Origins of cells and cranial placodes at the edge of the

Tuesday, April 17, 12 Gilbert6

Tuesday, April 17, 12 migration pathways

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Tuesday, April 17, 12 Neural crest migrates over and through the anterior sclerotome (of the somite), but not the posterior, due to repulsion from the posterior sclerotome.

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Tuesday, April 17, 12 Lineage tracing shows that neural crest cells are not committed to specific fates as they emigrate from the

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Tuesday, April 17, 12 Neural crest migration pathways and derivatives

Tuesday, April 17, 12 Trunk neural crest cells: Sensory neurons of dorsal root ganglia, Sympathetic neurons and ganglia

Tuesday, April 17, 12 Melanocytes from neural crest cells

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Tuesday, April 17, 12 cells that migrate past the BMP producing aorta, are induced to become neurons (of the sympathetic ganglia)

Tuesday, April 17, 12 Patterning of the hindbrain from Kiecker and Lumsden, 2005

The midbrain-hindbrain boundary acts The Hox code of r1-r8 as an “isthmic organizer

Tuesday, April 17, 12 Tuesday, April 17, 12 4 MOENS AND PRINCE

Fig. 3. CranialRhombomeres: neuroanatomy reflects the segmental repeating organization of branchiomotor neurons of the cranial nerves (n) also have a rhom- the zebrafish hindbrain. A: The individually identified reticulospinal neu- bomere-specific disposition revealed in live embryos by using a trans- rons are organizedmorphological in a ladder-like array along and the anterior-posterior genic line in which green fluorescent protein is under control of islet-1 extent of the hindbrain (anterior to the top). Note the large bilateral gene regulatory elements (Higashijima et al., 2000). Anterior is to the top. Mauthner neuronsorganizational with contralateral projections units in r4 (arrow). ofB: theThe hindbrain (here in the zebrafish) ilarly exit fromfrom even-numbered Moens and rhombomeres, Prince,2002 where the Hox genes play conserved roles in specifying seg- neuronal differentiation is similarly delayed in odd- ment identities. numbered rhombomeres (Bally-Cuif et al., 1998; Hi- gashijima et al., 2000) and where PATTERNING THE ANTERIOR–POSTERIOR similarly migrates from even-numbered rhombomeres AXIS OF THE ZEBRAFISH CENTRAL Tuesday, April 17, 12 (Schilling and Kimmel, 1994). A two-segment periodic- ity in cell adhesive properties has also been observed, Long before the hindbrain becomes segmented and and recent work in the zebrafish has uncovered a mo- hindbrain neuroanatomy is elaborated, the presump- lecular basis for these adhesive differences (see below; tive neurectoderm is broadly patterned along its ante- reviewed in Klein, 1999). rior-posterior axis into forebrain, midbrain, hindbrain, Each is unique. Differences in the and spinal cord. Indeed, as early as the beginning of size, number, and projections of the reticulospinal neu- gastrulation, largely nonoverlapping regions of the pre- rons in each segment illustrate another principle of sumptive neurectoderm are fated to give rise to distinct hindbrain organization, which is that, overlying the anterior-posterior identities (Fig. 4A; Kimmel et al., basic re-iterated pattern, are clear segment-specific dif- 1990; Woo and Fraser, 1995). The early gastrula re- ferences. For example, the prominent Mauthner neu- sembles a cap pulled halfway over a round head (the ron in rhombomere 4 has a much larger cell body and yolk), with a thickened edge—the margin or germ axon than do its segmental homologs in other rhom- ring—where the first mesendodermal precursors have bomeres (Fig. 3A). This picture of a basic segmental begun involuting. The dorsal side of the embryo is plan overlain by segment-specific differences is remi- marked by the embryonic shield, a thickening that is niscent of the body plan of the fly, where homeotic the equivalent of the organizer of the Xenopus embryo selector genes, the Hox genes, specify differences be- and the node of amniote embryos, which gives rise to tween initially similar segments. As we will discuss midline, or axial, mesendoderm (including further below, the vertebrate homologs of the fly Hox and prechordal plate). The cells in the lateral and ven- genes are expressed in rhombomere-restricted do- tral germ ring are fated to become nonaxial mesoderm mains, and work in several vertebrate systems has (including somites and blood) and endoderm. Cells fur- demonstrated that, although the segmenting mecha- ther from the margin (closer to the animal pole) give nisms appear to be different in flies and vertebrates, rise to ectoderm, with the neurectoderm mapping to fig.7 of Moens and Prince,2002

Tuesday, April 17, 12 Compartments (intrinsic segmentation) in the hindbrain Lumsden A, 2004

Rhombomeres (in the chick) are visible as local “bumps” of increased proliferation, and as units of axonal projection14

Tuesday, April 17, 12 Compartments (intrinsic segmentation) in the hindbrain Lumsden A, 2004

Rhombomeres become lineage restriction units, at about the ten somite

stage (stage X) 15

Tuesday, April 17, 12 Cranial neural crest, rhombomeres, and branchial arches

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Tuesday, April 17, 12 Cranial placodes and their derivatives, from Streit, 2008

Tuesday, April 17, 12 The placode and lens development an example of reciprocal interaction between and epithelium

Tuesday, April 17, 12