THE DEVELOPMENT OF THE BLOOD-BRAIN-BARRIER IN THE CHICK - STUDIES WITH EVANS BLUE AND HORSERADISH-PEROXIDASE

The development of the blood-brain barrier was microscopically examined in the optic tectum of the chick. The permeability of neural vessels to Evans blue and horseradish peroxidase decreases progressively during the period of incubation. Diffusion is massive on the 6th and 10th days of incubation and is reduced on the 14th day; on the 18th-21st day of incubation the vascular walls still allow Evans blue to diffuse but prevent extravasation of horseradish peroxidase completely. In one month old chickens the nervous substrate is free of both tracers.