Early steps in the development of the forebrain

Abstract

The tremendous complexity of the adult forebrain makes it a challenging task to elucidate how this structure forms during embryonic development. Nevertheless, we are beginning to understand how a simple epithelial sheet of ectoderm gives rise to the labyrinthine network of cells that constitutes the functional forebrain. Here, we discuss early events in forebrain development--those that lead to the establishment of the anterior neural plate and the regional subdivision of this territory into the different domains of the prospective forebrain.

Figure 1. Organization of the Rostral Neural Plate and Forebrain in Embryonic Zebrafish

(A and B) Cartoons of the rostral neural plate of a zebrafish embryo with anterior to the left. The approximate locations of cells destined to give rise to various territories are shown in different colors. The topological relationships of prospective forebrain domains are conserved between vertebrate species, although the relative sizes of the neural plate domains vary. In (A), axial midline neural tissue (blue) is shown moving rostrally within the neural plate deflecting cells within the eye field (orange) laterally (arrows) from where they will contribute to the left and right optic vesicles. The radial organization of the most rostral neural plate makes terminology of axes problematic. Telencephalon and eye field are both considered to be dorsal (alar) structures in the mature forebrain (G). On the neural plate, prospective telencephalon can be considered to be both anterior and/or lateral (dorsal) to the eye field. Prospective telencephalon can therefore be considered to be both a prospective dorsal, and the most rostral, neural plate domain. (C) Expression of two transcripts that mark the various territories in the rostral neural plate of a zebrafish embryo at same stage and in same orientation as shown in (B). (D–F) Lateral with rostral to the left (D and E) and frontal (F) views of embryonic zebrafish brains. (D) Early differentiating neurons visualized with an antibody to the neurotransmitter GABA. (E) Expression of shh (blue). In caudal regions, expression is restricted to the floorplate whereas in the midbrain expression includes other basal plate cells. In the diencephalon, shh expression extends dorsally along the zli, dividing the rostral prethalamic region from the more caudal thalamic and pretectal regions. Within the hypothalamus, only the anterior and dorsal regions express shh at this stage. In the text, we define the rostral limit of the floorplate as being located approximately at the caudal border of the hypothalamus (asterisk)–the rostral limit of shh expression in midline cells. Hypothalamus may well have specialized midline cells that run in continuity with more caudal floorplate. At gastrula and neural plate stages, it is not known if the different regions of the prospective hypothalamus are already specified. (F) Optic stalk tissue that connects the eyes to the brain is shown in continuity with a domain of cells that spans the midline. Cells in this medial location most likely originate from the medial parts of the eye field directly rostral to the prospective hypothalamus (see [B]), and differentiate as preoptic regions at the interface between telencephalon and diencephalon. (G) Schematic view of the front of the forebrain showing the relationship between the eye and the brain. Abbreviations: ac, anterior commissure; cb, cerebellum; cf, choroid fissure; d, diencephalon; e, epiphysis; ef, eye field; fp, floor plate; hy, hypothalamus; l, lens; le, prospective left eye; m, midbrain; nr, neural retina; os, optic stalk; p, pretectum; pa, pallium; pc, posterior commissure; pe, pigment epithelium; poc, postoptic commissure; pt, prethalamus; re, prospective right eye; sp, subpallium; t, telencephalon; th, thalamus; zli, zona limitans intrathalamica. Figure panels are adapted from Barth and Wilson (1995) and Macdonald et al. (1994) and unpublished data from N. Staudt and C.H.

Figure 2. Cell Movements Separate the Prospective Forebrain from Sources of Caudalizing Factors

(A–C) Dorsal views of fish embryos during gastrulation. In this and other panels, epi-blast/nonneural ectoderm is shown in green whereas neural ectoderm is shown in red/pink. The germ ring (blue) is a source of caudalizing signals (white arrowheads) and so as epiboly moves the germ ring vegetally (black arrows); the prospective forebrain becomes positioned further distant from the caudalizing signals. The prechordal plate mesendoderm migrates rostrally beneath the neural plate and is a source of antagonists of caudalizing signals. Prospective hypothalamic cells (*) also move rostrally within the neural plate. Although not illustrated on the figure, they lag behind the advancing prechordal plate (Varga et al., 1999). (D–G) Dorsal views of chick embryos prior to and during gastrulation. The pink star indicates the approximate rostral limit of the prospective forebrain within the epiblast. At very early stages (D and E), the prospective forebrain moves rostrally in response to signals from underlying hypoblast tissues. This distances the prospective forebrain from precursors of the primitive streak and organizer (blue star) and from caudalizing signals. By mid-late streak stage (F), the organization of tissues is similar to that shown for fish in (A). The node sits caudal to the rostral neural plate, and, as it regresses (black arrow), the prospective forebrain is distanced from caudalizing signals (white arrowheads). (H–J) Lateral views of sagittally bisected mouse embryos prior to and during gastrulation. The arrows indicate movements in the extraembryonic layer (yellow) that shift the AVE (orange) rostrally. This tissue is a source of signals (black arrowhead) that protect the adjacent prospective forebrain from the influence of caudalizing signals. The position of the future organizer/node is shown by the double asterisk. (D)–(J) are adapted from Foley et al. (2000), Stern (2001), and Lu et al. (2001) with modifications based on the fate map of Fernandez-Garre et al. (2002) and interpretations of Rubenstein et al. (1998). Abbreviations: AVE, anterior visceral endoderm; fb, prospective forebrain; n, node; nc, prospective notochord; pcp, prechordal plate mesendoderm; ps, primitive streak; A, anterior; P, posterior.

