Development of blood-cerebrospinal fluid barrier to horseradish peroxidase in the avian choroidal epithelium.

The penetration of horseradish peroxidase (HRP) from blood into ventricle via the avian choroid plexus was examined by electron microscopy in order to study the development of the blood-cerebrospinal fluid barrier (BCSFB). 1-20 mg HRP was injected into the allantoic vein of chick embryos between the 7th and 21st day of incubation. Until the 8th day (stage 34) the reaction products of injected HRP were observed in the interepithelial clefts at both luminal (ventricular) and abluminal sides. At the 9th day (stage 35), their penetration was blocked at most apical junctional complexes of the choroidal epithelia. At the 10th day (stage 36) and at every subsequent stage, HRP molecules were completely impeded at the apical tight junctions. These results show that the BCSFB to HRP in the avian choroidal epithelium is completely established by the 10th day, when molecules of HRP are still able to permeate between the capillary endothelia in the cerebellum and the spinal cord (Wakai and Hirokawa 1978).