The blood-brain barrier (BBB) is an important internal barrier. Herein, the electron microscope examination of duck BBB was performed during the brain development. Meanwhile, the genes/proteins of tight junctions (TJs) including zonula occludens-1, occludin, and claudin-5 in the duck brain were detected by Q-PCR and immunohistochemistry. The results showed the density of capillaries in the brain gradually increased during the embryonic period. The generation of the BBB and the specialization of its components occurred mainly in the embryonic stage. During this period, the endothelial cells (ECs) became thinner and pinocytic vesicles decreased; the TJs between EC membranes became longer and more electron-dense; the basement membrane surrounding ECs and pericytes gradually thickened; and the astrocyte foot processes appeared to wrap around the vessels. By the day of hatching (P1), the whole set of duck BBB structures was completely assembled and gradually improved in the subsequent growth process. Interestingly, compared with the cerebrum and cerebellum, the maturity level of the midbrain BBB was earlier seen during the embryonic stage. The expression of TJs increased during the embryonic period and remained stable by post-hatching. The study systematically investigated the histochemical and ultrastructural features of duck BBB during development and explored the corresponding relationship between structure and function.
