Conditions that may determine blood vessel phenotype in tissues grafted to brain

The hypothesis, that a blood vessel's phenotype is determined by the tissue it vascularizes and not by the vessel's source, does not hold for tissue beyond a certain period of development. In mature skeletal muscle grafted to choroid plexus of adult and 2-week-old rats, some vessels were choroidal or fenestrated (FV) rather than, according to the hypothesis, continuous (CV), like those of muscle. In E14 fetal muscle placed on the choroid plexus, approximately 80% of the grafts' capillaries were CV, like those of muscle. Most choroidal FV that entered the grafts were apparently changed to CV. By E16, about 70% of the graft vessels remained as FV rather than being converted. Thus, FV were changed to CV by a hypothetical conversion factor made, apparently, by E14 grafts but not by E16 grafts. In grafts from [3H]thymidine-labeled donors, an appreciable number of CV in E14 grafts were identified as intrinsic 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 source of graft FV. Vascular endothelial growth factor (VEGF) might convert CV into FV, yet VEGF and the mRNA for its receptor were present in choroid plexus but not in the grafts. Therefore, VEGF is not a conversion factor. The purported factor that changes FV to CV may be expressed in E14 muscle grafts but diminishes by fetal age E16 and beyond.