POSTNATAL REMODELING OF THE LEPTOMENINGEAL VASCULAR NETWORK AS ASSESSED BY INTRAVITAL FLUORESCENCE VIDEO-MICROSCOPY IN THE RAT

An intriguing characteristic of the ontogenic development of the cereb ral vasculature is the rapid differentiation of the neonatal leptomeni ngeal vascular plexus into the mature, adult network form. The physiol ogical and cellular mechanisms of this cerebrovascular remodeling proc ess are unclear. The objective of this work was to determine and corre late changes in vascular density, network pattern and flow velocity in leptomeningeal microvessels of the rat during postnatal development i n vivo. To this end, microvascular diameter, segment length, and vascu lar density of reconstructed leptomeningeal networks were measured fro m video-recordings of the microcirculation visualized through a crania l window in 0-15-day-old Sprague-Dawley rats. The velocity of erythroc ytes in the microvessels was measured by frame to frame tracking of fl uorescently labeled red blood cells. We found that surface vascular de nsity (total vessel length per area), node density and segment density (object per area) decreased significantly by the second week after bi rth. Anastomosing vascular polygons, characteristic to newborn network s, became less numerous and larger in diameter during the postnatal 2- week period, indicating progressive rarefaction of the networks. Vesse l diameter and red cell velocity showed transient increases at 1.5 wee ks. The velocity/diameter ratio (V/D), an index of wall shear rate, in creased by the age of 1.5 weeks and remained unchanged afterwards. The re was a negative correlation between V/D and diameter at 1 week; this relationship was reversed to a positive correlation at 2 weeks. We co nclude that postnatal remodeling of the leptomeningeal vascular networ k is associated with rarefaction and an adaptation of vessel caliber t o wall shear rate. These changes may contribute to arterio-venous diff erentiation and redistribution of blood flow from the superficial to t he intracortical vasculature in the developing brain.