The temporal degredation of bone collagen
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Forensic anthropologists are currently unable to reliably and quantitatively estimate the PMI of skeletonized remains. This pilot study was conducted to determine if bone collagen degrades at a predictive rate over time and if so, to determine if this knowledge could contribute to the creation of a quantitative and reliable method of estimating the PMI. The sample used in this study consisted of one 'Fresh' mature adult porcine long bone, ten 'modern' juvenile porcine long bones with known PMIs ranging from '2 months' to' 12 months' and ten 'archaeological' adult long bones of various taxa with known PMIs ranging from ≈ 60 to ≈ 1410 years. The method developed and tested involved the use of a histochemical stain which, when applied to embedded sections of bone, selectively stained collagenous proteins red and non-collagenous proteins green. Excess stain was rinsed from the section and the remaining stain was eluted from the sections and run through a Spectrophotometer at pre-specified optical density frequencies (540 and 605 nm) to yield an Absorbency value of the red and green dyes, respectively, from a given section. A standard curve was created such that a given OD value at a specific frequency (i.e. 540 or 605 nm) would correspond to a known concentration ofspecific frequency (i.e. 540 or 605 nm) would correspond to a known concentration of collagenous or non-collagenous proteins in the unknown section. The ratios of these concentrations were then calculated and statistical analysis was conducted to determine if a significant change or degradation in the ratio of bone protein concentrations occurred over time. Correlation and regression analyses were also performed to determine if the bone protein ratios were significantly correlated with time and, if so, how much of the correlation (or variance in the sample) could be contributed to this correlation. The results of the pilot study indicate that a statistically significant change took place over the time periods analyzed in the modem samples as well as the archaeological samples beyond ≈ 130 years. Additionally, while the rate of change in the archaeological sample beyond ≈ 130 years was more constant, a larger change in ratios of protein concentrations occurred in the first two months of the modem sample. The Pearson's correlation analyses performed revealed a significant negative correlation between the ratios of protein concentrations and time in both the modem and archaeological samples between ≈ 130 and ≈ 1410 years. However, the results of the regression analyses yielded only a moderate R² value, indicating that while time may have some predictive value in determining the age of a bone, other factors appear to have a significant effect in predicting the age of an unknown bone. In sum, while it is apparent that collagenous proteins are significantly correlated with time, other factors also clearly play a key role in the rate and process of bone degradation, or diagenesis. As a result, PMI estimations cannot currently be made solely upon the ratio of remaining collagenous and non-collagenous proteins in bone.
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