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Odontocetes (toothed whales, dolphins, and porpoises) have undergone a number of evolutionary modifications from their terrestrial ancestral state. Among these changes was a major increase in relative brain size. Several modern odontocete species possess encephalization levels second only to modern humans when brain-body allometry is taken into account (Ridgway and Brownson, 1984; Marino, 1998). An arguably equally dramatic transformation of odontocetes occurred in the anatomical structure and organization of their brains. Compared with many other mammalian brains, odontocete brain morphology is unusual in many respects. Researchers have stated that “…the lobular formations in the dolphin brain are organized in a pattern fundamentally different from that seen in the brains of primates or carnivores” (Morgane et al., 1980). Because of the fifty-five to sixty million year divergence between cetaceans and other mammals, odontocete brains represent a blend of early mammalian features along with unique derived characteristics (Ridgway, 1986, 1990; Glezer et al., 1988; Manger et al., 1998). The differences between odontocete and other mammalian brains of similar size are present at the level of cortical cytoarchitecture and immunohistochemistry (Garey et al., 1985; Garey and Leuba, 1986; Glezer and Morgane, 1990; Hof et al., 1992, 1995; Glezer et al., 1990, 1992a,b, 1993, 1998), cortical surface morphology (Jacobs et al., 1979; Morgane et al., 1980; Haug, 1987), noncortical structures and features (Tarpley and Ridgway, 1994; Glezer et al., 1995a,b), and ontogenesis (Oelschlager and Buhl, 1985; Buhl and Oelschlager, 1988; Oelschlager and Kemp, 1998).
Although there are a number of published descriptions of cetacean neuroanatomy (see Morgane et al., 1986; Ridgway, 1990; for reviews of this literature) there are only a handful of studies in which morphometric analyses were conducted in a systematic way permitting quantitative comparative analysis with other mammals (Jacobs et al., 1984; Johnson et al., 1984; Schwerdtfeger et al., 1984; Garey and Leuba, 1986; Johnson et al., 1994; Tarpley and Ridgway, 1994; Manger et al., 1998; Marino, 1998). Furthermore, with the exception of Morgane et al. (1980), Ridgway and Brownson (1984), Haug (1987), and Tarpley and Ridgway (1994) there are no systematic anatomical descriptions of whole cetacean brains and substructures at the qualitative level. There currently exists no comprehensive cetacean neuroanatomical atlas either in paper or electronic format on which to base studies of cetacean brain organization and function. This situation is mainly due to the time and practicality associated with the preparation of such large brain specimens. Magnetic resonance imaging (MRI) offers a means of observing the internal structure of the brain where traditional procedures of embedding, sectioning, staining, mounting, and microscopic examination of thousands of sections are not practical. Furthermore internal structures can be analyzed in their precise spatial interrelationships, which is difficult to accomplish after the spatial distortions often accompanying histological processing. This study presents an anatomically-labeled three-dimensional atlas, created from MRI images, of the brain of one of the most behaviorally studied odontocetes, the white whale (Delphinapterus leucas).
MATERIALS AND METHODS
The specimen is the postmortem brain, fixed in 10% buffered formalin, of an adult female white whale (Delphinapterus leucas) who died of natural causes. The whale had been involved in several behavioral studies including studies of its hearing (Awbrey et al., 1988). At death, the brain was extracted from the skull, weighed, and placed in neutral buffered formalin for 4 years before scanning. Fresh brain weight was 1,871 g. Fixed brain weight was 1,755 g at the time of scanning.
Magnetic Resonance Imaging
The brain was removed from the fluid and placed in a head coil ventral side down. Images of the entire brain were acquired in the coronal plane at 119 antero-posterior levels with a 1.5 T Siemens scanner (slice thickness = 1.3 mm, slice interval = 1.3 mm, isotropic 1 mm voxels, MP-RAGE sequence, field of view = 240 mm, matrix = 256 × 256). The scanning was done by Richard Buxton, PhD at the University of California, San Diego.
Three-Dimensional Reconstruction and Reformatting
Computer-generated three-dimensional reconstruction images were created by Timothy L. Murphy, using the software programs VoxelView and VoxelMath programs (Vital Images, Inc.) at the Laser Scanning Microscopy Laboratory at Michigan State University, Joanne Whallon, Director. The 3D rendered model, wherein details of internal and external morphology are represented in three-dimensional space, was then digitally resectioned in orthogonal planes to produce corresponding 0.9375 mm-thick “virtual” sections in the horizontal (163 “virtual” sections) and sagittal (236 “virtual” sections) planes.
