N‐cadherin expression in endothelial cells during early angiogenesis in the eye and brain of the chicken: relation to blood–retina and blood–brain barrier development*

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Abstract
The factors responsible for the induction and maintenance of blood–brain barrier properties are still undefined. The process of blood–brain barrier formation is thought to take place in a two‐stage manner: the initial commitment of vascular sprouts by neuroectodermal cells may be followed by the stabilization of barrier properties. In the present study, we investigated the expression pattern of neural (N)‐cadherin during early angiogenesis in the brain and the pecten oculi of the chicken. The pecten has been introduced previously as a model for the investigation of the formation and maturation of barrier properties in the central nervous system. Whereas perineural and choroid vessels remained immunonegative for N‐cadherin, vascular sprouts invading both the brain and the pecten primordium acquired anti‐N‐cadherin immunoreactivity. Confocal laser scanning and immunoelectron microscopy indicated that the antigen was located at the ablumenal endothelial membrane in contact with subendothelial cells. With the onset of barrier differentiation as determined by junctional restriction of the tight junction protein occludin, N‐cadherin labelling rapidly decreased. Specific intraneuroectodermal upregulation and decline of endothelial N‐cadherin was confirmed by in situ hybridization and suggests that N‐cadherin expression by cerebral and pecteneal endothelial cells represents an initial and transient signal which may be involved in the commitment of early blood vessels to express blood–brain and blood–retina barrier properties.