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.

中文翻译：

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在远脑发育过程中，星形胶质介导的半球间中线重塑仅对真人哺乳动物有效。

call体在人脑中形成主要的半球间连接，是真性（或胎盘）哺乳动物所特有的。然而，与这种结构的进化出现有关的发展事件仍然知之甚少。骨call形成的关键步骤是先通过胚胎星形胶质细胞对半球间裂进行重塑，然后再将其用作骨x轴突的允许底物，使它们能够越过半球中线。然而，尚不清楚星形胶质细胞介导的半球间重塑对于欧氏哺乳动物是独特的，因此可能与call体的系统发生起源有关，还是作为哺乳动物脑发育的一般特征。为了对此进行调查，我们对欧亚和非欧亚哺乳动物的半球间改建进行了比较分析，这些哺乳动物的谱系在call体进化之前就分支了。全脑MRI分析显示，有袋动物和单足类动物的半球间裂隙保留在成年期，与真人（小鼠）相反，在整个发育过程中，裂隙被明显地重塑。组织学分析进一步表明，尽管中线星形胶质细胞存在于发育中的有袋动物中，但这些细胞不会像中间小鼠那样通过中间的半球间缝隙相互插入。因此，发展中的有袋动物不会经历星形胶质介导的半球间重塑。