Abstract
Sonic hedgehog (SHH) is a secreted signaling factor that is implicated in the molecular patterning of the central nervous system (CNS), somites, and limbs in vertebrates. SHH has a crucial role in the generation of ventral cell types along the entire rostrocaudal axis of the neural tube. It is secreted early in development by the axial mesoderm (prechordal plate and notochord) and the overlying ventral neural tube. Recent studies clarified the impact of SHH signaling mechanisms on dorsoventral patterning of the spinal cord, but the corresponding phenomena in the rostral forebrain are slightly different and more complex. This notably involves separate Shh expression in the preoptic part of the forebrain alar plate, as well as in the hypothalamic floor and basal plates. The present work includes a detailed spatiotemporal description of the singular alar Shh expression pattern in the rostral preoptic forebrain of chick embryos, comparing it with FoxG1, Dlx5, Nkx2.1, and Nkx2.2 mRNA expression at diverse stages of development. As a result of this mapping, we report a subdivision of the preoptic region in dorsal and ventral zones; only the dorsal part shows Shh expression. The positive area impinges as well upon a median septocommissural preoptic domain. Our study strongly suggests tangential migration of Shh-positive cells from the preoptic region into other subpallial domains, particularly into the pallidal mantle and the intermediate septum.
Highlights
The central nervous system (CNS) is regionalized along anteroposterior (AP) and dorsoventral (DV) axes
The diagonal domain (Dg) contains a medial part of the complex of the bed nucleus of the stria terminalis (Bst), while the corresponding lateral part belongs to the adjacent pallidal domain
Preliminary remarks about telencephalic subpallial nomenclature In chick embryos, the telencephalic vesicle starts to evaginate at stages HH16/17 (Puelles et al, 1987a) and already expresses widely the FoxG1 transcription factor from HH14 onwards (Figures 1C,H,N)
Summary
The central nervous system (CNS) is regionalized along anteroposterior (AP) and dorsoventral (DV) axes. The subpallium is divided structurally in four distinct parallel territories disposed parallel to the septoamygdaloid dimension (Puelles et al, 2000) These are known as striatum, pallidum, anterior entopeduncular area and preoptic area (St, Pal, AEP, POA) and adopt a nested configuration (Puelles et al, 2000; Puelles and Rubenstein, 2003). The Dg/AEP concept essentially refers to a full radial histogenetic domain intercalated at the telencephalic stalk between Pal and POA. It extends from the neighborhood of the medial septum back to the subpallial amygdala. The Dg contains a medial part of the complex of the bed nucleus of the stria terminalis (Bst), while the corresponding lateral part belongs to the adjacent pallidal domain
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