Reliability of Placental Scar Counts in the Prairie Vole

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Abstract
Numbers of placental scars in fresh, fixed, and cleared uterine tracts were compared to known numbers of young from single- and multiple-bred female prairie voles (Microtus ochrogaster). The number of dark, distinct scars observed from the 5th through the 40th day postpartum correlated well with the number of young. From the 41st through the 70th day, fewer scars than young born were observed. At 80 days most scars were again discernible. Histological evaluation revealed that the lower correlations of scars with known young were explained by changes in the number, density and disposition of pigment-laden macrophages as they migrated through uterine tissue layers toward the mesometrium. Macrophage foci were obscured as they migrated through muscle layers, but were seen as distinct scars when in association with venous complexes between the muscle layers and within the mesometrium. Mesometrial scars appeared smaller and lighter with time but were still apparent at 200 days postpartum. If the approximate time of parturition is known, the time of sacrifice and the method of tissue preparation can be chosen to maximize the visibility of scars and obtain reliable estimates of total numbers of implanted young even in polyestrous species.