The Effect of Absence of the Corpus Callosum on the Position of the Hippocampus and on the Formation of Probst's Bundle KENNETH R. MAGEE AND ROBERT N. OLSON' Kresge Neurological Research Laboratory, University of Michigan Complete or partial agenesis of the fibers, the fimbria. Elliot Smith ('10) corpus callosum results in an alteration stressed that the position of the hippo- in anatomical developmcnt of the midline campus as the medial edge of the cerebral structures of the cerebral hemisphere. The cortex is an important feature of the brain influence of such maldevelopment on the in the ancestors of mammals and is prob- position of the hippocampus and its con- ably a fundamental feature in the develop- nections will be considered in this study. ment of all vertebrates. The dorsal com- In addition, agenesis of the corpus cal- missure of the marsupial contains only losum may allow structures existing in fibers of hippocampal origin (fig. 1). In normal brains to become more evident the bat, Nycotophilus, which represents a because they are not masked by the abun- transition from the acallosal to the callosal dant fiber distribution of the corpus cal- brain, the anterior part of the dorsal com- losum. Various other cerebral anomalies missure becomes invaded by pallial com- may often be seen in agenesis of the cor- missural fibers thus forming an anterior pus callosum but they will not be stressed extension of the dorsal commissure (Smith in this study for good reviews of the gen- 1897a). These pallial decussating fibers eral subject are available (Baker and do not develop to so great an extent as in Graves, '33; Carpenter and Druckemiller, higher animals but begin to influence the '53; Kirschbaum, '47; Bunts and Chaffee, position of the hippocampus. As in these '44; and Marburg, '49). lower forms, studies of higher mammals have demonstrated that the fibers which PERTINENT LITERATURE eventually form the corpus callosum are The embryologic and phylogenetic de- first seen as a bundle known as the tape- velopment of the midline anatomy of the turn covering the neocortical inner wall of human brain will first be reviewed. In a the archicortex (Tilney, '39). When the study of marsupials, Tilney ('39) con- fibers course medialward, they are pushed cluded, ". in spite of its extreme primi- against the anterior side of the alveus layer tiveness it (the marsupial) follows, stage of archicortex and then invade the ante- for stage, the fundamental principles rior part of the dorsal commissure matrix which underlie the growth of the brain or commissural bed of Elliot Smith (1897a). in all mammals." Tilney noted a precal- The first parts of the corpus callosum losal stage in all the mammals (including to develop are the corpus, the genu, and man) which he examined, that eventually the rostrum as these fibers are first seen develop a corpus callosum. in this region in front of the foramen of In the marsupial, the hippocampal for- Monro and the anterior commissure. The mation extends from the temporal pole to increasing number of fibers causes the the olfactory peduncle and forms the me- corpus callosum to increase in bulk and dial margin of the cortex (Ariens Kappers, in length mainly posteriorly but there is Huber and Crosby, '36, pp. 1412). This also some forward extension. The back- midline arc of hippocampus has the same ward extension of the corpus callosum is cortical makeup and general consistency throughout its extent. Along its inner mar- * Student fellow, National Institute of Neuro- gin runs a longitudinal bundle of nerve logical Diseases and Blindness. 371 372 KENNETH R. MAGEE AND ROBERT N. OLSON Fig. 1 Illustrating the position of the hippocampus on the medial surface of the hemi- sphere and the hippocampal commissure of the marsupial. Didelphis virginiuna. not accompanicd by a corresponding ex- located on its dorsal surface (Tilney, '39). pansion of the psalterium, which therefore The hippocampal formation is therefore becomes greatly thinned at its junction rolled back, down and out into the tem- with the splenium. The posterior expan- poral pole, into close relation with the cor- sion of the corpus callosum, interposed tex of the piriform lobe (Ariens Kappers, between hippocampal formation above and Huber and Crosby, '36, pp. 1413). A mid- hippocampal commissure below, forms the line hippocampal division persists, how- splenium (Smith, 1897a and '10). Related ever, forming the induseum griseum and to the increased development of inter- the white stria of Lancisi. Thus, even hemispheral communication in primates, in adult forms of higher primates, the the corpus callosum is enlarged and elon- hippocampus maintains the full arc from gated, primarily in the callosal trunk. The olfactory peduncle to temporal horn. This distance between genu and splenium is supracallosal hippocampus, however, is ap- disproportionately longer than in lower parently retarded in development by the mammals. The splenium