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Tela Choroidea of the Fourth Ventricle

Definition and Composition

The tela choroidea of the fourth ventricle is a thin, bilaminar membrane that forms the roof of the lower part of the fourth ventricle. It is composed of two layers fused together: an inner layer of ependymal cells (a single sheet of modified neuroepithelium) and an outer layer of pia mater with its associated blood vessels (vascular mesenchyme). The name comes from the Latin tela (web) and choroidea (membrane resembling the choroid of the eye), reflecting its delicate, vascular, web-like appearance.
  • Langman's Medical Embryology: "The roof plate of the myelencephalon consists of a single layer of ependymal cells covered by vascular mesenchyme, the pia mater. The two combined are known as the tela choroidea."
  • Medical Physiology: "The tela choroidea consists of a layer of ependymal cells covered by the pia mater and its associated blood vessels."

Embryological Development

During development, the roof plate of the myelencephalon (the caudal part of the hindbrain that gives rise to the medulla oblongata) remains extremely thin, retaining only a single layer of ependymal cells. This region does not generate neurons or glia in the same way as the rest of the neural tube. Instead, vascular mesenchyme from the overlying pia mater proliferates actively and closely invests this ependymal layer, forming the tela choroidea.
Because of active proliferation of the vascular mesenchyme, saclike invaginations project downward into the underlying ventricular cavity. These tuft-like infoldings of choroidal arteries of the pia differentiate into the choroid plexus of the fourth ventricle - the structure ultimately responsible for cerebrospinal fluid (CSF) production.
As summarized in The Developing Human: "This vascular membrane, together with the ependymal roof, forms the tela choroidea, the sheet of pia covering the lower part of the fourth ventricle. Because of the active proliferation of the pia, the tela choroidea invaginates the fourth ventricle, where it differentiates into the choroid plexus, infoldings of choroidal arteries of the pia."
An analogous process occurs at the roof of the third ventricle and the medial walls of the lateral ventricles, giving rise to the choroid plexuses of those ventricles as well.

Anatomical Relations

The fourth ventricle is a tent- or diamond-shaped CSF space located in the posterior fossa. Its floor is formed by the pons and medulla oblongata, and its roof is formed superiorly by the cerebellum (superior medullary velum) and inferiorly by the tela choroidea (inferior medullary velum + tela choroidea proper). The choroid plexus derived from the tela choroidea lies along only a portion of this tent-shaped roof and extends laterally into the foramina of Luschka, often projecting into the subarachnoid space.

Role in CSF Dynamics

The choroid plexus arising from the tela choroidea is the main CSF-producing structure of the fourth ventricle. The choroid epithelial cells are specialized ependymal cells with an apical border of microvilli and cilia projecting into the ventricular cavity. They are cuboidal in shape, bound by tight junctions, and receive a rich blood supply from the anterior and posterior choroidal arteries - blood flow per unit mass is approximately 10 times the average cerebral blood flow.
These cells secrete CSF in an isosmotic fashion, primarily driven by active Na⁺/K⁺-ATPase transport. The ventricular fluid produced is supplemented by contributions from the brain and spinal cord surfaces and the pia-arachnoid, eventually becoming CSF.
CSF then exits the fourth ventricle through three openings that arise because the thin roof evaginates in three locations and the resulting pouches rupture:
  • The single foramen of Magendie (median aperture) - in the midline roof
  • The paired foramina of Luschka (lateral apertures) - at the lateral recesses
Through these foramina, CSF enters the subarachnoid space (particularly the cisterna magna below the cerebellum), circulates around the brain and spinal cord, and is ultimately reabsorbed through arachnoid granulations (villi) projecting into the dural venous sinuses.

Signaling and Development

Beyond CSF production, the choroid plexus and CSF within the developing brain contain various signaling morphogens - including retinoic acid - that control the proliferation and differentiation of neuroprogenitor cells. This highlights that the tela choroidea and its derivative choroid plexus are not merely passive CSF factories but are active participants in brain development.

Clinical Relevance

  • Colloid cysts: These benign but potentially dangerous cysts (most commonly at the foramen of Monro in the third ventricle) originate from the infolding neuroepithelium of the tela choroidea.
  • Choroid plexus papillomas/carcinomas: Tumors arising from the choroid plexus epithelium, which is derived from the tela choroidea.
  • Hydrocephalus: Obstruction at the level of the fourth ventricle foramina (Magendie or Luschka) disrupts CSF egress from the ventricular system, leading to communicating or obstructive hydrocephalus. A developmental failure of these apertures to open is linked to Dandy-Walker malformation.
  • NF2 (Neurofibromatosis type 2): The NF2 gene promoter is notably active in the ependymal cell-containing tela choroidea, as well as in acoustic/trigeminal ganglia and the retinal pigmented epithelium.

Summary

The tela choroidea of the fourth ventricle is a delicate but functionally critical membrane. It is a bilateral fold of pia mater fused with ependyma that roofs the lower fourth ventricle, and it gives rise - through active vascular proliferation and invagination - to the choroid plexus. This structure secretes CSF, expresses developmental morphogens, and forms the apertures by which CSF exits the ventricular system into the subarachnoid space. Disruptions in its normal formation or function underlie several important neurological conditions.
Sources:
  • Langman's Medical Embryology (Myelencephalon)
  • The Developing Human: Clinically Oriented Embryology (Choroid Plexuses and CSF)
  • Medical Physiology (CSF fills the ventricles; Epithelial cells of the choroid plexus secrete the CSF)
  • Neuroanatomy through Clinical Cases, 3rd Ed. (CSF and Ventricular System)
  • Bradley and Daroff's Neurology in Clinical Practice (Extra-axial Brain Tumors; CSF Circulation Disorders)
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