The morphogenesis of the cetacean cerebellum

Quantitative examination of the bottlenose dolphin cerebellum

Neuroanatomical research into the brain of the bottlenose dolphin (Tursiops truncatus) has revealed striking similarities with the human brain in terms of size and complexity. However, the dolphin brain also contains unique allometric relationships. When compared to the human brain, the dolphin cerebellum is noticeably larger. Upon closer examination, the lobule composition of the cerebellum is distinct between the two species. In this study, we used magnetic resonance imaging to analyze cerebellar anatomy in the bottlenose dolphin and measure the volume of the separate cerebellar lobules in the bottlenose dolphin and human. Lobule identification was assisted by three-dimensional modeling. We find that lobules VI, VIIb, VIII, and IX are the largest lobules of the bottlenose dolphin cerebellum, while the anterior lobe (I-V), crus I, crus II, and the flocculonodular lobe are smaller. Different lobule sizes may have functional implications. Auditory-associated lobules VIIb, VIII, IX are likely large in the bottlenose dolphin due to echolocation abilities. Our study provides quantitative information on cerebellar anatomy that substantiates previous reports based on gross observation and subjective analysis. This study is part of a continuing effort toward providing explicit descriptions of cetacean neuroanatomy to support the interpretation of behavioral studies on cetacean cognition.

The dolphin brain--a challenge for synthetic neurobiology

Toothed whales (odontocetes) are a promising paradigm for neurobiology and evolutionary biology. The ecophysiological implications and structural adaptations of their brain seem to reflect the necessity of effective underwater hearing for echolocation (sonar), navigation, and communication. However, not all components of the auditory system are equally well developed. Other sensory systems are more or less strongly reduced such as the olfactory system and, as an exception among vertebrates, the vestibular system (the semicircular canals and vestibular nuclei). Additional outstanding features are: (1) the hypertrophy of the neocortex, pons, cerebellum (particularly the paraflocculus), the elliptic nucleus, the facial motor nucleus and the medial accessory inferior olive and (2) the reduction of the hippocampus. The screening of brain structures with respect to shared circuitry and shared size correlations resulted in central loops also known from other mammals which overlap in the cerebellum and serve in the integration and processing of sensory input. It is highly probable that for dolphin navigation the ascending auditory pathway, including the inferior colliculus and the medial geniculate body, is of utmost importance. The extended auditory neocortical fields project to the midbrain and rhombencephalon and may influence premotor and motor areas in such a way as to allow the smooth regulation of sound-induced and sound-controlled locomotor activity as well as sophisticated phonation. This sonar-guided acousticomotor system for navigation and vocalization in the aquatic environment may have been a major factor if not the key feature in the relative size increase seen in dolphin brains.

A novel method for in situ fixation of whale brains

A new method of in situ formalin fixation was used on 38 brains from minke whales (Balaenoptera acutorostrata). The method was developed because traditional ways of fixing brains are poorly suited to the collection of whale brains. The whole brain was preserved uncut in its meninges and then excised undamaged from the skull at a later opportunity. There was no handling of the brain in the fresh state. Fixation was started within a couple of hours post mortem. All brains were subjected to gross and light microscopy examination. The results showed that both the gross and microscopic architecture of the brains were adequately preserved, with no massive gross or histological changes due to insufficient fixation apparent. The occurrence of fixation artifacts was low. Microscopic examination showed well-preserved cells and myelin in all parts of the brain. We report the mean fixed weight of the minke whale brain as 2741 g, which is the lowest among the baleen whales. The cerebellum constituted 22% of the total brain weight, which conforms to findings in other baleen whales. This in situ method can probably be used without any particular modifications in other whale species and also in large terrestrial mammals.

Ontogenesis of the sperm whale brain

The development of the sperm whale brain (Physeter macrocephalus) was investigated in 12 embryos and early fetuses to obtain a better understanding of the morphological and physiological adaptations in this most exotic cetacean concerning locomotion, deep diving, and orientation. In male adult sperm whales, the average absolute brain mass and the relative size of the telencephalic hemisphere are the largest within the mammalia, whereas the ratio of the brain mass to the total body mass is one of the smallest. In the early sperm whale fetus, the rostral part of the olfactory system (olfactory nerves and bulbs) is lost, whereas the nervus terminalis seems to persist. Several components of the limbic system show signs of regression (hippocampus, fornix, mamillary body). In contrast, some components of the auditory system (trapezoid body, inferior colliculus) show marked enlargement in the early fetal period, thereby reflecting their dominant position in the adult. The cerebellum and pons grow slower than in most smaller toothed whales. The pyramidal tract develops poorly (reduction of the limbs), whereas marked growth of the striatum and inferior olive may be related to the animal's locomotion via trunk and tail. In the early fetal period, the trigeminal, vestibulocochlear, and facial nerves are the dominant cranial nerves (besides the vagus nerve). Whereas the number of axons in the vestibulocochlear nerve is high in adult, toothed whales and their diameters are considerable, the trigeminal nerve of the sperm whale may be the thickest of all cranial nerves and has the largest number of axons (innervation of the huge forehead region). A similar situation seems to exist for the facial nerve: It innervates the blowhole musculature that surrounds the very large spermaceti organ and melon (generation and emission of sonar clicks).

