How can we standardize peritoneal thickness measurements in experimental studies in rats?

The various methods of measuring peritoneal thickness in experimental studies in rats have yielded conflicting results. Also, no standard method exists to assess histologic findings in peritoneal morphology. We therefore undertook the present study to create a reproducible and standard method for assessing rat peritoneal histology in experimental studies.

Parietal peritoneal samples from 61 Wistar albino rats were used in the study. Excepting the skin, the whole abdominal wall from each rat was cut two-dimensionally (longitudinally and horizontally), fixed in formalin, and processed routinely for light microscopy. Slides were divided into two groups according to the direction of the inner abdominal muscle fibers in the sections. Longitudinal and horizontal sections of abdominal muscle were evaluated. For every section, one histopathology image was captured from a light microscope to an IBM-compatible computer. Peritoneal thickness (mean of the maximum and the minimum) and submesothelial area (SMA) were drawn on the image. A computer program then automatically performed measurements. Two different measurement methods were compared, based on the same sections.

The mean peritoneal thickness was 91 +/- 8 microm in the longitudinal sections and 75 +/- 7 microm in the horizontal sections (p < 0.05). Measurements of the SMA were found to be 47,762 +/- 4,374 microm2 for the longitudinal sections and 40,389 +/- 3,631 microm2 for the horizontal sections (p < 0.05). In both types of sections, a positive correlation (96% for longitudinal and 90% for horizontal) was found between the SMA and the peritoneal thickness (p < 0.01). The SMA measurements correlated significantly with functional properties [ratio of the dialysate concentration of glucose initially and after a 1-hour dwell (D1/D0 glucose), ultrafiltration, and protein loss; p < 0.01].

Peritoneal thickness can be measured as a mean of the minimum and maximum values. That measurement strongly correlates with submesothelial area. Both types of sections can be used, but the horizontal and longitudinal sections show systematic differences. All samples in a study should be taken using the same section pattern, either longitudinal or horizontal.