Tela Choroidea

Definition

• Medical Physiology (Boron & Boulpaep): "The tela choroidea consists of a layer of ependymal cells covered by the pia mater and its associated blood vessels."
• Langman's Medical Embryology: "The roof plate...consists of a single layer of ependymal cells covered by vascular mesenchyme, the pia mater. The two combined are known as the tela choroidea."

Composition

1. Ependymal layer - the inner epithelial cells lining the ventricle
2. Pia mater layer - the vascular connective tissue membrane derived from mesenchyme, carrying blood vessels

Locations in the Brain

Function - Formation of the Choroid Plexus

• The pia mater proliferates actively
• It invaginates through the ependymal roof into the ventricular cavity
• Saclike, tuft-like projections (called the choroid plexus) form inside the ventricle
• These choroid plexus tufts secrete CSF

CSF Production

1. Flows through the ventricular system (lateral --> third --> fourth ventricles via the cerebral aqueduct)
2. Exits the fourth ventricle through the foramen of Magendie (median aperture) and foramina of Luschka (lateral apertures)
3. Enters the subarachnoid space
4. Is absorbed into the venous system via arachnoid villi projecting into the dural venous sinuses

Clinical Relevance

• Colloid cysts arise from the infolding neuroepithelium of the tela choroidea and are located almost exclusively in the anterior third of the third ventricle at the level of the foramen of Monro (per Bradley and Daroff's Neurology in Clinical Practice).
• Tumors such as choroid plexus papillomas originate from the epithelium derived from the tela choroidea.
• The tela choroidea is also a site of NF2 (neurofibromatosis type 2) gene promoter expression, as the ependymal cells of the tela choroidea contain NF2-expressing cells.

Summary

Roof of the 4th Ventricle

Overall Shape

Components of the Roof

1. Superior (Upper) Part

• A thin sheet of white matter (myelinated nerve fibers) stretching between the two superior cerebellar peduncles
• Bridges the gap between the pons and the cerebellum in the upper portion
• The trochlear nerve (CN IV) - the only cranial nerve to emerge from the dorsal surface of the brainstem - decussates within and then exits from the superior medullary velum
• Clinically: infiltration by medulloblastoma can cause bilateral CN IV palsies

2. Posterior/Middle Part

• The anterior (inferior) surface of the cerebellum - specifically its vermis - forms the posterior roof
• The lingula of the cerebellar vermis is attached to the superior medullary velum
• The nodule of the vermis is attached inferiorly to the inferior medullary velum
• Localization in Clinical Neurology: "the cerebellum forms the roof of the fourth ventricle"

3. Inferior (Lower) Part

• A thin, paired (bilateral) crescentic sheet of white matter
• Attached to the nodule of the cerebellar vermis above and to the tela choroidea below
• Much thinner than the superior velum; some sources describe it as partly membranous

• A fold of pia mater fused with the ependymal lining
• Forms the roof of the lower (caudal) half of the 4th ventricle
• Contains the choroid plexus of the 4th ventricle, which hangs into the ventricular cavity
• The choroid plexus here is supplied by branches of PICA (posterior inferior cerebellar artery), AICA, and SCA (Grainger & Allison's)

Apertures (Openings) in the Roof

Summary Table

Clinical Relevance

• Medulloblastoma: arises from undifferentiated neuroectodermal cells of the roof (superior or inferior medullary velum and vermis); most common brain tumor in children (Bradley & Daroff's Neurology)
• Dandy-Walker malformation: ballooning of the posterior half of the 4th ventricle, often with non-patency of the foramen of Magendie, aplastic posterior vermis, and hydrocephalus
• Blake pouch cyst: derived from the posterior membranous area of the roof; normally communicates with subarachnoid space by 26 weeks gestation
• CN IV palsy: lesions of the superior medullary velum (tumor, trauma, demyelination) cause trochlear nerve injury since it decussates there

Image 1 - MRI of Cerebellum (Sagittal & Axial)

• 1 = Lingula (attached to SMV)
• 2 = Central lobule
• 3 = Culmen
• 4 = Declive
• 5 = Folium
• 6 = Tuber
• 7 = Pyramid
• 8 = Uvula
• 9 = Nodule (attached to inferior medullary velum)

Image 2 - MRI & CT showing Ventricular Apertures

Key Structures of the Roof (summarized from images)

Spinal Tracts

Classification

A. ASCENDING TRACTS (Sensory - carry signals UP to brain)

1. Anterolateral Pathways (pain, temperature, crude touch)

• 1st neuron: cell body in spinal (dorsal root) ganglion → enters posterior horn
• Axon collaterals ascend/descend 1-2 segments in Lissauer's tract before synapsing
• 2nd neuron: crosses obliquely over 2-3 segments via the anterior commissure to the contralateral anterolateral column → ascends to the thalamus
• 3rd neuron: thalamus → primary somatosensory cortex via posterior limb of internal capsule

2. Posterior Column - Medial Lemniscal Pathway (fine touch, vibration, proprioception)

• 1st neuron: spinal ganglion → enters posterior root → ascends ipsilaterally to caudal medulla
• Synapses at nucleus gracilis / nucleus cuneatus (caudal medulla)
• 2nd neuron: axons cross midline as internal arcuate fibers → form the medial lemniscus → ascend to VPL thalamus
• 3rd neuron: thalamus → somatosensory cortex

3. Spinocerebellar Tracts (subconscious proprioception to cerebellum)

B. DESCENDING TRACTS (Motor - carry signals DOWN from brain)

1. Lateral Motor System (controls limb movements)

• Cell bodies in primary motor cortex → corona radiata → posterior limb of internal capsule → crus cerebri (midbrain) → through pons → forms the pyramid in medulla
• At caudal medulla: ~90% of fibers decussate (pyramidal decussation) → form lateral corticospinal tract in contralateral lateral column
• Remaining ~10% stay ipsilateral → form anterior corticospinal tract
• Synapse on LMN cell bodies in the lateral anterior horn

2. Medial Motor System (controls axial/postural muscles)

Key Clinical Points

• Gray's Anatomy for Students, pp. e30-e32

Transverse Section at the Level of Pyramidal Decussation

Fig. C - Cervicomedullary Junction (THE Pyramidal Decussation Level)

DORSAL (Posterior) Region:

LATERAL Region:

VENTRAL (Anterior) Region:

For Comparison - Fig. B (Caudal Medulla, just above decussation):

• Pyramid is intact ventrally (not yet decussating)
• Medial lemniscus and sensory decussation (internal arcuate fibers) visible near the midline - this is where the posterior column 2nd neurons cross
• Inferior olivary nucleus (principal + medial accessory) large folded structure ventrally - disappears at decussation level
• Hypoglossal nucleus (CN XII) dorsomedially
• Nucleus ambiguus (CN IX, X, XI) in lateral tegmentum
• Solitary nucleus/tract (CN VII, IX, X) dorsolaterally

Key Difference: Pyramidal Decussation vs. Caudal Medulla

• Neuroanatomy through Clinical Cases (3rd Ed.), Fig. 14.5 (A-D)