Abstract
The prethalamic eminence (PThE), a diencephalic caudal neighbor of the telencephalon and alar hypothalamus, is frequently described in mammals and birds as a transient embryonic structure, undetectable in the adult brain. Based on descriptive developmental analysis of Tbr1 gene brain expression in chick embryos, we previously reported that three migratory cellular streams exit the PThE rostralward, targeting multiple sites in the hypothalamus, subpallium and septocommissural area, where eminential cells form distinct nuclei or disperse populations. These conclusions needed experimental corroboration. In this work, we used the homotopic quail-chick chimeric grafting procedure at stages HH10/HH11 to demonstrate by fate-mapping the three predicted tangential migration streams. Some chimeric brains were processed for Tbr1 in situ hybridization, for correlation with our previous approach. Evidence supporting all three postulated migration streams is presented. The results suggested a slight heterochrony among the juxtapeduncular (first), the peripeduncular (next), and the eminentio-septal (last) streams, each of which followed differential routes. A possible effect of such heterochrony on the differential selection of medial to lateral habenular hodologic targets by the migrated neurons is discussed.
Highlights
The prethalamic eminence (PThE) is a firm candidate to produce excitatory neurons invading neighboring septocommissural, subpallial and hypothalamic histogenetic units (Abellán et al 2010a; Bupesh et al 2011; Medina and Abellán 2012; Watanabe et al 2018; Alonso et al 2020)
We mapped schematically the location of labelled quail cell nuclei at ventricular and mantle zone levels (Fig. 3e’–l’) and we show in Fig. 4 correlative microphotographic details of framed areas (Fig. 4a–g; note sometimes the QCPNlabelled brown cell nuclei are partly hidden by the blue Tbr1 in situ hybridization (ISH) signal where it is massive, as occurs inside the PThE mantle; the brown label usually can be detected at high magnification)
At more advanced stages (HH29-30), once the cerebral peduncle enlarges, these initial streams separate into an intermediate juxtapeduncular component and a subpial peripeduncular component
Summary
The prethalamic eminence (PThE) is a firm candidate to produce excitatory neurons invading neighboring septocommissural, subpallial and hypothalamic histogenetic units (Abellán et al 2010a; Bupesh et al 2011; Medina and Abellán 2012; Watanabe et al 2018; Alonso et al 2020). Characteristic developmental gene markers of the PThE include Calb, Lhx, Lhx, VGlut, Tbr, Tbr and Pax (the latter is expressed selectively in ventricular PThE cells, as occurs in the telencephalic pallium) (see other markers in Bulfone et al 1995, 1999; Abbott and Jacobowitz 1999; Rétaux et al 1999; Puelles et al 2000; Abellán et al 2010b; Adutwum-Ofosu et al 2016; Ruiz-Reig and Studer 2017; Ruiz-Reig et al 2017) This molecular profile is consistent with a progenitor population producing glutamatergic neurons (Englund et al 2005; Hevner et al 2006). The glutamatergic identity of eminential neurons distinguishes the PThE from other more ventral alar prethalamic microzones, where the activity of master genes such as Dlx, Arx and Pax (in the mantle) lead to GABAergic neuronal phenotypes (Puelles et al 2000; Shimogori et al 2010)
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