The brain of the tree pangolin (Manis tricuspis). X. The spinal cord

The spinal cord of the tree pangolin is known to be very short compared to the overall length of the body and tail. Here, we provide a description of the tree pangolin spinal cord to determine whether the short length contributes to specific structural, and potentially functional, differences. The short spinal cord of the adult tree pangolin, at around 13 cm, terminates at the midthoracic level. Within this shortened spinal cord, we could identify six regions, which from rostral to caudal include the prebrachial, brachial, interramal, crural, postcrural, and caudal regions, with both the brachial and crural regions showing distinct swellings. The chemoarchitecture of coronal sections through these regions confirmed regional assignation, being most readily delineated by the presence of cholinergic neurons forming the intermediolateral column in the interramal region and the sacral parasympathetic nucleus in the postcrural region. The 10 laminae of Rexed were observed throughout the spinal cord and presented with an anatomical organization similar to that observed in other mammals. Despite the shortened length of the tree pangolin spinal cord, the regional and laminar anatomical organization is very similar to that observed in other mammals. This indicates that the functional aspects of the short tree pangolin spinal cord can be inferred from what is known in other mammals.

The brain of the tree pangolin (Manis tricuspis). VIII. The subpallial telencephalon

The current study provides a detailed architectural analysis of the subpallial telencephalon of the tree pangolin. In the tree pangolin, the subpallial telencephalon was divided into septal and striatopallidal regions. The septal region contained the septal nuclear complex, diagonal band of Broca, and the bed nuclei of the stria terminalis. The striatopallidal region comprised of the dorsal (caudate, putamen, internal and external globus pallidus) and ventral (nucleus accumbens, olfactory tubercle, ventral pallidum, nucleus basalis, basal part of the substantia innominata, lateral stripe of the striatum, navicular nucleus, and the major island of Calleja) striatopallidal complexes. In the tree pangolin, the organization and numbers of nuclei forming these regions and complexes, their topographical relationships to each other, and the cyto‐, myelo‐, and chemoarchitecture, were found to be very similar to that observed in commonly studied mammals. Minor variations, such as less nuclear parcellation in the bed nuclei of the stria terminalis, may represent species‐specific variations, or may be the result of the limited range of stains used. Given the overall similarity across mammalian species, it appears that the subpallial telencephalon of the mammalian brain is highly conserved in terms of evolutionary changes detectable with the methods used. It is also likely that the functions associated with these nuclei in other mammals can be translated directly to the tree pangolin, albeit with the understanding that the stimuli that produce activity within these regions may be specific to the life history requirements of the tree pangolin.

The brain of the tree pangolin (Manis tricuspis). IX. The pallial telencephalon

A cyto‐, myelo‐, and chemoarchitectonic analysis of the pallial telencephalon of the tree pangolin is provided. As certain portions of the pallial telencephalon have been described previously (olfactory pallium, hippocampal formation, and amygdaloid complex), we focus on the claustrum and endopiriform nuclear complex, the white matter and white matter interstitial cells, and the areal organization of the cerebral cortex. Our analysis indicates that the organization of the pallial telencephalon of the tree pangolin is similar to that observed in many other mammals, and specifically quite similar to the closely related carnivores. The claustrum of the tree pangolin exhibits a combination of insular and laminar architecture, while the endopiriform nuclear complex contains three nuclei, both reminiscent of observations made in other mammals. The population of white matter interstitial cells resembles that observed in other mammals, while a distinct laminated organization of the intracortical white matter was revealed with parvalbumin immunostaining. The cerebral cortex of the tree pangolin presented with indistinct laminar boundaries as well as pyramidalization of the neurons in both layers 2 and 4. All cortical regions typically found in mammals were present, with the cortical areas within these regions often corresponding to what has been reported in carnivores. Given the similarity of the organization of the pallial telencephalon of the tree pangolin to that observed in other mammals, especially carnivores, it would be reasonable to assume that the neural processing afforded the tree pangolin by these structures does not differ dramatically to that of other mammals.

The brain of the tree pangolin (Manis tricuspis). VII. The amygdaloid body

Here, we describe the cytoarchitecture and chemoarchitecture of the amygdaloid body of the tree pangolin. Our definition of the amygdaloid body includes the pallial portions of the amygdala, and the centromedial group that is a derivative of the subpallium and part of the extended amygdala. The remainder of the extended amygdala is not described herein. Within the amygdaloid body of the tree pangolin, we identified the basolateral group (composed of the lateral, basal, and accessory basal amygdaloid nuclei), the superficial, or cortical nuclei (the anterior and posterior cortical nuclei, the periamygdaloid cortex, and nuclei of the olfactory tract), the centromedial group (the central amygdaloid nucleus and the medial nuclear cluster), and other amygdaloid nuclei (the anterior amygdaloid area, the amygdalohippocampal area, the intramedullary group, and intercalated islands). The location within and relative to each other within the amygdaloid body and the internal subdivisions of these groups were very similar to that reported in other mammalian species, with no clearly derived features specific to the tree pangolin. The only variation was the lack of an insular appearance of the intercalated islands, which in the tree pangolin were observed as a continuous band of neurons located dorsomedial to the basolateral group similar in appearance to and almost continuous with the intramedullary group. In carnivores, the closest relatives of the pangolins, and laboratory rats, a similar appearance of portions of the intercalated islands has been noted.

