Tissue barriers define those internal and external compartments in which multicellular organisms can develop. Basically, tissue barriers consist of a simple epithelial sheet of structurally and functionally polarized cells and are shared by all metazoans, regardless of their diversity of body plans and lifestyles. Development and maintenance of polarized epithelia was a very early and sine qua non condition in metazoan evolution. Our research work focuses on the anatomical and molecular organization of some, very special barriers, such as the blood-brain barrier in embryonic development. Special attention is given to the construction and function of the barriers during development in the mammalian model.