The Chick Embryo
Chick eggs are readily available and embryos are easily accessible throughout embryogenesis. Embryos readily tolerate manipulation such as microsurgery. As a result of these attributes, the chick embryo has long been a favorite organism for experimental embryology. For a schematic view of key phases of early chick development, see Fig. 5. After the egg is fertilized, which occurs within the oviduct of the hen, shell components are added during the day-long journey through the oviduct prior to...
Migration Pathways of Cranial Neural Crest Cells
Cranial neural crest cells migrate beneath the surface ectoderm, above the paraxial cephalic mesoderm see Figs. 3 and 4B , although a few cells penetrate the paraxial mesoderm. FIGURE 3. Schematic lateral views of a generalized 20-30 somite-stage amniote embryo with the surface ectoderm removed except to show the positions of the cranial ectodermal placodes . Each tissue type from the embryo at the top is shown separately below, illustrating the relative positions of the migrating neural crest,...
The Mouse Embryo
The mouse, being a mammal, has an embryo that should be highly relevant for understanding development of the human embryo. Nevertheless, there are some caveats that make this model less than ideal. The fact that mouse development occurs within the maternal uterus and that the embryo is highly dependent upon its mother for respiration, nutrition, and the removal of its waste products makes the embryo relatively unsuitable for the kinds of embryological experimentation that have characterized...
The Zebrafish Embryo
Two large-scale mutagenesis screens propelled the zebra-fish embryo to the forefront of developmental biology Mullins and Nusslein-Volhard, 1993 Driever, 1995 . The combination of The discovery of the organizer in 1924 is one of the major milestones in developmental biology. This discovery has had a major influence on our thinking about the mechanisms underlying neural induction Spemann and Mangold, 1924 . The German scientists, Hans Spemann and Hilda Mangold, discovered that a region of the...
Separation of the First Second and Third Neural Crest Streams Amniotes
In chick and mouse embryos, there are neural crest cellfree zones adjacent to r3 and r5 Fig. 3 . It was suggested that neural crest cells at r3 and r5 die by apoptosis to generate adjacent neural crest-free zones Graham et al., 1993 . However, both r3 and r5 give rise to neural crest cells during normal development in both chick and mouse, though r3 generates fewer neural crest cells than other rhombomeres Sechrist et al., 1993 Kontges and Lumsden, 1996 Kulesa and Fraser, 1998 Trainor et al.,...
FoxD3 Promotes Neural Crest Cell Delamination at All Axial Levels
FoxD3 is essential for the formation of neural crest cell precursors section Snail Slug and FoxD3 Are Required for Neural Crest Precursor Formation , and it may also play a role in neural crest cell delamination. Ectopic expression of FoxD3 in the chick neural tube promotes neural crest cell delamination at all axial levels Dottori et al., 2001 . This is achieved without upregulating Slug or, apparently, RhoB section BMP4 Induces RhoB, Which Is Essential for Neutral Crest Cell Delamination ,...
Pharyngeal Arches and Neural Crest Streams
The patterning of cranial neural crest cell migration is intimately bound up with the segmental nature of both the hindbrain rhombomeres see Chapter 3 and the periphery pharyngeal arches . Pharyngeal arches are also known as branchial arches, from the Latin branchia gill , because in aquatic vertebrates the more caudal arches are associated with gills. However, pharyngeal is the more appropriate term, because all arches form in the pharynx, but not all arches support gills. Pharyngeal arches...
Placodes
Placodes are neuroectodermal thickenings that form outside of the boundaries of the CNS and contribute to the paired specialized sense organs olfactory nose, optic lens, otic or auditory ear, and lateral line system or to the anterior pituitary gland and cranial sensory ganglia Fig. 16 . Many early marker genes have been identified that are expressed in specific placodes such as Pax6, Otx2, and Sox3 in the lens placodes, Pax6 in the olfactory placodes Nkx5.1 Pax8, and Pax2 in the otic placodes...
Ventral Migration Pathway
In the chick, neural crest cells that delaminate opposite epithelial somites initially migrate ventrally between the somites. Once the sclerotome forms, they migrate exclusively through the rostral half of each sclerotome, leading to a segmental pattern of FIGURE 5. Schematic showing trunk neural crest cell migration pathways and derivatives also see Fig. 1C . Neural crest cells migrate ventrally through the sclerotome to form neurons and satellite glia in the dorsal root ganglia and...
The Ventricular Zone
The basic cellular organization of the VZ was recognized by both His 1889, 1897, 1904 and Ramon y Cajal 1894, 1909-1911 . Both of these giants of the field recognized that the VZ was several cell diameters in thickness and that there were mitotic figures adjacent to the ventricular surface Fig. 1 . His thought that the mitotic figures were germinal cells Keimzellen and that after mitotic division one daughter cell remained adjacent to the ventricular surface to divide again whereas the other...
Hindbrain
Just caudal to the isthmus, the neural swelling called the hindbrain or rhombencephalon develops see Fig. 6 . The rostral-most section of this vesicle r1 expresses En2 in a gradient peaking at the rostral margin the isthmus and forms the cerebellum under the influence of FGF8 and Wntl see above . The rhombencephalon is characterized early during development by its subdivision into anatomically identifiable rhombomeres. Rhombomeres 1-7 rl r7 form as identifiable bulges in the rhombencephalon...