Morphogenesis of the brain in the harbour porpoise

Morphogenesis of the brain in a cetacean species has been investigated by means of reconstructions from serial sections of successive prenatal stages of the harbour porpoise (Phocoena phocoena). Four specimens ranging from 10 to 46 mm crown-rump length (CRL) were selected and three-dimensional reconstructions of the developing brains were obtained with the plate model method. External and internal characteristics, established as criteria for staging embryonic development of primates and rodents, revealed that a common ontogenetic plan regarding the chronological sequence of morphogenetic events exists in mammalian orders as different as primates and odontocetes. Comparison of the 10-mm and 11.5-mm CRL harbour porpoise brains with those in other mammalian embryos of a similar ontogenetic stage (stages 16 and 17) showed a high degree of correspondence in morphological features. This ontogenetic age group therefore might still be considered as a generalized mammalian one. However, during succeeding morphogenesis of the Phocoena brain, qualitative and quantitative divergences from other mammalian groups became manifest, such as those found in the 24-mm CRL specimen (corresponding to mammalian stages 20, 21). Early foetuses of the harbour porpoise (46 and 65 mm CRL) already exhibited a variety of typical odontocete brain features, such as absence of olfactory bulb, thick cochlear nerve, and strong progression of brainstem structures. Morphogenesis of the harbour porpoise brain is discussed from a comparative perspective, incorporating the literature on the development of mammalian brains. Part of this study has been published in abstract form (Buhl and Oelschläger: Acta Anat. (Basel) 120:15-16 (Abstract), '84).

The subarcuate fossa and cerebellum of extant primates: comparative study of a skull-brain interface

The subarcuate fossa of the petrosal bone houses the petrosal lobule of the cerebellar paraflocculus. Although the subarcuate fossa can be extensive, little is known about its relative size and distribution in primates. Studies indicate parafloccular involvement with cerebellar areas coordinating vestibular, visual, auditory, and locomotor systems. Hypotheses have proposed a role for the paraflocculus in vestibular-oculomotor integration, caudal muscle control, autonomic function, and visual-manual predation. This study examines the morphology and relative extent of the subarcuate fossa/petrosal lobule in a range of living primates. Methods include study of postmortem specimens representing nine mammalian orders, and qualification of the volume of the subarcuate fossa and endocranial cavity in 155 dry primate crania of 36 genera. Results show that, in mammals, the size and morphology of the petrosal lobule is directly related to that of the subarcuate fossa. Craniometric analysis shows that the ratio of subarcuate fossa volume to endocranial volume is largest in lemuriforms. The largest ratio is in Microcebus and Hapalemur. Lorisids show a significant reduction in the size of the subarcuate fossa to almost 50% below the lemuriform mean. Tarsius is near the lemuriform mean. Among platyrrhines, the ratio is high, but significantly reduced compared to lemuiforms. The highest platyrrhine ratio is seen in Ateles, the lowest in Saimiri and Alouatta. Atelids are significantly elevated compared to cebids. In cercopithecids, the fossa is significantly reduced compared to platyrrhines. The trend toward reduction of the cercopithecid fossa is most pronounced in Theropithecus and least evident in Presbytis. In hominoids, the fossa is present only in Hylobates. In great apes and humans, other than Gorilla, the petromastoid canal occupies a similar location to the subarcuate fossa of other primates, but is not homologous to it. Neither the subarcuate fossa nor the petromastoid canal are present in Gorilla. A graded reduction of the subarcuate fossa/petrosal lobule is evident among primates which evolved later in time. The relative size of this cerebellar lobule within primates may reflect size-related factors and/or degree of neocortical evolution as these relate to usage of a specific sensory-mediated locomotor behavior. The subarcuate fossa may serve as an indicator to the differentiation of the petrosal lobule of the paraflocculus in fossil forms.

The ontogeny of the cerebellar fissures in the chick embryo

We have studied the chronology of appearance and the cortical changes which precede the fissures appearing between stages 34 to 40. In this paper we demonstrate the structural modifications of the cerebellar cortex defined as the anlagen of the fissures. The anlage of a fissure begins in a well-determined place of the cerebellar cortex. It begins with a thickening of the internal cortical cell layer, which finally folds. These modifications precede other similar ones which occur in the overlying external granular layer. On the other hand, these cortical structural modifications precede the transversal projection of the fissures. Chronological order of appearance of the anlagen of the fissures, related to the appearance of fissures on the surface of the cerebellum is also given.

Rotation induced increases of glucose uptake in rat vestibular nuclei and vestibulocerebellum

A new technique for the autoradiographic measurement of regional cerebral glucose consumption using tracer amounts of radioactive 2-deoxyglucose (2-DG) was employed to study the effects of vestibular inputs on local cerebral glucose metabolism. Rats rotated in the vertical plane showed localized increases of glucose utilization in the vestibular nuclei and several areas of the cerebellum: flocculus, nodulus, ventral uvula, and accessory paraflocculus. The changes in the cerebellum occurred both in the neuronal perikarya-rich granular layer and the neuropil-rich molecular layer. Differential changes resulting from rotation occurred within the granular layer of the nodulus, where at least 7 separate longitudinal zones of differing glucose consumption were seen. This type of longitudinal organization has been described previously in other areas of the vermis with other techniques.