Atlas of the Brain of a Hatchling (74 mm body length) Platypus
Introduction
Details on platypus biology can be found at the Platypus page of this website. Please see Platypus Brain for topography of the adult cerebral cortex.
Methods
The specimen illustrated here (AMNH201969; body length of 74 mm, head length of 29.3 mm) was kindly made available by Prof. Ulrich Zeller of the Museum für Naturkunde, Berlin (see Zeller, 1988, 1989). The fixation for the specimen is unknown. A body length (predehydration) of 74 mm and head length of 29.3 mm suggests a post-hatching age of approximately 24 days (Ashwell, 2013). The specimen had been decalcified and embedded in Paraplast before being sectioned frontally at a thickness of 10 µm and stained with Azan. The Azan stain is suboptimal for revealing details of brain nuclei and the contrast of grey matter with fibre bundles, but key nuclei are still visible.
General observations
The os carunculae is still present. The forebrain is dominated by large lateral and medial ganglionic eminences (lge, mge - generating the microneurons of the striatum and cortex) and surmounted by a thin pallium (developing cortex). The hypothalamus, diencephalon and midbrain have large fields of postmitotic neurons, but nuclear boundaries are indistinct. Many of the nuclei of the caudal rhombencephalon are present, but the cerebellum is still immature and unfoliated. The developmental stage corresponds approximately to an E16/17 laboratory rat, or an early fetal (10 to 12 weeks) human.
Trigeminal nerve branches
The behaviourally important trigeminal nerve and its divisions (5oph, 5max, 5mand and subdivisions) are very large. At least 3 major branches of the infraorbital branch of the maxillary division of trigeminal nerve can be seen (5max/inf1, 2, 3 in Plates 3 to 5). These provide sensory reception (electro- and mechanosensation) from the greater part of the upper bill.
Olfactory apparatus
Olfactory epithelium is present in the roof of the nasal cavity (olfepith in Plates 3 to 11). The vomeronasal organ (VNO in Plates 3 to 6) and vomeronasal nerve (vn in Plates 8 to 10) are also present. The main and accessory olfactory bulbs (MOB, AOB) are poorly developed (see Plate 12). The piriform (primary olfactory) cortex is rudimentary (Pir in Plates 13 to 18).
Cerebral isocortex (neopallium)
The cerebral isocortex is at an early intermediate stage of development in that there is a thin cortical plate (CxP in Plates 13 to 21) throughout the extent of the pallium, but the subventricular zone (SubV in Plates 14 to 21) for the generation of cortical and subcortical microneurons is still large and active.
A distinctive feature of the developing platypus cerebral cortex is the sheet of tissue that extends from the pallial/striatal angle around the curvature of the cortex to form a layer between the developing striatum and cortex (SubV/SubPl in Plates 14 to 20). This zone appears to have characteristics of both subventricular and subplate zones as seen in eutherian mammals, in that it contains active mitosis and is also traversed by ascending and descending connections with the cortex (Ashwell and Hardman, 2012a).
The main functional regions of the cerebral isocortex (S1, R, PV, C, V etc; see Krubitzer et al., 1995) have been indicated, but these are based on topographic position and inherently notional because of the immaturity of the cortex.
Hippocampus (archipallium)
The hippocampus (Hi in Plates 15 to 20) is very immature and the dentate gyrus cannot be distinguished, but fibres of the fimbria can be seen (fi in Plates 16 to 20).
Subpallial parts of the telencephalon
Putative nuclei of the amygdala, caudate, putamen and septum have been indicated, but these are necessarily notional because nuclear boundaries are indistinct with the Azan stain.
(Dorsal) thalamus
The dorsal thalamus (prosomere 2 of diencephalon) doesn’t show clear internal subdivisions so the labelling of constituent nuclei is necessarily notional and based on topography.
Hypothalamus
Although the Azan stain provides only poor cytoarchitectural contrast, it is possible to discern some major nuclei of the hypothalamus. These include the suprachiasmatic nucleus (SCh in Plate 16), arcuate nuclei (Arc in Plates 17, 18), the ventromedial nucleus of hypothalamus (VMH in Plate 16 to 18), and mammillary nuclei of the mammillary body (MB in Plate 19).
Midbrain and isthmus
Lamination of the superior colliculus (SC in Plates 21 to 23) is in progress (see component grey matter layers SuG, InG, DpG) but still poorly defined, and the component nuclei of the inferior colliculus are indistinct (DCIC, CIC in Plates 22, 23).
Cerebellum
The cerebellum is very immature, and the external granular (germinal) layer (egl in Plates 21 to 23) that produces the microneurons of the cerebellar cortex has only just begun its expansion over the cerebellar primordium. A posterolateral fissure (plf in Plate 23) separates the rostral cerebellum (spino- and pontocerebellum functional components) from the caudal vestibulocerebellum (nodulus and flocculus).
Rhombencephalon
The Azan stain allows only an indistinct delineation of medullary nuclei, but the major cranial nerve nuclei can be distinguished. The most striking feature of the brainstem is the large size of the trigeminal sensory column (Pr5pc, Pr5mc, Sp5O, Sp5I; see Plates 18 to 23), particularly the most rostral elements close to the trigeminal nerve (5n; see Plates 18 to 20) entry. See Ashwell et al. (2006) for details of adult trigeminal nuclei in this species and Ashwell and Hardman (2012b) for trigeminal nuclei development.
Acknowledgements
Acknowledgement is given to the American Museum of Natural History who provided this specimen for Prof. Zeller’s work. I would like to thank Prof Ulrich Zeller and Dr Peter Giere of the MfN, Berlin Germany, for access to the MfN collection and for all their help during the work.
References
Ashwell KW (2013) Embryology and post-hatching development of the monotremes. In KWS Ashwell (Ed.), Neurobiology of Monotremes: Brain Evolution in Our Distant Mammalian Cousins (1st ed., pp. 31-46). CSIRO.
Ashwell KW, Hardman CD (2012a) Distinct development of the cerebral cortex in platypus and echidna. Brain Behavior and Evolution 79, 57-72.
Ashwell KW, Hardman CD (2012b) Distinct development of the trigeminal sensory nuclei in platypus and echidna. Brain Behavior and Evolution 79, 261–274.
Ashwell KWS, Hardman CD, Paxinos G (2006) Cyto- and chemoarchitecture of the sensory trigeminal nuclei of the echidna, platypus and rat. Journal of Chemical Neuroanatomy 31, 81-107.
Krubitzer L, Manger P, Pettigrew J, Calford M (1995) Organization of somatosensory cortex in monotremes. In search of the prototypical plan. Journal of Comparative Neurology 351, 261–306.
Zeller U (1988) The lamina cribrosa of Ornithorhynchus (Monotremata, Mammalia) Anatomy and Embryology178, 513–519.
Zeller U (1989) Die Entwicklung und Morphologie des Schadels von Ornithorhynchus anatinus: (Mammalia, Prototheria, Monotremata). Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft 545, 1–188. Verlag Waldemar Kramer, Frankfurt.
