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We used magnetic resonance imaging to study the anatomy of cortical regions, nuclear groups, and major tracts in the brain of a monotreme, i.e., the short-beaked echidna (Tachyglossus aculeatus). Our specimens were from a collection held at the Australian Museum in Sydney and had been stored in formaldehyde solution for at least 70 years. Despite this, we were able to detect fine detail in the nuclear divisions of structures as well as in fiber tracts. In particular, we could detect the medial lemniscus as it approached the ventral posterior thalamic nucleus, subdivisions within the ventral posterior thalamic nucleus, lamination and subdivisions within the hippocampal formation, components of the olfactory pathways, and nuclei within the temporal amygdala. We were able to map the topography of subcortical white matter and relate it to cortical regions determined on the basis of physiology, as well as chemical and cytoarchitecture. As expected, dense aggregations of fibers were noted in association with the primary sensory areas of the isocortex (somatosensory, visual, and auditory) and connecting primary olfactory regions (intrabulbar anterior commissure and associated fibers). We found longitudinal fibers in the basal forebrain (medial forebrain bundle) and brainstem (corticopontine and corticospinal tracts), as well as a dense array of fibers associated with the vermal and paravermal zones of the anterior lobe of the cerebellum. We also observed previously unrecognized fiber systems, i.e., commissural connections between the paired frontal isocortical fields (dorsal Fr1), dense fibers to the retrosplenial association cortex, and prominent, paired longitudinal fiber bundles in the dorsal forebrain (longitudinal fasciculus) that intersected the dorsal anterior commissure. The connectome results are consistent with the known neuroanatomy of this monotreme and they extend our knowledge of the fiber topography within this unusual brain. Our findings demonstrate the feasibility of using this sort of imaging of archived brains to analyze the neuroanatomy of rare, endangered, and evolutionarily significant species.

The monotremes are a unique group of mammals whose young are incubated in a leathery-shelled egg and fed with milk from teatless areolae after hatching. As soon as they hatch, monotreme young must be able to maneuver around the nest or maternal pouch to locate the areolae and stimulate milk ejection. In the present study, the embryological collections at the Museum für Naturkunde, Berlin, have been used to follow the development of the monotreme cerebellum through incubation and lactational phases, to determine whether cerebellar circuitry is able to contribute to the coordination of locomotion in the monotreme hatchling, and to correlate cerebellar development with behavioral maturation. The structure of the developing monotreme cerebellum and the arrangement of transitory neuronal populations are similar to those reported for fetal and neonatal eutherians, but the time course of the key events of later cerebellar development is spread over a much longer period. Expansion of the rostral rhombic lip and formation of the nuclear and cortical transitory zones occurs by the time of hatching, but it is not until after the end of the first post-hatching week that deep cerebellar neurons begin to settle in their definitive positions and the Purkinje cell layer can be distinguished. Granule cell formation is also prolonged over many post-hatching months and the external granular layer persists for more than 20 weeks after hatching. The findings indicate that cerebellar circuitry is unlikely to contribute to the coordination of movements in the monotreme peri-hatching period. Those activities are most likely controlled by the spinal cord and medullary reticular formation circuitry.

The extant monotremes (platypus and echidnas) are believed to all be capable of electroreception in the trigeminal pathways, although they differ significantly in the number and distribution of electroreceptors. It has been argued by some authors that electroreception was first developed in an aquatic environment and that echidnas are descended from a platypus-like ancestor that invaded an available terrestrial habitat. If this were the case, one would expect the developmental trajectories of the trigeminal pathways to be similar in the early stages of platypus and short-beaked echidna development, with structural divergence occurring later. We examined the development of the peripheral trigeminal pathway from snout skin to trigeminal ganglion in sectioned material in the Hill and Hubrecht collections to test for similarities and differences between the two during the development from egg to adulthood. Each monotreme showed a characteristic and different pattern of distribution of developing epidermal sensory gland specializations (electroreceptor primordia) from the time of hatching. The cross-sectional areas of the trigeminal divisions and the volume of the trigeminal ganglion itself were also very different between the two species at embryonic ages, and remained consistently different throughout post-hatching development. Our findings indicate that the trigeminal pathways in the short-beaked echidna and the platypus follow very different developmental trajectories from the earliest ages. These findings are more consistent with the notion that the platypus and echidna have both diverged from an ancestor with rudimentary electroreception and/or trigeminal specialization, rather than the contention that the echidna is derived from a platypus-like ancestor.