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Spemann and others demonstrated that there may be separate head and tail organizers. This fact suggests that the very early inductive signals for neural development also influence the A-P axis. In a now classic experiment, Nieuwkoop see Chapter 1 transplanted small pieces of ectodermal tissue from one embryo into a host at various positions along the anterior-posterior axis. In all cases, the transplanted cells developed anterior neural structures. However, when the cells were transplanted in...
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receptors see BOX Biophysics Nuts and Bolts of Functional Maturation . These electrodes are brought within a few microns of a growth cone before it has contacted a myocyte. If the growth releases ACh, then the ACh receptor-coupled channels open, and current flows across the electrode. The release of transmitter is probably a general property of all growth cones. For example, a different neurotransmitter GABA is released from growth cones of mammalian CNS neurons Gao and van den Pol, 2000 ....
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FIGURE 2.1 The vertebrate brain and spinal cord develop from the neural tube. Shown here as lateral views upper and dorsal views lower of human embryos at successively older stages of embryonic development A,B,C . The primary three divisions of the brain A occur as three brain vesicles or swellings of the neural tube, known as the forebrain prosencephalon , midbrain mesencephalon , and hindbrain rhomben-cephalon . The next stage of brain development B results in further subdivisions, with the...
Dorsalventral Polarity In The Neural Tube
The early neural tube consists only of undifferentiated neural and glial progenitor cells. The neural tube is essentially a closed system, and the brain vesicles and developing spinal cord are fluid filled chambers. The surface of the tube, adjacent to the lumen, is known as the ventricular surface, since eventually the lumen of the neural tube goes on to form the ventricular system of the mature brain. The progenitor cells for neurons and glia of the CNS have a simple bipolar morphology and...
Border Patrol And Prevention Of Inappropriate Targeting
Once they have recognized and entered a target area, slowed down, and started to branch within, axons may be prevented from exiting the target area by repulsive cues at the perimeters. Sema3a, which we discussed in the last chapter, repels the growth cones of cutaneous sensory neurons. Analysis of knockout mice supports a critical role for Sema3a as an exclusion factor confining the peripheral ends of these axons to the correct target areas of the skin Taniguchi et al., A Pausing growth cone B...
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Transplanted foot pad tissue cholinergic target Transplanted foot pad tissue cholinergic target Transplanted parotid gland tissue noradrenergic target Transplanted parotid gland tissue noradrenergic target FIGURE 4.16 Control of transmitter phenotype by sweat glands in the footpad. A. A noradrenergic neuron begins to innervate developing sweat glands. As it does so, it switches and becomes cholinergic. B. Neurons that innervate hair follicles are noradrenergic, yet when a piece of footpad...
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Cell type Sensory Autonomic Schwann Pigment Smooth Neurons Neurons cells cells Muscle FIGURE 4.14 Different neural crest fates are promoted by a distinct set of extracellular signaling molecules. After Dorsky et al., 2000 Cell type Sensory Autonomic Schwann Pigment Smooth Neurons Neurons cells cells Muscle FIGURE 4.14 Different neural crest fates are promoted by a distinct set of extracellular signaling molecules. After Dorsky et al., 2000 mine them toward a sensory fate. It appears that the...
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FIGURE 8.11 A schematic showing the nectin-afadin adhesion system during the formation of a synapse in developing hippo-campal pyramidal neurons. The nectin-afadin system organizes adherens junctions cooperatively with the cadherin-catenin system. Nectin is an immunoglobulin-like adhesion molecule, and afadin is an actin filament-binding protein that connects nectin to the actin cytoskeleton. During development, nectin-1 and -3 localize at both the puncta adherentia junctions i.e., mechanical...
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FIGURE 3.26 Neuronal migration along glial fibers can be observed directly. In cerebellar neuron-glial co-cultures, the neurons associate with the glial processes and slowly move along them. Using time-lapse microscopy, this neuronal migration can be directly observed and quantified. This has provided an excellent assay for investigating the molecular basis for neuronal migration. Courtesy of M.E. Hatten FIGURE 3.26 Neuronal migration along glial fibers can be observed directly. In cerebellar...
