The lamina rostralis: modification of concepts concerning the anatomy, embryology, and MR appearance of the rostrum of the corpus callosum.

PURPOSE: To study the anatomy and embryology of the lamina rostralis, and to determine whether the rostrum is, as frequently stated, the last section of the corpus callosum to develop. METHODS: The rostrum was analyzed in dissected adult brains and on MR studies in 300 patients with a normal corpus callosum and in 84 patients with a hypogenetic corpus callosum. MR images of intact fetuses and photographs of dissected fetal and adult vertebrate brains were also analyzed. RESULTS: The rostrum extends from the genu to the upper end of the lamina terminalis and consists of two sections: a thick beaked segment and the thin lamina rostralis, which blends posteriorly with the lamina terminalis. During fetal development the lamina rostralis changes from a semivertical to a semihorizontal orientation. Many hypogenetic corpora callosi have a semivertical lamina rostralis. A rudimentary beaked segment can be present without a normal genu. CONCLUSIONS: The rostrum is not the last segment of the corpus callosum to develop. Rather, the lamina rostralis segment of the fetal rostrum is already present before the genu and splenium develop. Additionally, the beaked segment of the rostrum develops concurrently with maturation of the genu.