Choroid Plexus

Choroid Plexus is a papillary, frond-like vascularized neuroepithelial organ suspended within each of the four cerebral ventricles—the paired lateral ventricles of the telencephalon, the midline third ventricle of the diencephalon, and the fourth ventricle of the hindbrain—that secretes the majority of human cerebrospinal fluid (CSF) and forms the selective blood–cerebrospinal fluid barrier (BCSFB). Weighing only ~2 to 3 grams in total in the adult human brain, the four choroid plexuses expand their luminal surface area via branching villi and dense apical microvilli to roughly 150 to 200 cm², secreting 450 to 600 mL of CSF per day so that the ~150 mL total cranial and spinal CSF volume turns over three to four times every 24 hours.

Beyond providing hydromechanical buoyancy that reduces the effective weight of the ~1,400 g human brain to ~25–50 g in situ, the choroid plexus acts as an endocrine neurodevelopmental signaling hub and metabolic gatekeeper. Choroid plexus epithelial cells synthesize transthyretin (TTR, transporting thyroxine across the CSF), folate-binding exosomes, and insulin-like growth factor 2 (IGF-2) to regulate progenitor division, while clearing catabolites and xenobiotics via polarized solute carriers and ATP-binding cassette efflux pumps.

Roof-Plate BMP/Wnt Specification

Following dorsal closure of the embryonic neural tube, midline roof-plate signaling centers secrete high concentrations of bone morphogenetic proteins (BMP2, BMP4, BMP7) and canonical Wnt ligands (Wnt1, Wnt3a). Whereas adjacent cortical neuroepithelium expresses FOXG1 and LHX2 to initiate cortical neurogenesis, dorsal midline progenitors exposed to sustained BMP4 and Wnt signaling activate OTX2, MSX1, LMX1A, and the motile-ciliogenesis factor FOXJ1, differentiating into non-neurogenic cuboidal choroid plexus epithelial cells expressing abundant transthyretin (TTR) mRNA alongside invaginating mesenchymal vascular loops.

Anatomical Inversion Relative to the Blood–Brain Barrier

Although both the parenchymal blood-brain barrier (BBB) and the ventricular blood–CSF barrier (BCSFB) isolate central nervous system fluids from peripheral blood plasma, the anatomical site of the tight-junction barrier is inverted between the two systems:

1. Fenestrated Stromal Capillaries: Unlike parenchymal cerebral capillaries—which are non-fenestrated and sealed by claudin-5—stromal capillaries inside each choroid plexus villus are fenestrated (bearing 60–80 nm transendothelial pores bridged by PLVAP diaphragms). Water, ions, and plasma proteins filter freely into the subepithelial stroma patrolled by resident stromal Kolmer macrophages ontogenetically related to microglia.
2. Apical Epithelial Tight-Junction Seal: Resting on a collagen IV/laminin basement membrane, the cuboidal choroid plexus epithelium forms the selective BCSFB via apical-lateral tight junctions (zonulae occludentes) composed of barrier-forming claudin-1, claudin-3, claudin-11, and occludin paired with cation- and water-permeable claudin-2, establishing an intermediate transepithelial electrical resistance (TEER ≈ 100 to 250 Ω·cm²) suited for rapid isotonic fluid secretion.

Inverted Apical Na⁺/K⁺-ATPase, Carbonic Anhydrase, and NKCC1

Unlike renal or intestinal absorptive epithelia—which localize the sodium pump to the basolateral membrane—choroid plexus epithelial cells display reversed functional polarity, concentrating Na⁺/K⁺-ATPase pumps, Na⁺–K⁺–2Cl⁻ cotransporter 1 (NKCC1 / SLC12A2), and aquaporin-1 (AQP1) water pores on the apical (CSF-facing) microvillar brush border, powered by abundant cytoplasmic mitochondria (~12–15% of cell volume).

Unidirectional blood-to-CSF secretion (~0.35–0.40 mL/min in humans) couples basolateral uptake to apical extrusion across epithelial ion channels and carriers:

• Basolateral Uptake and Carbonic Anhydrase: Basolateral NCBE (SLC4A10) imports Na⁺ while AE2 (SLC4A2) exchanges intracellular HCO₃⁻ for stromal Cl⁻, fueled by carbonic anhydrase II and XII catalyzing CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻. Inhibiting carbonic anhydrase with acetazolamide lowers CSF output by ~50% to treat idiopathic intracranial hypertension.
• Apical Extrusion and Coupled Water Transport: Apical Na⁺/K⁺-ATPase extrudes 3 Na⁺ into the ventricle per 2 K⁺ imported alongside electrogenic NBCe2 HCO₃⁻ efflux and osmotic water permeation through AQP1. Annette B. Steffensen, Nanna MacAulay, and colleagues (2018) demonstrated that outward-directed apical NKCC1 cotransports ~500–590 H₂O molecules stoichiometrically per ion-transport cycle, driving roughly half of mammalian CSF secretion. The resulting low-protein (~15–45 mg/dL) CSF clamps extracellular K⁺ (~2.9 mM) and Mg²⁺ (~1.15 mM) before flowing into subarachnoid and glymphatic pathways.

