Editing Choroid plexus (section)

===Blood–cerebrospinal fluid barrier===
{{See also|Glymphatic system}}
The '''blood–cerebrospinal fluid barrier'''<!--This term redirects here and is bolded per MOS:BOLD--> (BCSFB) is a fluid–brain barrier that is composed of a pair of membranes that separate blood from CSF at the capillary level and CSF from brain tissue.<ref name="BCSF" /> The blood–CSF boundary at the choroid plexus is a membrane composed of [[epithelial cells]] and [[tight junction]]s that link them.<ref name="BCSF" /> There is a CSF-brain barrier at the level of the pia mater, but only in the embryo.<ref>{{Cite journal|last1=Saunders|first1=Norman R.|last2=Habgood|first2=Mark D.|last3=Møllgård|first3=Kjeld|last4=Dziegielewska|first4=Katarzyna M.|date=2016-03-10|title=The biological significance of brain barrier mechanisms: help or hindrance in drug delivery to the central nervous system?|journal=F1000Research|volume=5|pages=313|doi=10.12688/f1000research.7378.1|issn=2046-1402|pmc=4786902|pmid=26998242|quote=The embryonic CSF-brain barrier, shown in Figure 1(f). In the ventricular zone is a temporary barrier between the CSF and brain parenchyma. In early brain development, strap junctions are present between adjacent neuroepithelial cells; these form a physical barrier restricting the movement of larger molecules, such as proteins, but not smaller molecules. At later stages of development and in the adult brain, these strap junctions are no longer present when this interface becomes ependyma. |doi-access=free }}</ref>

Similar to the [[blood–brain barrier]], the blood–CSF barrier functions to prevent the passage of most blood-borne substances into the brain, while selectively permitting the passage of specific substances (such as nutrients) into the brain and facilitating the removal of brain metabolites and metabolic products into the blood.<ref name="BCSF" /><ref name="BBB and BCSFB review">{{cite journal | vauthors = Ueno M, Chiba Y, Murakami R, Matsumoto K, Kawauchi M, Fujihara R | s2cid = 22154007 | title = Blood-brain barrier and blood-cerebrospinal fluid barrier in normal and pathological conditions | journal = Brain Tumor Pathology | volume = 33 | issue = 2 | pages = 89–96 | date = April 2016 | pmid = 26920424 | doi = 10.1007/s10014-016-0255-7}}</ref> Despite the similar function between the BBB and BCSFB, each facilitates the transport of different substances into the brain due to the distinctive structural characteristics of each of the two barrier systems.<ref name="BCSF">{{cite book|vauthors=Laterra J, Keep R, Betz LA |title=Basic Neurochemistry: Molecular, Cellular and Medical Aspects|date=1999|publisher=Lippincott-Raven|location=Philadelphia|edition=6th|section-url=https://www.ncbi.nlm.nih.gov/books/NBK27998/|section=Blood–cerebrospinal fluid barrier|display-authors=etal}}</ref> For a number of substances, the BCSFB is the primary site of entry into brain tissue.<ref name="BCSF" />

The blood–cerebrospinal fluid barrier has also been shown to modulate the entry of leukocytes from the blood to the central nervous system. The choroid plexus cells secrete [[cytokine]]s that recruit [[macrophage|monocyte-derived macrophages]], among other cells, to the brain. This cellular trafficking has implications both in normal brain homeostasis and in [[neuroimmune system|neuroinflammatory processes]].<ref>{{cite journal | vauthors = Schwartz M, Baruch K | title = The resolution of neuroinflammation in neurodegeneration: leukocyte recruitment via the choroid plexus | journal = The EMBO Journal | volume = 33 | issue = 1 | pages = 7–22 | date = January 2014 | pmid = 24357543 | pmc = 3990679 | doi = 10.1002/embj.201386609 }}</ref>