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FIGURE 6.3 Some sympathetic neurons use a change in NT-3 expression to innervate their targets in the ear. A. Some SCG neurons project to and arborize in the pinna of a normal mouse. B. In NT3 knockout mice, these fibers do not invade the pinna. C. Restoration of targeting by injection of NT3 into the ear. Adapted from ElShamy et al., 1996 captures circulating GDNF and holds it to the target sites, thus creating a very high level of GDNF right around the target, which attracts innervation from...
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Newborn neurons send out processes, threadlike axons that carry information to target cells and dendritic processes that receive inputs from other neurons. Some neurons, local interneurons, have short axons and make connections to cells in their immediate vicinity, while others, projection neurons, send long axons to distant targets. There is tremendous divergence and convergence of wiring. On the sensory side, peripheral neurons send axons into the CNS where they usually diverge to project to...
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FIGURE 5.23 An experiment supporting the labeled pathway hypothesis. A. In a control embryo, the G growth cone, after crossing the midline, fasciculates with P-axons and not A-axons. B. When the P-neuron is ablated, the G growth cone stalls and does not fasciculate with the A-axons. After Raper et al., 1984 too, there is evidence that some axons use a labeled pathway mechanism. The tract of the postoptic commissure TPOC , for example, is a pioneering tract for axons from the pineal. The pineal...
Ganglion Mother Cell
FIGURE 4.6 Control of asymmetrical cell division in Drosophila. The Inscuteable complex is localized to the apical pole of the neuroblast where it orients the mitotic spindle and causes the basal localization of asymmetrically localized determinants such as Miranda and Numb. FIGURE 4.6 Control of asymmetrical cell division in Drosophila. The Inscuteable complex is localized to the apical pole of the neuroblast where it orients the mitotic spindle and causes the basal localization of...
Neural Crest Migration 1
genes as it migrates from the neural tube, and thus has a unique identity. This unique identity can be demonstrated by transplantation experiments where crest from one rhombomere is transplanted to the region of another, and its migration and further development are monitored Noden, 1983 . Crest cells that would normally populate the third arch were excised and replaced with first arch crest cells. The transplanted crest cells migrated into the third arch, but instead of making neck cartilage,...
Survival Depends On The Synaptic Target
pyknotic cells Figure 7.6 . At first glance, it may not be clear why so few pyknotic neurons are observed during the period of maximal neuron elimination. This is due to the rapid removal of cell debris, which has been variously estimated to occur in as little as 3 hours. The magnitude of cell death is impressive. The data in Figure 7.6 show that the motor neuron population innervating frog hind limbs declines from an initial size of about 4000 to a final size of 1200. Thus, well over half of...
Dorsal Neural Tube And Neural Crest
The experiments of Harrison and others showed that removal of the notochord resulted in a neural tube without much dorso-ventral polarity. This implies that the dorsal neural tube is in some way the default condition, whereas the ventral structures require an additional signal to develop their fates. However, in the last few years it has become apparent that the dorsal neural tube also requires signals for its appropriate development. Before the neural tube closes, the future dorsal neural tube...
Cerebral Cortex Histogenesis
In the next section, the histogenesis of two specific regions of the CNS the cerebral cortex and the cerebellum will be highlighted histogenesis of these structures has been the subject of intense study for many years. As noted in the previous chapter, the cerebral hemispheres develop from the wall of the telen-cephalic vesicle. The neuroepithelial cells initially span the thickness of the wall, and as they continue to undergo cell division, the area of the hemispheres expands. At this early...
Hyperdorsalized Embryo
FIGURE 1.16 Interactions between the animal and vegetal cells of the amphibian embryo are necessary for induction of the meso-derm. A. The regions of the amphibian embryo that give rise to these different tissue types are shown. The animal pole gives rise to epidermal cells and neural tissue, the vegetal pole gives rise to endodermal derivatives, like the gut, while the mesoderm blue arises from the equatorial zone. B. If the animal cap and vegetal hemispheres are isolated from one another,...
Chemospecificity And Ephrins
In the early 1940s, Roger Sperry cut the optic nerve of a newt, rotated the detached eye 180 in its orbit, and assayed the visuomotor behavior of the animal after its nerve had regenerated. The newts, and in subsequent studies, frogs, behaved as if their visual world were back-to-front and upside-down when a lure was presented in front of them, they wheeled rapidly to the rear instead of striking forward and when the lure was presented above the animals struck downward in front of them and got...
