Editing Choroid plexus (section)

=== Microanatomy ===
The choroid plexus consists of a layer of [[cuboidal epithelium|cuboidal epithelial cells]] surrounding a core of [[capillary|capillaries]] and [[Stroma (tissue)#Types|loose connective tissue]].<ref name="Lun"/> The [[epithelium]] of the choroid plexus is continuous with the [[ependymal cell]] layer (ventricular layer) that lines the ventricular system.<ref>{{cite book|vauthors=Javed K, Reddy V, Lui F |title= Neuroanatomy, Choroid Plexus|publisher= StatPearls|date=1 January 2022|pmid= 30844183|url= https://www.ncbi.nlm.nih.gov/books/NBK538156/}}</ref> Progenitor ependymal cells are monociliated but they [[Cellular differentiation|differentiate]] into multiciliated ependymal cells.<ref name="Delgehyr">{{cite book |last1=Delgehyr |first1=N |last2=Meunier |first2=A |last3=Faucourt |first3=M |last4=Bosch Grau |first4=M |last5=Strehl |first5=L |last6=Janke |first6=C |last7=Spassky |first7=N |chapter=Ependymal cell differentiation, from monociliated to multiciliated cells |title=Methods in Cilia & Flagella |series=Methods in Cell Biology |date=2015 |volume=127 |pages=19–35 |doi=10.1016/bs.mcb.2015.01.004 |pmid=25837384|isbn=9780128024515 }}</ref><ref name="pmid29437557">{{cite journal | vauthors = van Leeuwen LM, Evans RJ, Jim KK, Verboom T, Fang X, Bojarczuk A, Malicki J, Johnston SA, van der Sar AM | title = A transgenic zebrafish model for the in vivo study of the blood and choroid plexus brain barriers using claudin 5 | journal = Biology Open | volume = 7 | issue = 2 | pages = bio030494| date = February 2018 | pmid = 29437557 | pmc = 5861362 | doi = 10.1242/bio.030494 }}</ref> Unlike the ependyma, the choroid plexus epithelial layer has [[tight junctions]]<ref name="Hall">{{cite book|title=Guyton and Hall textbook of medical physiology|date=2011|publisher=Saunders/Elsevier|isbn=978-1-4160-4574-8|edition=12th|location=Philadelphia, Pa.|page=749|last1=Hall|first1=John}}</ref> between the cells on the side facing the ventricle (apical surface). These tight junctions prevent the majority of substances from crossing the cell layer into the cerebrospinal fluid (CSF); thus the choroid plexus acts as a blood–CSF barrier. The choroid plexus folds into many villi around each capillary, creating frond-like processes that project into the ventricles. The villi, along with a brush border of microvilli, greatly increase the surface area of the choroid plexus.{{citation needed|date=April 2017}} CSF is formed as plasma is filtered from the blood through the epithelial cells. Choroid plexus epithelial cells actively transport sodium ions into the ventricles and water follows the resulting osmotic gradient.<ref name=GUYTONHALL2005p764>{{cite book |first1=Arthur C. |last1=Guyton |first2=John Edward |last2=Hall | name-list-style = vanc |title=Textbook of medical physiology |year=2005 |publisher=W.B. Saunders |location=Philadelphia |isbn=978-0-7216-0240-0 |edition=11th |pages=764–7}}</ref>

The choroid plexus consists of many capillaries, separated from the ventricles by choroid epithelial cells. Fluid filters through these cells from blood to become cerebrospinal fluid. There is also much [[active transport]] of substances into, and out of, the CSF as it is made.{{cn|date=March 2025}}