The Pallium in Reptiles and Birds in the Light of the Updated Tetrapartite Pallium Model

142

This essay reviews step by step the conceptual changes of the updated tetrapartite pallium model from its tripartite and early tetrapartite antecedents. The crucial observations in mouse material are explained first in the context of assumptions, tentative interpretations, and literature data. Errors and the solutions offered to resolve them are made explicit. Next, attention is centered on the lateral pallium sector of the updated model, whose definition is novel in incorporating a claustro-insular complex distinct from both olfactory centers (ventral pallium) and the isocortex (dorsal pallium). The general validity of the model is postulated at least for tetrapods. Genoarchitectonic studies performed to check the presence of a claustro-insular field homolog in the avian brain are reviewed next. These studies have indeed revealed the existence of such a complex in the avian mesopallium (though stratified outside-in rather than inside-out as in mammals), and there are indications that the same pattern may be found in reptiles as well. Peculiar pallio-pallial tangential migratory phenomena are apparently shared as well between mice and chicks. The issue of whether the avian mesopallium has connections that are similar to the known connections of the mammalian claustro-insular complex is considered next. Accrued data are consistent with similar connections for the avian insula homolog, but they are judged to be insufficient to reach definitive conclusions about the avian claustrum. An aside discusses that conserved connections are not a necessary feature of field-homologous neural centers. Finally, the present scenario on the evolution of the pallium of sauropsids and mammals is briefly visited, as highlighted by the updated tetrapartite model and present results.

Section: The Tripartite Modelsupporting

confidence: 61%

Comments on the Updated Tetrapartite Pallium Model in the Mouse and Chick, Featuring a Homologous Claustro-Insular Complex

Puelles

2017

142

“…In contrast to our opinion, some authors think that both the MEC and LEC derive from the medial pallium [Puelles et al, 2017], which would help to explain those similarities. However, the latter view cannot explain the clear differences of the MEC and LEC (mentioned above) regarding gene expression and enhancer activity during early development.…”

Section: Mouse Ec: Two Divisions Two Embryonic Origins?contrasting

confidence: 48%

Contribution of Genoarchitecture to Understanding Hippocampal Evolution and Development

Medina

Abellán

Desfilis³

2017

120

The hippocampal formation is a highly conserved structure of the medial pallium that works in association with the entorhinal cortex, playing a key role in memory formation and spatial navigation. Although it has been described in several vertebrates, the presence of comparable subdivisions across species remained unclear. This panorama has started to change in recent years thanks to the identification of some of the genes that regulate the development of the hippocampal formation in the mouse and help to delineate its subdivisions based on molecular features. Some of these genes have been used to try to identify subdivisions in chicken and lizards comparable to those of the mammalian hippocampal formation and the entorhinal cortex. Here, we review some of these data, which suggest the existence of fields comparable to the dentate gyrus, CA3, CA1, subiculum, as well as medial and lateral parts of the entorhinal cortex in all amniotes. We also analyze available data suggesting the existence of serial connections between these fields, speculate on the possible existence of auto-associative loops in CA3, and discuss general principles governing the formation of the connections.

“…A recent series of investigations has established a framework for comparing brain regions across vertebrates by using conserved developmental gene expression [Smith-Fernández et al, 1998;Puelles et al, 2000;Medina and Abellán, 2009;Puelles and Ferran, 2012]. This research program has reinforced the idea that mammalian hippocampal and parahippocampal regions originate from and form a distinct cortical sector (the medial pallium), which suggested that the lateral entorhinal cortex may have a uniquely different developmental origin [Abellán et al, , 2014Puelles et al, 2017]. Most studies have focused on cortical development in mice and chickens, two distantly related species.…”

Section: The Hippocampus In Pieces and Patchesmentioning

confidence: 98%

“…Also, most regions of the mammalian medial pallium, at least between CA1 and the entorhinal cortex [Slomianka et al, 2011], contain multiple cellular layers. Because reptilian cortex contains only one principal cell layer, the interpretation of layer-specific markers remains difficult [Puelles et al, 2017]. In summary, it could be that finer subdivisions of the reptilian hippocampal region remain to be uncovered (e.g., with molecular methods), that single reptilian hippocampal regions correspond to multiple mammalian areas, or that some mammalian hippocampal areas do not have an equivalent in reptiles.…”

Section: A Model Of the Brainmentioning

confidence: 99%

“…This second model ascribes (approximately) the medial half of the reptilian dorsal cortex to the hippocampal formation ( Fig. 2 , medium green) [Edinger, 1908;Smith, 1919;Schepers, 1948;Northcutt, 1969;Kirsche, 1972;Schwerdtfeger and Germroth, 1990;Puelles et al, 2017]. A third model considers an even larger part of the reptile dorsal cortex as being part of the hippocampal formation ( Fig.…”

Section: A Model Of the Brainmentioning

confidence: 99%

See 1 more Smart Citation

On the Value of Reptilian Brains to Map the Evolution of the Hippocampal Formation

Reiter

Liaw²,

Yamawaki³

et al. 2017

Our ability to navigate through the world depends on the function of the hippocampus. This old cortical structure plays a critical role in spatial navigation in mammals and in a variety of processes, including declarative and episodic memory and social behavior. Intense research has revealed much about hippocampal anatomy, physiology, and computation; yet, even intensely studied phenomena such as the shaping of place cell activity or the function of hippocampal firing patterns during sleep remain incompletely understood. Interestingly, while the hippocampus may be a ‘higher order' area linked to a complex cortical hierarchy in mammals, it is an old cortical structure in evolutionary terms. The reptilian cortex, structurally much simpler than the mammalian cortex and hippocampus, therefore presents a good alternative model for exploring hippocampal function. Here, we trace common patterns in the evolution of the hippocampus of reptiles and mammals and ask which parts can be profitably compared to understand functional principles. In addition, we describe a selection of the highly diverse repertoire of reptilian behaviors to illustrate the value of a comparative approach towards understanding hippocampal function.