Astroglial-mediated remodeling of the interhemispheric midline during telencephalic development is exclusive to eutherian mammals

Abstract

The corpus callosum forms the major interhemispheric connection in the human brain and is unique to eutherian (or placental) mammals. The developmental events associated with the evolutionary emergence of this structure, however, remain poorly understood. A key step in callosal formation is the prior remodeling of the interhemispheric fissure by embryonic astroglial cells, which then subsequently act as a permissive substrate for callosal axons, enabling them to cross the interhemispheric midline. However, whether astroglial-mediated interhemispheric remodeling is unique to eutherian mammals, and thus possibly associated with the phylogenetic origin of the corpus callosum, or instead is a general feature of mammalian brain development, is not yet known. To investigate this, we performed a comparative analysis of interhemispheric remodeling in eutherian and non-eutherian mammals, whose lineages branched off before the evolution of the corpus callosum. Whole brain MRI analyses revealed that the interhemispheric fissure is retained into adulthood in marsupials and monotremes, in contrast to eutherians (mice), in which the fissure is significantly remodeled throughout development. Histological analyses further demonstrated that, while midline astroglia are present in developing marsupials, these cells do not intercalate with one another through the intervening interhemispheric fissure, as they do in developing mice. Thus, developing marsupials do not undergo astroglial-mediated interhemispheric remodeling. As remodeling of the interhemispheric fissure is essential for the subsequent formation of the corpus callosum in eutherians, our data highlight the role of astroglial-mediated interhemispheric remodeling in the evolutionary origin of the corpus callosum.

Keywords: Corpus callosum; Interhemispheric fissure; Interhemispheric remodeling; Telencephalic commissure formation.

Fig. 1

The IHF is retained into adulthood in both marsupials and monotremes. Comparison of horizontal T1-weighted structural MRI images from adult mouse (a), dunnart (b) and platypus (c) brains. Brackets indicate the distribution of the IHF, hippocampal commissure (HC) and corpus callosum (CC) along the midline. Note separation of the septum in dunnart and platypus brains, compared to the fused septum in mouse brains (red arrowheads)

Fig. 2

Developmental remodeling of the IHF does not occur in marsupials. a Immunofluorescence for Laminin (green), counterstained with DAPI (blue) shows the length of the IHF and the extent of fused septum (brackets) in horizontal sections of the forebrain from E16 and adult mice (upper panels), as well as P24 and adult dunnarts (lower panels). Insets in right-hand panels show Laminin expression (white) at adult stages within the IHF. b Schematic of midline and IHF length measurements taken to quantify normalized IHF length. c Quantification of normalized IHF length in E16 and adult mouse brains, compared with normalized IHF length in dunnarts at the equivalent stages (P24 and adult). Data are represented as means ± SEM (n-values within bars). Scale bars within insets = 100 μm

Fig. 3

Astroglia do not initiate IHF remodeling in marsupials. Immunofluorescence was performed for Gap43 (red) and Laminin (green) to compare commissure formation and IHF development, and for Glast (red) and Gfap (green) to compare midline glial development in mice (a, c, and e) and dunnarts (b, d, and f) at equivalent developmental stages. The plane of each horizontal section (red dotted line) is represented by a mid-sagittal schematic on the right of each panel in a – f. Brackets indicate the distribution of the IHF (green) and commissural axons along the interhemispheric midline (yellow). Arrowheads indicate the distribution of MZG and MZG-like cells (red arrowheads) as well as the position of the third ventricle (3V; white arrowheads) along the interhemispheric midline. AC, anterior commissure; CC, corpus callosum; Fx, fornix; HC, hippocampal commissure