The normal and abnormal genu of the corpus callosum: an evolutionary, embryologic, anatomic, and MR analysis.

PURPOSE To define the normal and abnormal genu of the corpus callosum by examining its evolution and embryology and by analyzing its normal and abnormal appearance on MR images. METHODS A reference line was drawn from the mamillary body through the anterior commissure and corpus callosum-the MAC line. This line was used to evaluate the genu in adult mammal brains, in human fetal brains, on MR images of 1800 patients with normal corpora callosi, and on MR images of 113 patients with callosal anomalies. RESULTS In primates, increased frontal lobe size is associated with an anteriorly shifted genu. In human fetal development, the anterior body of the corpus callosum develops before the definitive genu. The normal human genu always projects in front of the MAC line. In none of the 113 patients with callosal anomalies was there only a normal genu. CONCLUSIONS The human corpus callosum develops bidirectionally, not from front to back. The MAC line is a useful frame of reference to study the evolution and embryology of the genu and to distinguish the normal from the abnormal genu of the human corpus callosum.