In early vertebrate development, organizer regions-groups of cells that signal to and thereby influence neighboring cells by secreted morphogens-play pivotal roles in the establishment and maintenance of cell identities within defined tissue territories. The midbrain-hindbrain organizer drives regionalization of neural tissue into midbrain and hindbrain territories with fibroblast growth factor 8 (FGF8) acting as a key morphogen. This organizer has been extensively studied in chicken, mouse, and zebrafish. Here, we demonstrate the enrichment of FGF8-expressing cells from human pluripotent stem cells (hPSCs), cultured as attached embryoid bodies using antibodies that recognize \"Similar Expression to Fgf\" (SEF) and Frizzled proteins. The arrangement of cells in embryoid body subsets of these cultures and the gene expression profile of the FGF8-expressing population show certain similarities to the midbrain-hindbrain organizer in animal models. In the embryonic chick brain, the enriched cell population induces formation of midbrain structures, consistent with FGF8-organizing capability.

This essay reexamines molecular evidence supporting the existence of the \'preisthmus\', a caudal midbrain domain present in vertebrates (studied here in the mouse). It is thought to derive from the embryonic m2 mesomere and appears intercalated between the isthmus (caudally) and the inferior colliculus (rostrally). Among a substantial list of gene expression mappings examined from the Allen Developing and Adult Brain Atlases, a number of quite consistent selective positive markers, plus some neatly negative markers, were followed across embryonic stages E11.5, E13.5, E15.5, E18.5, and several postnatal stages up to the adult brain. Both alar and basal subdomains of this transverse territory were explored and illustrated. It is argued that the peculiar molecular and structural profile of the preisthmus is due to its position as rostrally adjacent to the isthmic organizer, where high levels of both FGF8 and WNT1 morphogens must exist at early embryonic stages. Isthmic patterning of the midbrain is discussed in this context. Studies of the effects of the isthmic morphogens usually do not attend to the largely unknown preisthmic complex. The adult alar derivatives of the preisthmus were confirmed to comprise a specific preisthmic sector of the periaqueductal gray, an intermediate stratum represented by the classic cuneiform nucleus, and a superficial stratum containing the subbrachial nucleus. The basal derivatives, occupying a narrow retrorubral domain intercalated between the oculomotor and trochlear motor nuclei, include dopaminergic and serotonergic neurons, as well as a variety of peptidergic neuron types.

The objective of this study was to induce the production of isthmic organizer (IsO)-like cells capable of secreting fibroblast growth factor (FGF) 8 and WNT1 from human embryonic stem cells (ESCs). The precise modulation of canonical Wnt signaling was achieved in the presence of the small molecule CHIR99021 (0.6 μM) during the neural induction of human ESCs, resulting in the differentiation of these cells into IsO-like cells having a midbrain-hindbrain border (MHB) fate in a manner that recapitulated their developmental course in vivo. Resultant cells showed upregulated expression levels of FGF8 and WNT1. The addition of exogenous FGF8 further increased WNT1 expression by 2.6 fold. Gene ontology following microarray analysis confirmed that IsO-like cells enriched the expression of MHB-related genes by 40 fold compared to control cells. Lysates and conditioned media of IsO-like cells contained functional FGF8 and WNT1 proteins that could induce MHB-related genes in differentiating ESCs. The method for generating functional IsO-like cells described in this study could be used to study human central nervous system development and congenital malformations of the midbrain and hindbrain.

Pluripotent stem cells (PSCs) possess self-organizing abilities in 3D culture. This property has been demonstrated in recent studies, including the generation of various neuroectodermal and endodermal tissues. For example, PSCs are able to differentiate into specific type of neural tissues, such as the neocortex and the optic cup, in response to local positional information brought about by signals during embryogenesis. In contrast, the generation of cerebellar tissue from PSCs requires a secondary induction by a signaling center, called the isthmic organizer, which first appears in the cell aggregate in 3D culture. Such developmental complexity of cerebellum has hampered establishment of effective differentiation culture system from PSCs, thus far.We recently reported that cerebellar neurons are generated from human PSCs (hPSCs). In this chapter, we describe an efficient protocol for differentiation of 3D cerebellar neuroepithelium from hPSCs. We also describe the protocols for further differentiation into specific neurons in the cerebellar cortex, such as Purkinje cells and the granule cells.

Gap junctions (GJs) are integral membrane proteins that enable the direct cytoplasmic exchange of ions and low molecular weight metabolites between adjacent cells. They are formed by the apposition of two connexons belonging to adjacent cells. Each connexon is formed by six proteins, named connexins (Cxs). Current evidence suggests that gap junctions play an important part in ensuring normal embryo development. Mutations in connexin genes have been linked to a variety of human diseases, although the precise role and the cell biological mechanisms of their action remain almost unknown. Among the big family of Cxs, several are expressed in nervous tissue but just a few are expressed in the anterior neural tube of vertebrates. Many efforts have been made to elucidate the molecular bases of Cxs cell biology and how they influence the morphogenetic signal activity produced by brain signaling centers. These centers, orchestrated by transcription factors and morphogenes determine the axial patterning of the mammalian brain during its specification and regionalization. The present review revisits the findings of GJ composed by Cx43 and Cx36 in neural tube patterning and discuss Cx43 putative enrollment in the control of Fgf8 signal activity coming from the well known secondary organizer, the isthmic organizer.

Historically, the molecular and cellular mechanisms of cerebellar development were investigated through structural descriptions and studying spontaneous mutations in animal models and humans. Advances in experimental embryology, genetic engineering, and neuroimaging techniques render today the possibility to approach the analysis of molecular mechanisms underlying histogenesis and morphogenesis of the cerebellum by experimental designs. Several genes and molecules were identified to be involved in the cerebellar plate regionalization, specification, and differentiation of cerebellar neurons, as well as the establishment of cellular migratory routes and the subsequent neuronal connectivity. Indeed, pattern formation of the cerebellum requires the adequate orchestration of both key morphogenetic signals, arising from distinct brain regions, and local expression of specific transcription factors. Thus, the present review wants to revisit and discuss these morphogenetic and molecular mechanisms taking place during cerebellar development in order to understand causal processes regulating cerebellar cytoarchitecture, its highly topographically ordered circuitry and its role in brain function.