MACROMOLECULAR PERMEABILITY OF CHICK WING MICROVESSELS - AN INTRAVITAL CONFOCAL STUDY

The development of the vertebrate limb requires the formation of a nor mal vasculature to nurture the soft and hard tissue phenotypes. The pa ttern of embryonic limb bud vessels has been extensively studied, but little is known about the permeability characteristics of the developi ng circulation. In the present study, the microvascular endothelial ce ll phenotype was examined by in vivo confocal microscopy following the systemic injection of a graded series of fluorescent dextrans (40,000 , 70,000, 150,000 molecular weight) into chick embryos at stages 21-23 in order to determine how selective is the endothelial lining of micr ovessels as a partition between the blood vessels and the interstitium . Videodensitometry, over a gray scale range of 0-255, was used to qua ntitate the amount of tracer found within the interstitial compartment of the limb. The tracers of larger molecular weight (70,000, 150,000) were confined exclusively to the vascular lumina, whereas that of sma ller molecular weight (40,000) was found to cause perivascular brighte ning due to extravasation into the surrounding interstitium. The repor ted differences in permeability were not dependent upon the stage of t he embryo used in this study, but were due to the size of the tracer. These data indicate that embryonic wing microvessels demonstrate perms electivity to macromolecular efflux across the endothelium. The presen t results provide a basis for additional studies concerned with the dy namic characteristics of the limb microvasculature and challenge our c oncepts about the role of diffusible morphogens in vertebrate limb dev elopment.