The blood-brain barrier maintains central nervous system homeostasis and limits the entry of blood-borne substances that could alter neuronal function and survival. The barrier exists predominantly at the endothelium of cerebral vascular microvessels. The cerebral vascular endothelium becomes highly specialized during the formation of the neurovascular unit early in embryonic development. The blood-brain barrier is present and functional early in fetal life. The tightness of the barrier gradually increases throughout gestation and in the newborn period. Alterations in the basolateral environment of the cerebral microvasculature can modify the blood-brain barrier properties by modulating the expression of the endothelial tight junctions and other biochemical properties of the cerebral vascular endothelium. Maturation of the blood-brain barrier late in gestation correlates with increases in endogenous corticosteroids and with exposure to exogenous corticosteroids. Several adverse fetal and neonatal conditions can alter the structure and function of the blood-brain barrier. Impairment of blood-brain barrier function in the perinatal period could increase the entry of bilirubin and other neurotoxic substances from the systemic circulation into the brain, thereby exacerbating and/or causing damage to the developing brain.

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