Quantitative determination of morphometric indices of the total and Perfused capillary network of the newborn pig brain

The purpose of this study was to determine if the newborn pig brain had a reserve of unperfused capillaries during normoxia. To accomplish this, a method was developed to determine the volume fraction, surface area, and number of both total and perfused capillaries in the newborn pig brain. Newborn pigs of either sex, 2-7 d old, were used. FITC-dextran, molecular weight 147 000, was used as a plasma marker to visualize the perfused capillaries. Alkaline phosphatase staining was used to stain the total capillary bed of the brain. Our results showed that FITC-dextran stayed within the vascular compartment, as it was not seen in areas that were not subsequently visualized with alkaline phosphatase staining. Eighty-four to 86% of the alkaline phosphatase-stained capillaries could be visualized with perfusion markers (india ink or FITC-dextran) in different brain regions. Similar results were obtained in two animals using a basement membrane stain, silver methenamine. The total volume fraction of capillaries (mm3/mm3) was cortex 0.055 ± 0.012, cerebellum 0.062 ± 0.011, and medulla 0.039 ± 0.012. Capillary surface area (mm2/mm3) of different brain regions averaged cortex 23.2 ± 1.8, cerebellum 24.8 ± 2.5, and medulla 15.8 ± 2.9. The total number of capillaries (per mm2) was cortex 375 ± 37, cerebellum 329 ± 37, and medulla 216 ± 32. The time course of filling of the capillaries indicated that approximately 50% were perfused at 6 s, which increased to over 80% at 12 s and remained unchanged thereafter. We found that FITC-dextran (molecular weight 147 000) was a suitable plasma marker to visualize the perfused capillaries, and alkaline phosphatase staining could mark the total capillary bed in the newborn pig brain. We conclude that the capillaries in the newborn pig brain fill rapidly and that this organ lacks a capillary reserve at rest.