Blood-Brain Barrier and Glial Interfaces

The blood-brain barrier is the selective interface that separates the circulating blood from the central nervous system parenchyma. It is formed primarily by brain capillary endothelial cells joined by tight junctions, supported by pericytes, a basement membrane, and the end-feet of astrocytes. Together with neurons these elements form the neurovascular unit, in which glial cells help establish and regulate the barrier and couple blood flow to neural activity.

Definition

The blood-brain barrier is the selectively permeable interface, formed by tight-junction-coupled brain endothelial cells together with pericytes, basement membrane, and astrocyte end-feet, that controls the passage of substances between blood and the central nervous system.

Scope

This topic covers the cellular architecture of the blood-brain barrier and the glial interfaces of the central nervous system: the tight-junction endothelium, pericytes and basement membrane, astrocyte end-feet and the glia limitans, and the concept of the neurovascular unit including neurovascular coupling. It is reference-educational and does not give clinical guidance.

Core questions

• Which cells form the blood-brain barrier and what does each contribute?
• How do astrocyte end-feet relate to the barrier and to the glia limitans?
• What is the neurovascular unit?
• How is local blood flow coupled to neural activity?

Key concepts

• Blood-brain barrier
• Endothelial tight junctions
• Pericytes and basement membrane
• Astrocyte end-feet
• Glia limitans
• Neurovascular unit
• Neurovascular coupling

Mechanisms

Brain capillary endothelial cells are sealed by continuous tight junctions that greatly restrict paracellular passage, so most molecules must cross by regulated transcellular transport; pericytes embedded in the shared basement membrane and astrocyte end-feet that ensheath the vessel both contribute to inducing and maintaining these barrier properties (Daneman & Prat, 2015; Abbott et al., 2006). The astrocyte end-feet, together with the glia limitans they form at the brain surface, link the vasculature to the neural tissue. In the broader neurovascular unit, signaling among neurons, astrocytes, and vascular cells adjusts local blood flow to meet metabolic demand, a process termed neurovascular coupling (Iadecola, 2017; Attwell et al., 2010).

Clinical relevance

Blood-brain barrier integrity is central to understanding central nervous system drug delivery, edema, neuroinflammation, and stroke, and barrier breakdown features in many neurological conditions. This entry describes normal structure and physiology for educational reference and is not a basis for diagnosis or treatment.

History

The barrier was inferred from late nineteenth- and early twentieth-century dye experiments by Paul Ehrlich and Edwin Goldmann, which showed that dyes injected into the blood spared the brain while those injected into the cerebrospinal fluid stained it. Electron microscopy later localized the barrier to the tight-junction endothelium, and the contributions of pericytes and astrocyte end-feet were progressively recognized. The integrative concept of the neurovascular unit, reviewed by Iadecola (2017), reframed the barrier as one function of a coordinated cellular ensemble.

Key figures

• Paul Ehrlich
• Edwin Goldmann
• Costantino Iadecola

Seminal works

• daneman-2015
• abbott-2006
• iadecola-2017

Frequently asked questions

What actually forms the seal of the blood-brain barrier?

The seal is formed by tight junctions between brain capillary endothelial cells, which restrict movement of substances between the cells; pericytes, the basement membrane, and astrocyte end-feet help induce and maintain this property.

What is the neurovascular unit?

The neurovascular unit is the functional ensemble of neurons, astrocytes, and vascular cells (endothelium and pericytes) that together form the blood-brain barrier and adjust local blood flow to neural activity.