Neural Crest Derivatives
Neural crest cells form a startling array of different cell types, including cartilage and bone in the head, teeth, endocrine cells, peripheral sensory neurons, all peripheral autonomic neurons enteric, postganglionic sympathetic, and parasympathetic neurons , all peripheral glia, and all epidermal pigment cells Fig. 1 . The neural crest origin of these cells has been determined by a variety of ablation and cell-labeling experiments, some of which are described in detail in the section on...
Dorsolateral Migration Pathway
Neural crest cells that migrate along the dorsolateral pathway, between the somites and the ectoderm, give rise to epidermal melanocytes in all vertebrates. In chick embryos, melanocytes only differentiate after they have invaded the ectoderm, while in amphibians, melanocytes often differentiate during migration see, e.g., Keller and Spieth, 1984 . In Xenopus, the subectoder-mal pathway is only a minor pathway for pigment cells, as most pigment cell precursors follow the ventral pathway...
The Subventricular Zone
The SVZ was first recognized by Schaper and Cohen 1905 by the presence of mitotic figures in a location distant from the lateral ventricles. It was first shown definitively to have proliferating cells using 3H-thymidine label in vitro using slabs of human brain Rakic and Sidman, 1968 . The proliferating cells of the SVZ differ in two major ways from those of the VZ Fig. 4 . First, the nuclei of proliferating cells of the SVZ do not move during the cell cycle, but reflecting the fact that cells...
Closure of the Neural Groove
Bending brings the tips of the neural folds into close contact at the site of the dorsal midline of the embryo. During closure, the two tips attach and fuse. Each component of the tip must fuse correctly, such that the non-neural epithelium forms a continuous sheet overlying the newly formed roof plate of the neural tube and the associated neural crest. The exact mechanism of this concluding step of neurulation is not well understood, and the molecules that mediate adhesion, epithelial...
Posteriorizing Signals Wnts and FGFs
A posteriorizing signal derived from the paraxial meso-derm enables rostral neural plate tissue to form neural crest cells in the chick Muhr et al., 1997 and establishes Pax3 expression at the neural plate border in both chick and Xenopus embryos Bang et al., 1997, 1999 . In the chick, this posteriorizing activity is mediated by Wnt family members, in particular Wnt8c and Wnt11, in conjunction with permissive FGF signaling Nordstrom et al., 2002 . Paraxial mesoderm produces several other...
Bending of the Neural Plate
Bending involves the establishment of localized deformations of the cells of the neuroepithelium and the subsequent elevation of the two flanks of the neuroepithelium, converting it from the neural plate to the neural groove. Bending is actually driven by two distinct types of movement furrowing and folding Colas and Schoenwolf, 2001 . Furrowing is a behavior intrinsic to the hinge points within the neuroepithelium. There are three hinge points within the neural plate a single median hinge...
The Output from the Ventricular Zone
The output from the PVE is the population of neurons and other cell types that populate the mature brain and the cells that seed the SVZ. A priori, the mechanisms that control this output depend on four factors, the number of proliferating cells, the length of the cell cycle, the period of time that the proliferating population exists, and the proportion of daughter cells that exit vs remain in the proliferating population at each pass through the cell cycle. By definition the beginning of the...
SnailSlug and FoxD3 Are Required for Neural Crest Precursor Formation
The Snail superfamily of zinc finger transcriptional repressors contains two major families Snail and Scratch Nieto, 2002 . In vertebrates, the Snail family is further subdivided into Snail and Slug subfamilies, both of which are essential during two stages of neural crest formation 1 The formation of neural crest cell precursors within the neuroepithelium, and 2 delamination of cranial neural crest cells section Snail Family Members Promote Cranial Neural Crest Cell Delamination . In Xenopus,...
The Xenopus Embryo
A large body of literature exists on the development of the amphibian embryo. Indeed, two of the most important findings regarding the embryogenesis of the vertebrate nervous system the discovery of the organizer and the elucidation of its role in neural induction Spemann and Mangold, 1924, 2001 and the discovery of the molecular mechanisms of neural induction Sasai and De Robertis, 1997 Nieuwkoop, 1999 Weinstein and Hemmati-Brivanlou, 1999 were obtained using amphibian embryos. These will be...
Output of the Subventricular Zone
The SVZ produces both neurons and glia, but the types of cells produced differ both regionally and temporally. For example, FIGURE 15. Expression domains of Dlx1, Dlx2, Dlx5, and Dlx6 during mouse brain development. Top Schema of a transverse section through the E12.5 mouse telencephalon showing the combined expression of Dlx transcripts. Most cells in the subpallial telencephalon express Dlx1, Dlx2, Dlx5, or Dlx6 at some stage of their differentiation. The arrows indicate the migration from...
The Rhombic Lip And The External Granule Cell Layer Of The Cerebellum
The external granule cell layer of the cerebellum is unique among the proliferating populations of the CNS in that it is adjacent to the pial surface rather than the ventricular surface Fig. 17 . The external granule cell layer was first recognized as the source of the granule cells of the cerebellum near the end of the 19th century Obersteiner, 1883 Schaper, 1897a, b Ramon y FIGURE 17. The external granule cell layer EGL lies beneath the pial surface of the developing cerebellum. These stem...
The Dentate Gyrus
The subhilar region of the dentate gyrus is a specialized proliferative population that produces the granule cells of the dentate gyrus. The presence of a proliferating population of stem and progenitor cells in the dentate gyrus of mammals was first described in the mouse Angevine, 1964, 1965 . This proliferating population initially arises from the VZ of the medial wall of the lateral ventricle, that is, near the anlage of the dentate gyrus, and migrates into the future position of the...