The indusium griseum: anatomic study with potential application to callosotomy.

BACKGROUND: Although the indusium griseum (IG) is often seen by the neurosurgeon, almost nothing exists in the literature regarding its anatomic structure. Some have postulated that this structure is a remnant of the hippocampus, and some have found memory deficits in patients after callosotomy. OBJECTIVE: To further investigate the anatomy of the IG in humans through gross and histological analysis. METHODS: The IG from 10 adult cadaveric brains underwent microdissection and immunohistochemical analysis. RESULTS: Grossly, the IG was on average 2 cm in width over the body and genu of the corpus callosum (CC) and traveled in intimate contact anteriorly over the lamina terminalis, and posteriorly it was adherent to the splenium of the CC. Histologically, the IG is a thin layer of hypocellular glial tissue interposed between the pia and the CC. Glial cells composed the cellular constituents of the IG, and compared with the underlying CC, the IG was hypomyelinated. The fibers/axons of the IG travel in a perpendicular plane compared with those of the CC, and the IG varied in thickness from one area to the other and was occasionally discontinuous. CONCLUSION: The IG is a glial membrane with no neuronal content or obvious connections to the hippocampus. Based on this study, transection of this membrane with callosotomy should not be the reason for postoperative memory loss seen in some of these patients. Future studies aimed at elucidating the function of the IG in humans are now warranted.