile strength of the fibres and decrease resistance to occlusal forces. The easy extractability of the teeth is explained by the greater susceptibility to lathyrogens of the fibres in the alveolar-related part of the periodontal ligament, compared with those in the other parts.
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