Figure 4.

The localisation of BDA3000 (biotin-dextran) in neocortex of opossums at P5 (A,B), P2 (C–E) and P12 (F) 20–25min (except A which is 40min) after an intraperitoneal injection. A and B are light micrographs of vibratome sections and C–F electron micrographs.

A) Staining is seen within blood vessels and towards the inner and outer surfaces of the cortex.

B) At high magnification it can be seen that two different pial vessels meet and grow into the brain together. Staining can only be seen inside the blood vessels and none in the surrounding tissue.

C) Cross section of a blood vessel which shows that the reaction product is abundant inside the lumen of the vessel, but is not visible in the immediately surrounding tissue. The arrowheads point to the intercellular clefts of the endothelial cell.

D) One of the interendothelial clefts from C (boxed in C) is shown at higher magnification. Arrowhead points to site of the tight junction.

E) Although vesicles that contain the reaction product (arrowhead) are present in the endothelial cells, these are not common.

F) A similar cleft to D at an older age (P12). Note in D and F that the reaction product is only present at the most luminal end of the cleft demonstrating that the tight junctions (arrowheads) between the endothelial cell restrict the movement of the tracer from blood into neural tissue.

Scale bars are 200µm in A, 25µm in B, 2µm in C, 200nm in D&E and 400nm in F.