Early Development

The forebrain is derived from the anteriormost transverse domain of the neural plate that is generated during gastrulation (Fig. 1). Tissues adjacent to and within the neural plate produce molecules that regulate regional specification and morphogenesis of the CNS. Prospective subdivisions of the brain are specified through several mechanisms. Anteroposterior patterning generates transverse subdivisions: forebrain, midbrain, hindbrain, and spinal cord (Fig. 1A). Dorsoventral patterning generates longitudinally aligned domains, called floor plate, basal plate, alar plate, and roof plate (Fig. 1B). Within the forebrain, neuromeric theories postulate that there are further transverse subdivisions, which will be discussed later. Finally, regionally distinct molecular properties of the neuroepithelium and local signals from adjacent

Encyclopedia of the Human Brain Volume 2

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Figure 1 Early development of the forebrain. (A) Schema of transverse domains specified at the neural plate stage; the transverse boundaries are represented by black lines. Note that telencephalic and eye vesicles are separate outgrowths. (B) Schema of longitudinal domains at the same stage (boundaries in black; transverse and other limits in gray). (C) Schema of cross section through the neural plate showing the longitudinal domains and the relative position of the main tissues that regulate dorsoventral patterning, the notochord and the nonneural ectoderm. These exert ventralization and dorsalization effects, respectively, which are symbolized below. (D) Side view of the closed neural tube. The forebrain appears rostral to the mesencephalon and is composed of the caudal diencephalon, rostral diencephalon, eye vesicle, and telencephalon; the latter is shown as it starts to evaginate. (E) Dorsal view of the same stage as in D showing unfinished anterior closure of the neural tube and the bulging eye vesicles. (F). At a later stage, a dorsal view illustrates the stalks of the eye vesicles, the bilateral telencephalic vesicles, and the choroidal plexus primordium at the forebrain roof (cp; stippled area). (G) Schema of the closed neural tube in cross section and its longitudinal domains. Development of its wall is schematized below; a proliferating ventricular zone, a differentiating mantle zone, and a subpial layer of white matter (growing axons) can be distinguished.

Figure 1 Early development of the forebrain. (A) Schema of transverse domains specified at the neural plate stage; the transverse boundaries are represented by black lines. Note that telencephalic and eye vesicles are separate outgrowths. (B) Schema of longitudinal domains at the same stage (boundaries in black; transverse and other limits in gray). (C) Schema of cross section through the neural plate showing the longitudinal domains and the relative position of the main tissues that regulate dorsoventral patterning, the notochord and the nonneural ectoderm. These exert ventralization and dorsalization effects, respectively, which are symbolized below. (D) Side view of the closed neural tube. The forebrain appears rostral to the mesencephalon and is composed of the caudal diencephalon, rostral diencephalon, eye vesicle, and telencephalon; the latter is shown as it starts to evaginate. (E) Dorsal view of the same stage as in D showing unfinished anterior closure of the neural tube and the bulging eye vesicles. (F). At a later stage, a dorsal view illustrates the stalks of the eye vesicles, the bilateral telencephalic vesicles, and the choroidal plexus primordium at the forebrain roof (cp; stippled area). (G) Schema of the closed neural tube in cross section and its longitudinal domains. Development of its wall is schematized below; a proliferating ventricular zone, a differentiating mantle zone, and a subpial layer of white matter (growing axons) can be distinguished.

tissues regulate the outgrowth of vesicles from the forebrain, such as the bilateral eye and telencephalic vesicles (secondarily also the olfactory bulbs), the neurohypophysis, and the epiphysis (pineal gland) (Figs. 1D-1F).

Dorsoventral patterning is regulated by nonneural tissues flanking the neural plate [the dorsalizing nonneural ectoderm (NNE) in Figs. 1B and C] and below the neural plate midline (the ventralizing notochord and prechordal mesendoderm) (N; Fig. 1C). Anteroposterior patterning is less fully understood, but it involves tissues underlying the neural plate, tissues at the anterior and posterior limits of the neural plate, as well as a later-appearing patterning

Figure 2 Topological schema of transverse and longitudinal subdivisions of the vertebrate forebrain (FB), midbrain (MB), and rostral hindbrain (HB). The longitudinal zones are indicated at the far left (R, roof plate; A, alar plate; B, basal plate; F, floor plate); the alar/basal boundary is represented by the thick black line with oblique white stripes. The transverse boundaries are vertical lines; black lines separate the main brain vesicles (identified underneath) and dashed lines separate the neuromeric subdivisions (prosomeres p1-p6), which are identified under the alar/basal boundary line. Various other specific names for different alar or basal territories are indicated. The choroidal plexi (cp) are marked by vertical stripes at the top. Note the position of the evaginated telencephalic vesicle (T) and the unevaginated telencephalon impar (Ti). The optic tract (ot) is schematized as a longitudinal domain extending from the optic chiasm (oc) to the optic tectum. See Table I for other abbreviations.

Figure 2 Topological schema of transverse and longitudinal subdivisions of the vertebrate forebrain (FB), midbrain (MB), and rostral hindbrain (HB). The longitudinal zones are indicated at the far left (R, roof plate; A, alar plate; B, basal plate; F, floor plate); the alar/basal boundary is represented by the thick black line with oblique white stripes. The transverse boundaries are vertical lines; black lines separate the main brain vesicles (identified underneath) and dashed lines separate the neuromeric subdivisions (prosomeres p1-p6), which are identified under the alar/basal boundary line. Various other specific names for different alar or basal territories are indicated. The choroidal plexi (cp) are marked by vertical stripes at the top. Note the position of the evaginated telencephalic vesicle (T) and the unevaginated telencephalon impar (Ti). The optic tract (ot) is schematized as a longitudinal domain extending from the optic chiasm (oc) to the optic tectum. See Table I for other abbreviations.

center at the midbrain-hindbrain junction. These stepwise processes generate a two-dimensionally regionalized specification map of prospective forebrain subdivisions (Fig. 2) Table I.

During the patterning stage, morphogenetic processes start to generate the shape of the forebrain. Neurulation folds the neural plate into the neural tube (Figs. 1C and G) and rapid cell divisions expand its surface area. The position-specific production of neurons and glia increases the thickness of the neural tube along the ventriculopial axis, which is the third dimension of the brain (Fig. 1G, arrows). Note that the internal, fluid-filled cavity of the neural tube and adult brain is called ventricular space, which is lined by the pseudostratified neuroepithelium; the outside of the neural tube contacts through a basement membrane the surrounding mesenchyme, which matures into a thin meningeal sheet called the ''pia.'' Thus, the term "pial" means superficial, whereas "ventricular" means internal; most cell divisions occur in the ''ventricular zone'' (vz) of the neuroepithelium (Fig. 1G). Cell-type specification and differentiation processes generate postmitotic cells that migrate away from their site of origin in the vz toward the pial surface, under which they form the mantle zone (mz; Fig. 1G). Here, nuclei and laminar structures are gradually assembled. Many neurons and some glia cells only undergo radial migrations to the mz (Fig. 1G, arrows), thus maintaining a fixed position relative to their site of origin. This is probably essential for the generation of topographic connectivity maps between different brain regions. On the other hand, some neurons and oligodendrocytes undergo tangential migrations away from their primary sites of entrance in the mantle layer, frequently into specific target loci. Tangential migrations enable certain cell types that can only be formed in specific neural tube areas to become functionally integrated in local circuitry elsewhere.

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