From the Guidelines

The choroid plexus is a small, fluid-filled structure within the choroid of the lateral ventricles of the brain.

Located in the lateral ventricles of the brain
Appears as echolucent cysts within the echogenic choroid on ultrasound
May be single or multiple, unilateral or bilateral, and most often are <1 cm in diameter
Identified in approximately 1% to 2% of fetuses in the second trimester of pregnancy

Associated with trisomy 18, but also commonly found in fetuses with euploidy
Presence of a choroid plexus cyst does not alter the risk of trisomy 21 1
Neurodevelopmental outcomes in children with euploidy born after a prenatal diagnosis of choroid plexus cysts have not shown differences in neurocognitive ability, motor function, or behavior 1

Ultrasound characteristics of choroid plexus cysts (size, complexity, laterality, and persistence) should not be used to modify risk further 1
Choroid plexus tumors can arise in association with the ventricular system (intraventricular/paraventricular) 1

From the Research

The choroid plexus is an epithelial-endothelial vascular convolute within the ventricular system of the vertebrate brain 2.
It consists of epithelial cells, fenestrated blood vessels, and the stroma, which may contain various cell types and a rich extracellular matrix 2.
The choroid plexus is mainly involved in the production of cerebrospinal fluid (CSF) by using the free access to the blood compartment of the leaky vessels 2.

The choroid plexus is responsible for producing most of the CSF that fills the brain ventricles and the subarachnoid space, which is renewed approximately 2-3 times daily 3.
It contains an impressive number of receptors and transporters, both in the apical and basolateral sides of the epithelial cells, and is a key structure for the communication between the brain and the periphery 3.
The choroid plexus plays a role in regulating brain metabolism and function, and its activity influences brain development and function 4, 5.

The choroid plexus is involved in the regulation of neural stem cell behavior and neurogenesis during brain development 6.
It is a site of origin of extrinsic regulation of neural stem cells, and regulated secretion of signaling molecules from the choroid plexus into CSF can regulate neural stem cell behavior 6.
The choroid plexus is involved in various neurological disorders, including neurodegenerative, inflammatory, infectious, traumatic, neoplastic, and systemic diseases 2, 3.