A rich capillary plexus develops in the connective tissue around the neural tube of the 6-8 mm embryo. The roofs of the prosencephalon and rhombencephalon are thin, composed of ependyma that becomes invaded by the capillaries. Ependyma plus pia mater is called tela choroidea; vascularized tela choroidea is choroid plexus (red).

In the adult, cerebrospinal fluid (CSF) is formed by choroid plexus in the 1) body and inferior horns of the lateral ventricles, 2) roof of the 3rd ventricle of the diencephalon, and 3) roof of the 4th ventricle in the medulla. Apertures in the 4th ventricle (foramina of Magendi and Luschka), which develop during the 3rd month, permit the flow of CSF from within the ventricles to subarachnoid spaces surrounding the brain and spinal cord.

The ventricle follows the cerebral hemisphere as it makes its “C”-shaped curve. The choroid fissure and its associated choroid plexus is at first on the thinned medial wall on the dorsal aspect of the developing hemisphere. Later, it continues into the roof of the medial aspect of the inferior horn of the lateral ventricle.

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