Assessment of elephant claustrum by combined histological analysis and high-resolution micro-CT

Analysis of the brain architecture of the three extant elephant species is challenging, because of the vast size of their brains. We identified the elephant claustrum in histological Nissl-stained sections from small parts of an Asian (Elephas maximus) and an African savanna elephant (Loxodonta africana) brain. We find that the elephant claustrum is organized into islands of widely differing volume and cell numbers. We attempted to resolve these islands in virtual elephant brain sections from a 3 T Magnetic Resonance (MR) scanner, but found that the resolution was insufficient for such an analysis. We then transferred one hemisphere of an adult female African elephant brain into an ascending alcohol series. After degassing, we scanned the entire hemisphere in a micro-computed tomography (micro-CT) scanner with a resolution of 67 µm³ and parts of the hemisphere with a resolution of 26 µm³. Such scans provided sufficient resolution to estimate the total volume of the elephant claustrum in one hemisphere: 1453 mm³, corresponding to 0.22 % of cortical gray matter volume. In conjunction with our histological data, we estimate that the elephant claustrum in the same hemisphere contains 7.61 million neurons, or 0.27 % of cortical neurons (2869.86 million neurons). These values fit well with known cortico-claustral allometric relationships. Although elephant claustrum structure is widely distributed and organized into irregular islands, its volume follows the typical mammalian pattern, and micro-CT scans provide sufficient resolution to resolve small structures in large brains.