Recent advances in peritoneal morphology: the milky spots in peritoneal dialysis

Milky spots are submesothelial lymphoid structures, essential for the maturation of resident peritoneal macrophages, for peritoneal defense, and for all peritoneal inflammatory and immune processes. We evaluated the number and size of milky spots in omentum of rats subjected to dialysis for 15, 30, and 60 days and in omentum of non dialyzed control rats (5 rats per group). After 15 days of dialysis, the number (4.2 +/- 1.5/cm2) and mean size (0.13 +/- 0.04 mm2) of milky spots were significantly lower than in the control group (7.6 +/- 2.3/cm2, p < 0.03; 0.25 +/- 0.04 mm2, p < 0.01). After 30 days of dialysis, values returned to a level similar to that in controls (6.8 +/- 1.9/cm2 and 0.20 +/- 0.04 mm2). After 60 days of dialysis, values were significantly greater than in all other groups (11.8 +/- 2.2/cm2 and 0.41 +/- 0.07 mm2, p < 0.03). The early decrease in milky spots seems to be due to washing of the peritoneum and replacement of resident white cells at the expense of the white cell population in the milky spots. At 30 days, a process of adaptation seems to establish functional equilibrium. The increase in milky spots after 60 days of dialysis may be due to the chronic inflammatory stimulus of dialysis solutions with poor biocompatibility.