Figure 3. Signals that Regulate Regionalization of the Rostral Neural Plate

(A–C) Schematics of the neural plate (gray), nascent mesendoderm in the germ ring (pale blue), organizer (dark blue), and midline neural tissue (blue in [C], [D], and [E]) at successive stages of gastrulation. Anterior (including prospective forebrain) is to the top. The panels are based upon neural development in fish but similar events occur in other species. Arrows indicate various secreted signals, and although we show directionality, there is no evidence that these signals only act in the directions indicated. (A) During early gastrulation, caudalizing signals (including Wnts, RA, and Fgfs) arise from the germ ring. Antagonists to at least some of these signals are produced in the organizer and its early derivatives (prechordal plate mesendoderm). Bmp signals from nonneural ectoderm encroach into the neural plate at this and later stages. (B) By mid- to late gastrulation, Wnt antagonists and the secreted protein Tiarin are secreted from cells at and/or near the anterior margin of the rostral neural plate. Expression of Fgfs (not shown) and Wnts is initiated in neural plate domains caudal to the prospective forebrain. At this stage, midline neural tissue is extending rostrally in the neural plate (outlined) and may already be a source of Hh and Nodal signals (not shown). (C) By neural plate stage, Fgfs are expressed in the ANB and Hh and Nodal ligands are expressed in prospective hypothalamic tissue (blue) that has extended close to the rostral limit of the neural plate. (D and E) Schematics showing some of the signals arising from tissues beneath the neural plate (top layer) that potentially influence forebrain development. Mesoderm and axial mesendoderm (prechordal plate and notochord) are shown in blue, and endoderm and extraembryonic tissues are shown in yellow. (D) During gastrulation, axial mesendoderm produces both Nodal and Hh proteins that signal to the overlying neural plate. The effects of Nodal activity upon midline neural tissue are moderated by the activity of Nodal antagonists (not shown, see Feldman et al., 2002). Endodermal and extraembryonic tissues are probably also a source of signals (not shown) that influence neural plate development in all species. (E) By neural plate stage, axial mesendoderm has reached the rostral limit of the anterior neural plate. In addition to Nodals and Hhs, prechordal plate mesendoderm also expresses Bmps whereas the more caudal axial tissues express Bmp antagonists. Abbreviations: am, prospective axial mesendoderm; d, prospective diencephalon; e, eye field; mb, prospective midbrain; nne, nonneural ectoderm; np, neural plate; pm, paraxial mesoderm; t, telencephalon.

Figure 4. Nodal and Hh Signals Affect Hypothalamic Induction and Patterning

(A–C) Frontal views of a wild-type zebrafish embryo (A) and two embryos (B and C) homozygous for mutations affecting Nodal signaling. In the mutants, the hypothalamic tissue that is normally located between the eyes is absent and the retinae are partially (B) or completely (C) fused. (D and E) Lateral views of living wild-type zebrafish embryos in which wild-type cells or cells unable to receive Nodal signals (oep^−/−) were transplanted into the prospective hypothalamus during gastrulation. Both wild-type and mutant cells have extended rostrally along the ventral neural tube contributing to both floorplate and hypothalamus. However, the cells unable to receive Nodal signals (green in [D] and red in [E]) are excluded from the posterior and ventral regions of the hypothalamus. The ventral midline of the CNS is indicated by dots. (F and G) Lateral views of brains of a wild-type zebrafish embryo (F) and an embryo (G) homozygous for a mutation in the smoothened gene (smu^−/−) in which Hh signaling is severely reduced. Expression of shh (red) in dorsal and anterior hypothalamic tissue is reduced in the mutant, whereas emx2 expression (blue) in posterior and ventral hypothalamus is expanded. Abbreviations: da, anterior and dorsal hypothalamus; fp, floorplate; l, lens; pv, posterior and ventral hypothalamus; r, retina; t, telencephalon; zli, zona limitans intrathalamica. Figure panels adapted from Masai et al. (2000) and Mathieu et al. (2002).