CSF and BBB: Lecture 4

The Choroid Epithelium and CSF Formation

• The choroid epithelium forms a selective tight-junction barrier that regulates the passage of ions and molecules across the epithelial cell barrier.
• The choroid epithelium controls the composition of CSF that enters the ventricles.
• CSF is created from blood plasma, consisting of 99% water, different ions, and almost no proteins.
• Micronutrients such as Vit B6, folates, and Vit C are present in CSF.

CSF Formation and Circulation

• Water in choroid epithelial cells combines with intracellular CO2 to form H+ and HCO3-.
• H+ is exchanged for extracellular Na+ from the blood at the basal surface.
• Na+ is pumped out through the apical surface into the ventricles.
• Cl- and HCO3- move into the ventricles to neutralize the excess positive charge.
• Water diffuses into the ventricles to maintain osmotic balance.

Functions of CSF

• CSF protects the brain and spinal cord against impact upon their surrounding bony walls.
• CSF acts as an effective waste control system, removing potentially harmful cellular metabolites.
• CSF acts as a distribution system for some peptide hormones and growth factors.
• CSF maintains a consistent extracellular microenvironment for neurons and glia.

Ventricular System and CSF Circulation

• The ventricular system consists of four expanded regions forming ventricles, interconnecting foramina, and tubes.
• Lateral ventricles are oriented longitudinally in each hemisphere and connected with the third ventricle.
• The third ventricle is connected with the fourth ventricle through the mesencephalic aqueduct.
• The fourth ventricle is connected with the subarachnoid space and central canal of the spinal cord.
• CSF flows from the ventricles, through the interventricular foramina, into the subarachnoid space, and is absorbed into the venous sinuses through arachnoid villi.

Barriers and Transport

• Barriers exist between the blood and CSF, and the brain fluid, with different concentrations of constituents.
• Many larger molecules hardly pass from the blood into the CSF or interstitial fluids of the brain.
• Astrocytes form perivascular end feet around the capillary endothelium, interacting with endothelial cells to maintain the blood-brain barrier (BBB).
• The BBB has specific carrier molecules for transport, and is highly permeable to water, CO2, O2, and lipid-soluble substances.
• Glucose and large neutral amino acids use facilitated diffusion to cross the BBB.

Meninges and Ventricular System

• The brain and spinal cord are surrounded by three layers of connective tissue called meninges: dura mater, arachnoid, and pia mater.
• The dura mater is the outermost meningeal layer, fused with the inner surface of the skull.
• The arachnoid is a thin, loose connective tissue layer with a spiderweb appearance.
• The pia mater is the innermost meningeal layer, adhering to the surface of the CNS.
• The ventricular system is lined by ependymal cells, which create CSF flow through coordinated cilia beating.

Hydrocephalus and CSF Circulation

• Hydrocephalus is an abnormal dilation of the ventricular system from an increased volume of CSF.
• CSF can flow from the subarachnoid space to the venous outflow system, but not in the reverse direction.
• The rate of CSF formation, flow, and absorption are sufficiently high to cause its replacement several times daily.