Corpus callosum

The corpus callosum (plural: corpora callosa) is the largest of the commissural fibers, linking the cerebral cortex of the left and right cerebral hemispheres. It is the largest white matter tract in the brain.

Summary

• located inferior to the cerebral cortices, and superior to the thalamus
• connects left and right cerebral hemispheres
• majority of vascular supply from the pericallosal arteries

Gross anatomy

The corpus callosum is approximately 10 cm in length and is C-shaped (like most supratentorial structures) in a gentle upwardly convex arch. It is thicker posteriorly.

It is divided into four parts (from anterior to posterior):

• rostrum ⁹
  • ^​thicker beaked segment
  • thinner lamina rostralis (continuous with the lamina terminalis)
• genu
• trunk/body
• splenium

Relations

Immediately above the body of the corpus callosum, lies the interhemispheric fissure in which runs the falx cerebri and branches of the anterior cerebral vessels. The superior surface of the corpus callosum is covered by a thin layer of grey matter known as the indusium griseum.

On either side, the body is separated from the cingulate gyrus by the callosal sulcus.

Attached to the concave undersurface of the corpus callosum is the thin vertical septum pellucidum anteriorly, and the fornix and its commissure posteriorly.

Fiber tracts

Although the corpus callosum can be seen as a single large fiber bundle connecting the two hemispheres, a number of individual fiber tracts can be identified. These include:

• genu: forceps minor connects medial and lateral surfaces of the frontal lobes
• rostrum: connecting the orbital surfaces of the frontal lobes
• trunk (body): pass through the corona radiata to the surfaces of the hemispheres
• trunk and splenium: tapetum; extends along the lateral surface of the occipital and temporal horns of the lateral ventricle
• splenium: forceps major; connect the occipital lobes

These connections can also be divided into:

• homotopic connections: those that link similar regions on each side (e.g. visual fields of motor/sensory areas of the trunk)
• heterotopic connections: those that link dysimilar areas

Arterial supply

The corpus callosum (CC) has a rich blood supply, relatively constant and is uncommonly involved by infarcts. The majority of the CC is supplied by the pericallosal arteries (the small branches and accompanying veins forming the pericallosal moustache) and the posterior pericallosal arteries, branches from the anterior and posterior cerebral respectively. In 80% of patients, additional supply comes from the anterior communicating artery, via either the subcallosal artery or median callosal artery.

• subcallosal artery (50% of patients) is essentially a large version of a hypothalamic branch, which in addition to supplying part of the hypothalamus also supplies the medial portions of the rostrum and genu

• median callosal artery (30% of patients) can be thought of as a more extended version of the subcallosal artery, in that it travels along the same course, supplies the same structures but additionally reaches the body of the corpus callosum

• posterior pericallosal artery (also known as the splenial artery) supplies a variable portion of the splenium. Its origin is inconstant, arising from P3 or branches thereof

Venous drainage

Various small veins draining the central parts of the corpus callosum drain into the internal cerebral veins, in turn draining into the straight sinus. Tributaries of the internal cerebral veins draining the corpus callosum include ¹⁰:

• callosal veins
• callosocingulate veins
• subependymal veins
• septal veins

In addition, pericallosal veins follow their arterial counterpart over the superficial surface of the corpus callosum:

• posterior pericallosal veins (splenial veins) drain the splenium and drain directly into the straight sinus or great cerebral vein (of Galen)
• anterior pericallosal veins drain the genu and rostrum and drain into the anterior cerebral vein

Development

The development of the corpus callosum occurs between the 12^th and 16-20^th weeks of gestation ⁶.

Traditionally, it was believed that development begins in the genu and progresses posteriorly with the rostrum appearing last.

More recent studies, including using MR tractography, cast some doubt on this assertion, instead suggesting that the anterior body develops first and then continues bidirectionally, with the anterior portions (genu) developing earlier/more prominently than the posterior portions (splenium) ^7,8. This is not, however, universally accepted ¹¹. 

Myelination of the corpus callosum occurs in the opposite direction, from the splenium forwards.

Variant anatomy

• dysgenesis of the corpus callosum

History and etymology

From the Latin, corpus meaning "body" and callosum, "hard" or "tough". 

Related pathology

• lesions of the corpus callosum