Morphogen Secretion and Xenobiotic Efflux

During fetal brain development, CSF-borne factors bathe primary cilia on ventricular radial glia. Maria K. Lehtinen, Christopher A. Walsh, and colleagues (2011) showed that choroid-plexus-secreted IGF-2 peaks during mid-corticogenesis to stimulate apical progenitor proliferation alongside TTR-thyroxine, FOLR1 folate vesicles, Sonic hedgehog, and Slit2. Simultaneously, OAT1/OAT3, OATP1A2, PEPT2, LRP1/LRP2, and ABC efflux pumps (P-gp / ABCB1, MRP1 / ABCC1, MRP4 / ABCC4) clear organic anions, β-lactam antibiotics, and soluble amyloid-β from CSF into blood.

Self-Organizing Human Choroid Plexus Organoids

Conventional unguided cortical organoids develop sporadic choroid-plexus-like epithelial patches along their periphery without forming sealed fluid compartments. In 2020, Laura Pellegrini, Madeline A. Lancaster, and colleagues generated self-organizing human choroid plexus organoids by pulsing three-dimensional cerebral organoid precursors derived from induced pluripotent stem cell lines with dorsalizing BMP4 and the GSK3β-inhibiting canonical Wnt activator CHIR99021 between days 10 and 15 of differentiation. Patterned organoids elaborate papillary, multiciliated TTR⁺/OTX2⁺/FOXJ1⁺/AQP1⁺ cuboidal epithelia sealed by continuous apical claudin-1/2/3/11 and ZO-1 tight junctions around large, self-contained fluid-filled cysts holding hundreds of microliters of colorless human in-organoid CSF (iCSF). Quantitative mass spectrometry confirmed that iCSF excludes 66-kDa serum albumin while secreting authentic human TTR, IGF-2, clusterin, and Niemann-Pick NPC2 proteins across fetal-to-postnatal maturation stages. These human barrier organoids quantitatively predict clinical unbound CSF-to-plasma drug partition coefficients (K_p,uu,CSF)—separating CNS-penetrant levodopa, bupropion, and diazepam from peripherally restricted atenolol and revealing slow intraventricular accumulation of the neurotoxic clinical candidate BIA 10-2474—while modeling ACE2/TMPRSS2-mediated SARS-CoV-2 neurotropism and tight-junction breakdown at the human blood–CSF barrier.

How does the blood–CSF barrier differ structurally from the blood–brain barrier?

At the parenchymal blood–brain barrier, non-fenestrated brain microvascular endothelial cells are sealed directly by high-resistance claudin-5 tight junctions (TEER ≈ 1,500–6,000 Ω·cm²). Inside the choroid plexus, stromal capillaries are fenestrated and permeable; the selective blood–CSF barrier is formed instead by apical claudin-1/2/3/11 tight junctions linking cuboidal choroid plexus epithelial cells (TEER ≈ 100–250 Ω·cm²).

How does acetazolamide reduce intracranial pressure?

Acetazolamide inhibits carbonic anhydrase II and XII inside choroid plexus epithelial cells, slowing intracellular conversion of CO₂ and H₂O into H⁺ and HCO₃⁻. Depleting intracellular HCO₃⁻ and H⁺ starves basolateral Na⁺ and Cl⁻ exchangers, cutting net apical ion and water transport into the ventricles by ~40% to 55%.

Why are human choroid plexus organoids useful for CNS drug screening?

Rodent choroid plexus differs from humans in efflux and uptake transporter abundances (P-gp, MRP1/4, OATPs), causing interspecies errors in CSF drug penetration estimates. Because human choroid plexus organoids form sealed tight-junction cysts containing syringe-accessible, authentic human CSF-like fluid, researchers can measure quantitative human blood-to-CSF drug permeability directly in vitro.

1. Lehtinen, M. K., Zappaterra, M. W., Chen, X., Yang, Y. J., Hill, A. D., Lun, M., ... & Walsh, C. A. (2011). The cerebrospinal fluid provides a proliferative niche for neural progenitor cells. Neuron, 69(5), 893–905. https://doi.org/10.1016/j.neuron.2011.01.023
2. Damkier, H. H., Brown, P. D., & Praetorius, J. (2013). Cerebrospinal fluid secretion by the choroid plexus. Physiological Reviews, 93(4), 1847–1892. https://doi.org/10.1152/physrev.00004.2013
3. Lun, M. P., Monuki, E. S., & Lehtinen, M. K. (2015). Development and functions of the choroid plexus–cerebrospinal fluid system. Nature Reviews Neuroscience, 16(8), 445–457. https://doi.org/10.1038/nrn3921
4. Steffensen, A. B., Oernbo, E. K., Stoica, A., Gerkau, N. J., Barbuskaite, D., Tritsaris, K., ... & MacAulay, N. (2018). Cotransporter-mediated water transport underlying cerebrospinal fluid formation. Nature Communications, 9(1), 2167. https://doi.org/10.1038/s41467-018-04677-9
5. Pellegrini, L., Bonfio, C., Chadwick, J., Begum, F., Skehel, M., & Lancaster, M. A. (2020). Human CNS barrier-forming organoids with cerebrospinal fluid production. Science, 369(6500), eaaz5626. https://doi.org/10.1126/science.aaz5626