Analyzing the corpus callosum.

The corpus callosum has been recognized and studied for centuries. Although this structure appears to have been mentioned by Galen in the second century AD, the first available anatomic description appears to have been rendered by Vesalius in “De Humani Corporis Fabrica” (1543, translated by Clarke and O’Malley [1]). As reviewed by Njiokiktjien (2), speculation on the function of the corpus callosum began during the 17th century, but it was not until the late 19th century, after it was discovered that the white matter of the brain is composed of separate bundles of axons, that neuroscientists became aware that callosal axons connect homologous areas in the cortices of the cerebral hemispheres. Afterward, in the early years of the 20th century, studies of patients with disconnection syndromes were published (3–6), and the function of the corpus callosum became much better defined. Accurate clues to the embryology of the corpus callosum came much later, with the most important early work done by Abbie (7), Loeser and Alvord (8, 9), and Rakic and Yakovlev (10), as noted in the article by Kier and Truwit (11) in this issue of AJNR. More recent work on the mechanisms by which the callosal axons migrate across the midline of the developing brain and navigate through the milieu of the developing brain to reach their final synaptic connections has been largely performed by Silver and coworkers (12–15) and suggests that callosal axons are guided across the midline by molecular clues on the surface of glial cells. These theories have been reviewed recently in the AJNR (16). The article by Kier and Truwit (11) in the present issue of the AJNR deals with the appearance of the normal corpus callosum and in particular the location of certain portions of the corpus callosum with respect to the mamillary bodies and the anterior commissure during development. The article has a number of interesting aspects, although this commentary will deal with only a few of them. As the authors point out, it was established long ago that the initial callosal axons cross in the midline near the region of the foramina of Monro. This is discussed both in the paper by Abbie (7) and in the paper by Rakic and Yakovlev (10), both referenced by the authors. Kier and Truwit have not modified this concept; instead they have focused on the location of the point of initial callosal crossing with respect to what they call the MAC line, a line defined by two points, the mamillary bodies and the anterior commissure on a (presumably) midline sagittal magnetic resonance image. Kier and Truwit have defined the callosal genu as the portion of callosal fibers that lies anterior to the MAC line. They do not define the thickness of the midline image that they use, nor do they discuss the portion of the anterior commissure or mamillary bodies (anterior edge, posterior edge, middle) that should be used for drawing the line or the potential effects of averaging the two mamillary bodies. They do not discuss whether the structures chosen as the points for the creation of the MAC line might be important. The authors justify their definition of the genu by its ease of application on sagittal MR images and by their statement that the boundaries of the components of the corpus callosum are not defined in the anatomic literature. Although some definitions of the genu have been presented in the neuroscience (17) and neuropathology (18) literature, the authors have chosen not to use these. In fact, defining the genu by the MAC line seems to be as reasonable as the other definitions that have been proposed. Kier and Truwit have performed an extraordinary amount of work to complete this project and they are to be commended for that. The mere thought of reviewing 1800 cases of normal corpora callosi is exhausting! Nonetheless,