Ultrastructural basis of the vessel wall differentiation in the chick embryo optic tectum.

The mode and the temporal sequence of the growth and differentiation processes of the intraneural blood vessels have been ultrastructurally analyzed in optic tecta of 4, 8, 12, 16 and 19 incubation day chick embryos and of chicks 15 days after hatching. The findings suggest that the morphological maturation of the various components of the vascular wall and perivascular spaces, leading to the formation of the blood-brain barrier, unfold according to a spatio-temporal gradient, radially extending from the surface of the tectum rudiment.