| Objective:It is a prior issue in forensic practice to estimate the time since injury and to distinguish ante-mortem and post-mortem injuries. Great efforts were applied to these areas. The research of time since injury is mainly focused on brain and skin. Since skin is the largest human organ which is directly contacted with the environment and is most likely to get hurt, its injury is the most common type in forensic practice. To study the relationship of Substance P (SP) and time since injury, we used model of rat skin injury, by applying immunohistochemistry technique and image analysis system to study the dynamic expression and the postmortem stability of SP at different time during incised rat skin healing processs. By doing so, we expect to find a novel and additional reference marker. Wound age estimation and to judge antemortem injuries postmortem injuries have been the key problems in the practice forensic medicine. And they are also the focus issue and hot problem in the forensic pathology's research .Nowadays the research about wound age estimation mainly on the brain and skin injurys, the skin is the largest organ of human and directly contact with the outside world. So the skin is most vulnerable to injury part of body. Therefore, the skin injurys are the most common injury in the practice forensic medicine. This experiment, we established the model of skin incision on the skin of rats. Using of immunohistochemical techniques, we observe the relation between the neuropeptide substance P (Substance P, SP) and the injury of time. We observe the changes of SP expression with injury in the healing process of the incised wounds on skin of rats. And the rats were put into different temperatures environment after they death. Then analyze the stability of SP expression in different environment. To provide a new and additional reference marker.Methods:In this experiment, the experimental rats were randomly divided into experimental groups and normal control group. The experimental groups were further divided into antemortem incisied group, postmortem incised group and postmortem stability groups at different environment temperature(-20℃, 4℃and 30℃). Rats were put to death at the arranged time point after the models were founded. Sample the skin at the side of incision, including the margin areas, by the size of 2 cm×2 cm. Using immunohistochemistry technique and image analysis system to study the dynamic expression and the postmortem stability of SP at different time during incised rat skin healing processs. Counting each slide for SP-positive cell rate from 5 different zones randomly choosed under 400×microscope, then calculating a mean value. The test data collected were analyzed by Excel and the statistical package of SPSS13.0 software.Results: Positive stain of SP was detected in the neutrophil cells 1h after injury and the SP-positive rate reached maximum on the 4th day, then it began to decrease. The expression of SP in the damage zone and the surrounding skin tissues was significantly higher than the normal control group for 8 days after injury(P <0.05); Weak SP-expressions, which have no obviously changes, in the epidermic cells, sweat gland cells and endothelial cells were detected in the postmortem injury groups and normal control groups. No significant differences of SP-expressions were found among each subgroups of postmortem stability group under the environment of -20℃, 4℃and 30℃degree, compare to the time immediately after death (P>0.05).Conclusion : The SP-expression during incised rat skin healing processs is time-dependent, and the SP is quite stable after death. So SP may be used as a useful reference marker to estimate the time since injury.
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