Developmental changes of HT7 expression in the microvessels of the chick embryo brain.

The neurothelin/HT7 antigen is a chick-specific, cell-surface glycoprotein expressed by the brain endothelial cells and widely utilized in experimental studies as a marker of barrier-provided vessels. Previous immunohistochemical studies have demonstrated that HT7 is already expressed in the embryonic brain vessels and that it is first detectable on embryonic day 10 and developmentally regulated. In the present study, the vascular expression of HT7 was investigated in different regions of the central nervous system from the 5th day right up to the latest stage of the chick embryo development. The study was carried out utilizing a monoclonal antibody anti-HT7, which was detected by both enzymatic and fluorescent immunohistochemical methods. The observations demonstrated the presence of HT7-stained vessels as early as embryonic day 6 in the rhombencephalon and mesencephalon, and at embryonic day 9 in the prosencephalon. Regional differences were also evidenced within the rhombencephalon and mesencephalon, since the endothelial antigen HT7 was expressed earlier in the brain stem (tegmentum of the medulla oblongata, pons and mesencephalon) than in the cerebellum and optic tectum. The caudo-cranial and ventro-dorsal gradients of HT7 expression were temporally and spatially related to the development of the brain vessels. The early detection of HT7 staining in the choroid plexus epithelium and perineural vessel endothelium, sites of the blood-cerebrospinal fluid barrier and pial barrier, respectively, has confirmed this antigen to be a precocious marker for all the barrier systems in the brain.