Competence And Histogenesis
The cells of the cerebral cortex are generated near the ventricular surface of the closed neural tube. As described in Chapter 3, the cell divisions of cortical neuroblasts give rise to postmitotic neurons that FIGURE 4.17 Specification of peripheral glia. A. Normally crest cells of the DRG produce neurons and gia see text and if placed in culture, crest cells give rise to both neurons N and glia G . B. Removal of Nrg-1 or addition of BMP2 increase the number of neurons. C. Addition of the...
Early Embryology Of Metazoans
The development of multicellular organisms varies substantially across phyla nevertheless, there are some common features. The cells of all metazoans are organized as layers. These layers give rise to the various organs and tissues, including the nervous system. These layers are generated from the egg cell through a series of cell divisions and their subsequent rearrangements Figure 1.2 . The egg cells of animals are typically polarized, with an animal pole and a vegetal pole. This polarity is...
Defasiculation
In order to enter a target area or to find a target cell with which to make synapses, it is often first necessary to exit from a tight bundle. Nerves, tracts, columns, bundles, and fasciculi often travel past a variety of potential targets. As they do so, specific axons or groups of axons peel off of these common pathways, so that they can enter the target. In the last chapter, we saw that homophilic adhesion molecules such as N-CAM and its homologs can cause similar axons to fasciculate...
The Growth Cone
Growth cones were recognized more than a hundred years ago by the famous Spanish neuro-anatomist, Ramon y Cajal, as expansions at the tips of axons in fixed embryonic material. He imagined the growth cone as a sort of soft battering ram that extending axons used to force their way through the packed cells of the embryonic brain Ramon y Cajal, 1890 Figure 5.6A . In 1910, Ross Harrison took pieces of embryonic neural tube and put them into tissue culture where he saw axons tipped with growth...
The Generation Of Neurons And Glia
The nervous system contains both neurons and glia, and both basic types of cells are produced in highly stereotypic ratios. What factors control the relative ratios of neurons and glia in the brain Early in the development of the CNS, many, if not all, of the progenitor cells have the capacity to generate both neurons and glia. Retroviral lineage studies have shown that, for many regions of the nervous system, neurons, astrocytes, and oligodendrocytes can arise from a single infected progenitor...
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FIGURE 2.19 Prosomeric model of forebrain development longitudinal and transverse patterns of gene expression that subdivide the neural tube into a grid of regional identities. The expression of some of these genes is shown for the mouse embryo at two different stages of development. Two genes of the emx class are expressed in the telencephalon, one in the anterior half of the cerebral hemispheres emx1 and the other in the posterior half of the hemispheres emx2 . Analysis of the expression...
Interactions With Neighboring Tissues In Making Neural Tissue
The three basic layers of the embryo the endo-derm, mesoderm, and ectoderm arise through the complex movements of gastrulation. These movements also create new tissue relations. For example, after gastrulation in the frog, presumptive mesoderm FIGURE 1.11 Development of the chick embryo. The blastoderm area opaca sits on top of the large yolk and is the result of a large number of cleavage divisions. At the start of gastrulation, cells move posteriorly arrows and migrate under the area opaca....
Linking Induction To Proneural Activity
Finally, we would like to link up the processes of neural induction, covered earlier in the chapter, with the proneural bHLH Notch pathway genes discussed in the second half of the chapter. To look for links between these processes, Sasai and colleagues used differential screening for genes upregulated in the animal caps after chordin treatment. One family of genes found in this type of screen, members of the Sox family, were found to be expressed very early in neural plate, soon after...
Conservation Of Neural Induction
Even more fascinating than the identification of three candidate neural-inducing factors in a relatively short period of time is that these three factors may all act by a related mechanism and that this mechanism appears to be at least partially conserved between ver FIGURE 1.21 Dissociation of animal cap cells prior to gastrula-tion causes most of them to differentiate into neurons in culture. Animal caps can be cultured intact left or dissociated into single cells by removing the Ca 2 ions...
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FIGURE 1.7 The neuroblasts of the Drosophila separate from the ectoderm by a process known as delamination. The neuroblasts enlarge relative to the surrounding cells and squeeze out of the epithelium. The process occurs in several waves after the first set of neuroblasts has delaminated from the ectoderm, a second set of cells in the ectoderm begins to enlarge and also delaminates. The delami-nating neuroblasts then go on to generate several neurons through a stereotypic pattern of asymmetric...
Dan H Sanes Thomas A Reh William A Harris
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