Anatomical Labeling and Nomenclature
All identifiable anatomical structures of the white whale brain were labeled in the originally-acquired coronal plane images as well as in the images from the “virtual” sectioned brain in the sagittal and horizontal planes. The nomenclature used is from Morgane et al. (1980). As a guide to the identification of structures, the MRI scans and the sections from the three-dimensional reconstruction of the whale brain were compared with the few published illustrations (or images of real stained) sections through the white whale brain and photographs of the whole brain (Yablokov et al., 1964; Morgane et al., 1980). They were also compared with similar MRI scans and “virtual” sections and three-dimensional reconstructions from the scans of brains of bottlenose dolphins (Tursiops truncatus) (Morgane et al., 1980). All were compared with complete alternate series of sections from brains of bottlenose dolphins, stained, respectively, for cell bodies (Nissl method), and for myelinated fibers in the same three orthogonal planes (coronal, sagittal, and horizontal). These stained section series are from the Yakovlev-Haleem collection at the National Museum of Health and Medicine and the Welker collection at the University of Wisconsin-Madison.
Three-dimensional reconstructions of the white whale's whole brain were produced from the original scans in the coronal plane. Figure 1 displays an image (or view) of the posterior-ventral surface of a computer-generated three-dimensional reconstruction of the whole brain and a labeled illustration of the image. Figures 2 and 3 also display three-dimensional reconstructions of the whole brain and the brain digitally recut in the horizontal and sagittal planes, respectively. These three-dimensional reconstructions clearly display many noted characteristics of the cetacean brain that diverge from most other terrestrial mammalian brains (Morgane et al., 1980). The foreshortened orbital lobes are evident in Figure 3 as is the pronounced bitemporal width of the brain in Figures 1 and the last “virtual” cut in Figure 2. The labeled “virtual” cut in Figure 3 shows the mesencephalic and pontine flexures reminiscent of brainstem flexure patterns in the embryonic state of most terrestrial mammals. These flexures remain present in adult cetacean brains.
Anatomically-Labeled Two-Dimensional MRI Sections
Figure 4a–l displays a rostral-to-caudal sequence of originally-acquired 1.3 mm-thick coronal MRI brain sections at 22 mm intervals along with a labeled schematic illustration of each section. Figure 5 displays a dorsal-to-ventral sequence of reconstructed “virtual” horizontal sections (0.9375 mm thick) and a labeled schematic of each section. Figure 6 displays a lateral-to-medial sequence of reconstructed “virtual” sagittal sections, also 0.9375 mm thick, through the left hemisphere at 18.75 mm intervals (with the last two images 10.3 mm apart) and a labeled schematic for each section.
The high level of convolution of the cortex is evident in almost all of the figures. The extreme depth and density of cortical sulci are particularly evident in Figure 6 (c–f). These images also display an orbital-to-occipital gradient of increased sulcation concordant with the increased elaboration of the occipital-parietal region over the orbital region. This occipital-parietal elaboration is evident in Figure 6 (g–j) in the striking triple-tiered arrangement of limbic, paralimbic, and supralimbic arcuate cortical lobules divided by the deep limbic and paralimbic clefts. This specific combination of occipital-parietal organization and elaboration is distinct from other mammals.
In contrast to the distinctive cortical features, the odontocete brain generally resembles other mammalian brains on a subcortical level. The volumetric proportions of various subcortical features, however, reveal even more of the distinctive adaptations and allometric rearrangements associated with odontocete evolution. As seen in Figure 1, the olfactory bulbs are absent. In contrast, auditory processing areas are enlarged (though visual structures are not necessarily reduced). These include the proportionately large inferior colliculus compared with the superior colliculus as seen most clearly in Figure 4 (g,h).
In keeping with behavioral and electrophysiological evidence for a high degree of hemispheric independence (Viamonte et al., 1968; Mukhametov et al., 1977; Mukhametov, 1984), the corpus callosum is small relative to the massive hemispheres, consistent with quantitative findings in other odontocete species and qualitative observations of the white whale brain (Tarpley and Ridgway, 1994). This is apparent in most of the figures but particularly in Figure 6 (g–l).