The Mammalian Locus Coeruleus Complex-Consistencies and Variances in Nuclear Organization

Descriptions of the nuclear parcellation of the locus coeruleus complex have been provided in approximately 80 mammal species spanning the phylogenetic breadth of this class. Within the mammalian rostral hindbrain, noradrenergic neurons (revealed with tyrosine hydroxylase and dopamine-ß-hydroxylase immunohistochemistry) have been observed within the periventricular grey matter (A4 and A6 nuclei) and parvicellular reticular nucleus (A5 and A7 nuclei), with the one exception to date being the tree pangolin, where no A4/A6 neurons are observed. The alphanumeric nomenclature system, developed in laboratory rodent brains, has been adapted to cover the variation observed across species. Cross-species homology is observed regarding the nuclear organization of noradrenergic neurons located in the parvicellular reticular nucleus (A5 and A7). In contrast, significant variations are observed in the organization of the A6 neurons of the locus coeruleus proper. In most mammals, the A6 is comprised of a moderate density of neurons, but in Murid rodents, primates, and megachiropteran bats, the A6 exhibits a very high density of neurons. In primates and megachiropterans, there is an additional moderate density of A6 neurons located rostromedial to the high-density portion. These variations are of importance in understanding the translation of findings in laboratory rodents to humans.

Nuclear organization of catecholaminergic neurons in the brains of a lar gibbon and a chimpanzee

Using tyrosine hydroxylase immunohistochemistry, we describe the nuclear parcellation of the catecholaminergic system in the brains of a lar gibbon (Hylobates lar) and a chimpanzee (Pan troglodytes). The parcellation of catecholaminergic nuclei in the brains of both apes is virtually identical to that observed in humans and shows very strong similarities to that observed in mammals more generally, particularly other primates. Specific variations of this system in the apes studied include an unusual high-density cluster of A10dc neurons, an enlarged retrorubral nucleus (A8), and an expanded distribution of the neurons forming the dorsolateral division of the locus coeruleus (A4). The additional A10dc neurons may improve dopaminergic modulation of the extended amygdala, the enlarged A8 nucleus may be related to the increased use of communicative facial expressions in the hominoids compared to other primates, while the expansion of the A4 nucleus appears to be related to accelerated evolution of the cerebellum in the hominoids compared to other primates. In addition, we report the presence of a compact division of the locus coeruleus proper (A6c), as seen in other primates, that is not present in other mammals apart from megachiropteran bats. The presence of this nucleus in primates and megachiropteran bats may reflect homology or homoplasy, depending on the evolutionary scenario adopted. The fact that the complement of homologous catecholaminergic nuclei is mostly consistent across mammals, including primates, is advantageous for the selection of model animals for the study of specific dysfunctions of the catecholaminergic system in humans.

The brain of the tree pangolin (Manis tricuspis). VI. The brainstem and cerebellum

The brainstem (midbrain, pons, and medulla oblongata) and cerebellum (diencephalic prosomere 1 through to rhombomere 11) play central roles in the processing of sensorimotor information, autonomic activity, levels of awareness and the control of functions external to the conscious cognitive world of mammals. As such, comparative analyses of these structures, especially the understanding of specializations or reductions of structures with functions that have been elucidated in commonly studied mammalian species, can provide crucial information for our understanding of the behavior of less commonly studied species, like pangolins. In the broadest sense, the nuclear complexes and subdivisions of nuclear complexes, the topographical arrangement, the neuronal chemistry, and fiber pathways of the tree pangolin conform to that typically observed across more commonly studied mammalian species. Despite this, variations in regions associated with the locus coeruleus complex, auditory system, and motor, neuromodulatory and autonomic systems involved in feeding, were observed in the current study. While we have previously detailed the unusual locus coeruleus complex of the tree pangolin, the superior olivary nuclear complex of the auditory system, while not exhibiting additional nuclei or having an altered organization, this nuclear complex, particularly the lateral superior olivary nucleus and nucleus of the trapezoid body, shows architectonic refinement. The cephalic decussation of the pyramidal tract, an enlarged hypoglossal nucleus, an additional subdivision of the serotonergic raphe obscurus nucleus, and the expansion of the superior salivatory nucleus, all indicate neuronal specializations related to the myrmecophagous diet of the pangolins.

The brain of the tree pangolin (Manis tricuspis). IV. The hippocampal formation

Employing a range of standard and immunohistochemical stains we provide a description of the hippocampal formation in the brain of the tree pangolin. For the most part, the architecture, chemical neuroanatomy, and topological relationships of the component parts of the hippocampal formation of the tree pangolin were consistent with that observed in other mammalian species. Within the hippocampus proper fields CA1, 3, and 4 could be identified with certainty, while CA2 was tentatively identified as a small transitional zone between the CA1 and CA3 fields. Within the dentate gyrus evidence for adult hippocampal neurogenesis at a rate comparable to other mammals was observed. The subicular complex and entorhinal cortex also exhibited divisions typically observed in other mammalian species. In contrast to many other mammals, an architecturally and neurochemically distinct CA4 field was observed, supporting Lorente de Nó's proposed CA4 field, at least in some mammalian species. In addition, up to seven laminae were evident in the dentate gyrus. Calretinin immunostaining revealed the three sublamina of the molecular layer, while immunostaining for vesicular glutamate transporter 2 and neurofilament H indicate that the granule cell layer was composed of two sublamina. The similarities and differences observed in the tree pangolin indicate that the hippocampal formation is an anatomically and neurochemically conserved neural unit in mammalian evolution, but minor changes may relate to specific life history features and habits of species.