Contribution of Genoarchitecture to Understanding Hippocampal Evolution and Development

Abstract

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.

Molecular anatomy of the alligator dorsal telencephalon

“…In this study, we provided evidence for a highly conserved tri‐partite molecular organization present in the hippocampus of the mouse and the medial cortex of a reptile. Furthermore, these data strongly suggest that the crocodilian medial cortex alone is homologous to the mammalian hippocampus, and undermine the proposal that the reptile dorsal cortex and medial cortex are together homologous to the hippocampus (Desfilis, Abellan, Sentandreu, & Medina, ; Medina, Abellan, & Desfilis, ).…”

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

“…A third model considers an even larger part of the reptile dorsal cortex as being part of the hippocampal formation ( Fig. 2 , light green) [Kappers, 1921;Rose, 1923;Filimonoff, 1964;Lohman and Smeets, 1991;Medina et al, 2017]. The differences between these models do not appear to stem from major differences in experimental methods, except maybe the use of different model animal species.…”

Comparative Contemplations on the Hippocampus

“…This thus indicates that functionally different types of input may be mapped onto different hippocampal domains along the transverse axis, a prediction that was shown to be correct in CA1 in rats with respect to spatial information carried by firing properties of neurons [Henriksen et al, 2010]. It remains to be established whether comparable functional differences exist in other clades, but recent gene expression patterns during embryological development indicate that in birds and lizards, LEC and MEC might be identifiable [Abellan et al, 2014;Medina et al, 2017]. Whether these different entorhinal domains in birds and lizards show connectional differences, comparable to those seen in mammals, is open to further study.…”

“…In his seminal paper on the EC [Ramón y Cajal, 1902], he stated twice that the connections between the EC and the hippocampal formation are so conspicuous that they necessarily imply the functional solidarity of both centers. We will, however, not deal extensively with the comparative aspects of the EC in this paper (for more details, see Medina et al [2017]). …”