The cerebellum is large relative to the hemispheres. This is especially evident in Figure 4 (g–j), and Figure 5 (i,j), and also in the sagittal images. As shown in Figure 4 (e,f) the cerebral peduncles are high on the lateral surface of the caudal diencephalon and through the entire midbrain, rather than on the basal inferior or ventral surface as in most mammals. The basal surface is instead occupied by a large mass of gray matter that appears to be continuous with the ventral striatum and the dorsal and ventral pallidum of the forebrain reaching from these structures to the pontine nuclei caudally. The approximation of apparent basal ganglia and pons may be a consequence of the flexing of the diencephalon and the midbrain bringing the brain stem into contact with the basal forebrain, or it may represent a specialized development of the basal ganglia.
This study presents the first MRI-based, anatomically-labeled, three-dimensional atlas of the brain of the white (beluga) whale (Delphinapterus leucas). In addition, we have constructed three-dimensional models of the white whale brain and produced “virtual” horizontal and sagittal sections from these original images. These reconstructed images allow for the visualizing of a range of distinctive white whale brain features from various orientations by preserving the gross morphological and internal structure of the specimen. Because there are none of the distortions associated with histological processing, we have a more realistic view of the brain as it was in situ.
Many cortical features are easily identified from the original MRI scans and “virtual” images. These include the distinctive lobular formations, gyral and sulcal patterns, and general gradient of elaboration in the parietal, occipital, and temporal regions. Subcortical allometry, including that of both gray and white matter structures, is easily assessed as well. Our findings are consistent with what has been noted in the few existing histological studies of the odontocete brain. Moreover, because we are able to preserve the internal structure of the specimen, neuroanatomical studies of brains from MRI set the stage for much-needed accurate and reliable morphometric analyses of various brain structures in odontocetes. These studies are underway.
There is a deep evolutionary divergence of the Order Cetacea (of which Odontoceti is a suborder) from other mammalian lines. Furthermore, cetacean evolution is characterized by distinctive environmental pressures associated with a fully aquatic existence versus a terrestrial lifestyle. These related attributes make the comparative study of structure-function relationships in cetacean brains, compared with those of other mammals, uniquely valuable for improving our understanding of the parameters of mammalian brain evolution.
The brain of the white whale as revealed in this study is characterized by similar morphological trends as those found in the bottlenose dolphin and other cetaceans (Morgane et al., 1980). Although there are differences among cetacean brains, these differences are relatively minor compared with the striking dissimilarities to brains of other mammals. The most obvious difference between cetacean brains and those of other mammals is in the gross morphological configuration of the whole structure and the lobules of the cerebral hemispheres. These are well-visualized in MRI scans. Evolution of overall brain shape in cetaceans may have been partly due to migration of the blowhole and telescoping of the skull, i.e., antorbital elongation and postorbital compression. This in turn may account for the distinctive construction of the midbrain, i.e., the corticopontine, corticobulbar and corticospinal fibers travel high on the lateral surface whereas the ventral surface is occupied by a large continuous mass of gray matter extending from the diencephalon rostrally to the pontine nuclei caudally. There may be distinctive organizational features of the basal ganglia that also contribute to this uniquely cetacean architecture.
There is also adequate evidence that many of the anatomical changes in the cetacean brain represent changes in function, e.g., loss of olfactory structures and enlargement of acoustic structures. Similar, convergent changes in function, along with their neuroanatomical correlates, are observed in several brains of unrelated clades, such as many bats and primates (Johnson et al., 1984; Johnson et al., 1994). In general, the cetacean brain possesses some common mammalian features in combination with specialized and highly unusual features, the function of which we have barely begun to understand.
If we are to eventually understand the functional significance of this mosaic of typical mammalian and uniquely cetacean features, the structural organization of the cetacean brain must be further elucidated. This can be rapidly and effectively accomplished by MRI-based studies of neuroanatomy. In comparison, already-existing data from the more traditional methods of sectioning and staining are very time-intensive, expensive, and vulnerable to spatial distortion compared with the data acquired by MRI. Studies like the present one are crucial for establishing the structural basis of and templates for future functional studies using non-invasive neuroimaging techniques to investigate the neurobiological basis of cetacean cognition and behavior.
We wish to thank R.C. Switzer III for discussions and identifications of the basal ganglia in the cetacean specimens. We also thank Joanne Whallon for use of the Voxel View programs and Silicon Graphics, Inc. workstations at the Laser Scanning Microscopy Laboratory at Michigan State University, Cheryl Short for technical assistance, Rick Buxton for conducting the MRI scanning at the University of California, San Diego, and Patsy Bryan for excellent artwork. This study was supported by an Emory University University Research Committee Award.