What is the rostrum of the corpus callosum in the cranium?

What is the Rostrum of the Corpus Callosum?

The rostrum is the most anterior and inferior segment of the corpus callosum, extending from the genu downward to the upper end of the lamina terminalis, consisting of a thick beaked segment anteriorly and a thin posterior portion called the lamina rostralis. 1

Anatomical Structure and Location

The rostrum comprises two distinct anatomical components that differ in thickness and orientation 1:

• The beaked segment: A thick anterior portion that projects downward from the genu of the corpus callosum 1
• The lamina rostralis: A thin posterior segment that blends with the lamina terminalis and transitions from a semivertical to semihorizontal orientation during fetal development 1

The entire corpus callosum measures approximately 7.8 cm in length from rostrum to splenium, with the rostrum representing the most inferior anterior extension 2.

Developmental Considerations

Contrary to older anatomical teaching, the rostrum is not the last segment of the corpus callosum to develop—the lamina rostralis portion is already present before the genu and splenium form, while the beaked segment develops concurrently with genu maturation. 1

Key developmental features include:

• The lamina rostralis undergoes significant reorientation from semivertical to semihorizontal position during fetal maturation 1
• In hypogenetic (partially formed) corpus callosum, the lamina rostralis often retains a semivertical orientation, indicating arrested development 1
• A rudimentary beaked segment can exist even without normal genu formation 1

Clinical Imaging Relevance

Corpus callosum segmentation on MRI divides the structure into seven subregions 3, 4:

1. Rostrum (most anterior-inferior)
2. Genu
3. Rostral body
4. Anterior midbody
5. Posterior midbody
6. Isthmus
7. Splenium (most posterior)

When evaluating corpus callosum pathology on MRI, the rostrum should be assessed using 3D T1 volumetric sequences to detect structural changes, thinning, or absence in congenital malformations. 5, 6

Common imaging pitfalls:

• The thin lamina rostralis can be mistaken for absence or hypoplasia on suboptimal imaging planes 1
• Proper midsagittal alignment is essential for accurate rostral assessment, as off-midline cuts may not capture this small structure 3
• The rostrum's connection to the lamina terminalis requires careful distinction from periventricular structures 1

Pathological Significance

Rostral abnormalities occur in partial agenesis of the corpus callosum, where the rostrum may be absent or malformed while other callosal segments remain intact. 6, 1

The rostrum contains small-diameter fibers that myelinate later in development, making this region potentially vulnerable to:

• Neurodegenerative processes affecting late-myelinating structures 4
• Developmental malformations when early embryonic formation is disrupted 6, 1
• White matter diseases that preferentially affect thin callosal segments 7