CONDITIONS THAT MAY DETERMINE BLOOD-VESSEL PHENOTYPE IN TISSUES GRAFTED TO BRAIN

The hypothesis, that a blood vessel's phenotype is determined by the t issue it vascularizes and not by the vessel's source, does not hold fo r tissue beyond a certain period of development. In mature skeletal mu scle grafted to choroid plexus of adult and 2-week-old rats, some vess els were choroidal or fenestrated (FV) rather than, according to the h ypothesis, continuous (CV), like those of muscle. In E14 fetal muscle placed on the choroid plexus, similar to 80% of the grafts' capillarie s were CV, like those of muscle. Most choroidal FV that entered the gr afts were apparently changed to CV. By E16, about 70% of the graft ves sels remained as FV rather than being converted. Thus, FV were changed to CV by a hypothetical conversion factor made, apparently, by E14 gr afts but not by E16 grafts. In grafts from [H-3]thymidine-labeled dono rs, an appreciable number of CV in E14 grafts were identified as intri nsic to the muscle. When hosts were labeled to verify the origin of FV in their nonlabeled donor grafts, only a few FV, to date, were tagged . The FV must have come from host choroid plexus, the only available s ource of graft FV. Vascular endothelial growth factor (VEGF) might con vert CV into FV, yet VEGF and the mRNA for its receptor were present i n choroid plexus but not in the grafts. Therefore, VEGF is not a conve rsion factor. The purported factor that changes FV to CV may be expres sed in E14 muscle grafts but diminishes by fetal age E16 and beyond. ( C) 1995 Academic Press, Inc.