THE MICROVASCULAR SYSTEM IN ISCHEMIC CORTICAL-LESIONS

In the rat, normal blood flow can be restored in the territory of the occluded artery after an arterial occlusion. This event has been attri buted to changes in the collateral vessels supplying the territory of the occluded artery. Since only a limited amount of data is available about the plasticity of the microvascular system after a cortical isch emic lesion, in the present study we have evaluated whether the restor ation of blood flow to normal levels in the territory of the middle ce rebral artery after permanent ischemia is due only to flow through pre existing collateral vessels or also to the development of new microves sels. Middle cerebral artery occlusion was performed in 45 rats. After 24 h of ischemia, magnetic resonance imaging was used to select 16 ra ts with cortical lesions of similar size and location. After 2 weeks, vascular corrosion casts were obtained from 8 rats by injection of low -viscosity resin and observed by scanning electron microscopy. A corre lative light and electron microscopy study was performed using the rem aining 8 rats. Two different patterns of vascular modifications were f ound, one dorsal and one ventral to the lesion. The dorsal portion of the lesion was vascularized by collateral arteries originating from th e anterior or posterior cerebral arteries. Collateral trunks showed a meandering course, mainly in the occipital pole. In the ventral portio n of the lesion a complex microvascular system was found characterized by an intense vascular proliferation. The arterioles showed a paralle l, candelabrum-like pattern with dichotomic branching. Contraction rin gs were frequently seen. The capillaries showed a sinusoid-like struct ure, with a large lumen and a continuous endothelium with many micropi nocytotic vesicles. A peripheral ring-shaped venous sinus was composed of a network of flat vessels. These results give the first comprehens ive description of the microvascular modifications in a focal model of infarct and suggest that the restoration of blood flow to normal leve ls described in the territory of the middle cerebral artery after perm anent ischemia may be due not only to flow through collateral vessels but also to the development of a new vascular system originating mainl y from branches of the middle cerebral artery before the occlusion poi nt.