Hey everyone! It's Nicole from Kenhub again, and in this tutorial, we're going to be talking about the choroid plexus.
So we're going to begin this tutorial on the choroid plexus with our midsagittal section of the brain. And just to point out a few structures before we start, here in the forebrain, we have, of course, our cerebral cortex with its sulci; we have our corpus callosum here, our fornix here, and our thalamus and interthalamic adhesion just here. And those are all, of course, part of the forebrain and the midbrain which surrounds the choroid plexus.
So, the choroid plexus is part of the ventricular system of the brain which is the system responsible for the production and circulation of cerebrospinal fluid or CSF. And the choroid plexus is a richly innervated invagination of pia mater into the four ventricles of the brain and these are the paired lateral ventricles, the third ventricle, and the fourth ventricle shown down here in the cerebellum.
And as we return to our image of the choroid plexus, it's important to note that the choroid plexus consists of two components – the highly vascularized connective tissue of the pia mater and a covering or plexus of choroid epithelium. And this simple cuboidal epithelial covering is continuous with the ependyma – the glial cell membranous lining of the ventricles – who forms a blood-cerebrospinal fluid barrier which helps to control the brain's internal environment. And the primary function of the choroid plexus is synthesis of cerebrospinal fluid. And note that this image shows the choroid plexus of the third ventricle which is highlighted here in green and the choroid plexus of the fourth ventricle is in this area down here below. And in this image, we can see the choroid plexus of the lateral ventricle in green.
So, the choroid plexus is the site of continuous synthesis of cerebrospinal fluid in all four ventricles though it occurs primarily in the choroid plexus of the lateral ventricles. And the fluid is formed by a process involving diffusion and active transfer from the arterial blood supply of the pia mater which is then transferred from the vascular system of the choroid plexus through the choroid epithelium and into the ventricles. And we're going to have a little bit of a chat about the circulation and function of the cerebrospinal fluid over the next few slides.
So, once the cerebrospinal fluid is secreted into the lateral ventricle, the cerebrospinal fluid travels through the interventricular foramen – which just popped up in your screen in green for those of you who didn't see it – and through the interventricular foramen it travels into the third ventricle and then through the cerebral aqueduct into the fourth ventricle. And it leaves the ventricular system and enters the subarachnoid space through the three apertures of the fourth ventricle which we can see here – so two lateral apertures on both sides here and a single median aperture. And cerebrospinal fluid flows superiorly within the subarachnoid space and eventually passes into the dural venous sinuses for reabsorption into the venous system by the arachnoid villi.
And the choroid plexus-synthesized cerebrospinal fluid contains water and other plasma components, amino acids and glucose which nourishes brain tissue. It's presence in the subarachnoid space cushions the brain and provides shock absorbance in instances of mild to severe head trauma. Additionally, since the brain does not contain lymphatic vessels, cerebrospinal fluid acts as a conduit for the removal of waste products from the brain.
In addition to synthesizing cerebrospinal fluid, the choroid plexus forms a blood-cerebrospinal fluid barrier which helps to maintain homeostasis of the internal environment of the brain and the central nervous system in general especially during development. The blood-cerebrospinal fluid barrier which is composed of the choroid epithelium of the choroid plexus separates the blood flow around the brain from the cerebrospinal fluid. And this barrier is said to maintain ionic homeostasis because it controls the movement of small ions and other substances from the blood side of the barrier to the cerebrospinal fluid side.
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