is therefore car- introduction of the corpus callosum for ried caudad in such a way as to bear it does not grow or advance in cellular with it the hippocampal commissure and, structure and remains very small in com- at the same time, cause an elongation of parison to the hippocampus associated the longitudinal fibers of the fimbria (Til- with the temporal horn. ney, '39). The corpus callosum also influences the This caudal extension of the corpus cal- development of the fornix, as has been losum and development of the neopallial mentioned. The fornix becomes elongated regions of the hemisphere carrics with it with the pushing backward and downward the growing structures of the archicortex of the hippocampus by the corpus callosum ABSENCE OF CORPUS CALLOSUM 373 (Ariens Kappers, Huber and Crosby, '36, systems, it would seem quite probable that pp. 1428). Although most fibers of the in agenesis of the corpus callosum: fornix are ventral to the corpus callosum, longitudinal association fibers and some fibers of the fornix (fimbria) system bundles, some of which are not are dorsal to the corpus callosum (E. usually distinguished, become more Smith, 1896, 1897a and b). Some of the fas- distinct; cicles from the medial stria of Lancisi, the location of the hippocampus cross through the corpus callosum to enter and its connections is altered; early interruption or destruction of the septum pellucidum where they join the commissural bed can be the in the distribution of the subcallosal for- cause of true agenesis (Marburg, nix fibers. Some fibers from the hippo- '49). campal flexure (that part of the hippo- following material is mesented campus arching over the caudal part of with these obseriations in mind.L In addi- the splenium) course around the splenium tion, the composition of the so-called before they perforate the corpus callosum. Probst's bundle (longitudinal callosal bun- These fibers which perforate the corpus dle) will be noted and discussed later. This callosum to reach the fornix are called the bundle is apparent in many cases of agen- superior fornix. It is now believed that esis of the corpus callosum and appears some of these fibers may also arise from as a primarily longitudinal fiber system. the gyrus cinguli or possibly neighboring Its exact nature has been the subject of corticaI areas (Valenstein and N-auta, '59). much disagreement. There are also fibers which pass around the genu of the corpus callosum to reach DISCUSSION OF MATERIAL the septum, some of which arise from the Study 1: A 20-year-old woman had a medial striae Lancisii and become a part psychosis and jerking movements of the of the precommissural fornix (Ariens extremities. A diagnosis of Huntington's Kappers, Huber and Crosby, '36, pp. 1430). chorea was considered but not proven. As with the longitudinal fibers which Birth and developmental history and the cause of death were not available. make up the fornix systems (the fascicles Examination of the brain disclosed com- of the striae of Lancisi, etc.), the major pIete agenesis of the corpus callosum. The association bundles, which connect differ- anterior commissure was present as was ent lobes of the same hemisphere, are in the fornix which, however, was separated close anatomical relationship with the cor- throughout its full extent. Olfactory tracts pus callosum. The fibers which make up were present. The septum pellucidum was these bundles vary in number and kind absent. The gyrus cinguli was not present, (short or long association fibers) at differ- although the regions of cortex which usu- ent parts of the bundle. Longitudinal asso- ally make up this gyrus may have been ciation fibers exist which are not con- represented by portions of the convolu- densed into the bundles but run among tional pattern. Sulci radiated from what the corpus callosal fibers, being perpendic- in a normal brain wouId have been the ular to them. With the absence of such callosal sulcus. The parieto-occipital sulci a large structure as the corpus callosum, did not meet the calcarine fissures. it is easily seen how the longitudinal fibers The hippocampal formation was of par- of the fornix longus, the fornix superior, ticular interest and had not taken its nor- and the association fibers of the hemis- mal adult position (as in the adult) but pheres may come into a close relationship remained in a more medial and dorsal posi- to form a bundle. tion, lying next to the medial wall of the Considering the ways in which the de- lateral ventricle. Figure 2 shows the hippo- velopment of the corpus callosum affects campus in this position, superior to the the position of the hippocampus and for- inferior colliculus. Some of the fibers of nix, and the manner in which develop- Probst's bundle apparently turned down- ment of this large neopallial commissure ward to the hippocampus but they were not can obscure the finer longitudinal fiber of sufficient number to account for more 374 KENNETH R.