The Questions of Developmental Biology

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"Life's splendid drama" Until this decade, "many invertebrate biologists saw the reconstruction of relationships among the phyla as an insoluble dilemma. . . . Indeed, as late as 1990, a comprehensive summary concluded that the relationships between most of the higher animal groups were entirely unresolved" (Erwin et al. 1997). However, in the 1990s, a broad consensus on the general form of a phylogenetic tree of life began to emerge among paleontologists, molecular biologists, and developmental geneticists (see Winnepenninckx et al. 1998; Davidson and Ruvkun 1999; Erwin 1999). This consensus (one representation of which is shown in Figure 22.1A) came about from (1) improved methods of analyzing DNA, taking into account its variation within groups of animals, (2) new data on conserved regulatory gene sequences such as the Hox genes, which are usually stable within phyla but can diverge between phyla, (3) morphological evidence for the related nature of some structures that had once been thought to be distinct, and (4) computer programs that can sort out enormous amounts of data, not privileging any particular set of relationships over others. The results were surprising to many scientists. 1. The animal kingdom can be divided into Porifera (sponges), Cnidaria and Ctenophora (jellyfish and comb jellies), and the Bilateria. The Porifera lack any coherent epithelium or any symmetry. The Cnidaria and Ctenophora are diploblastic (with two epithelial layers, lacking mesoderm) and have radial symmetry. 2. The Bilateria are triploblastic (with true endoderm, mesoderm, and ectoderm), have bilateral symmetry and can be divided into two groups, the deuterostomes and the protostomes. The deuterostomes include the echinoderms and the chordates. (Echinoderm larvae are originally bilateral.) Deuterostomes form their anuses from their blastopores. They probably arose from a tornaria larva similar to that of the pluteus. 3. The protostomes can be divided into two groups, the Ecdysozoa (animals whose bodies are covered by an exoskeleton, which therefore molt as they grow) and the Lophotrochozoa (animals that have most or all of their soft tissues in contact with the environment and which generally use cilia in feeding or locomotion). Nematodes and flatworms (which lack true coeloms) had formerly been considered basal groups, perhaps ancestral to both the protostomes and the deuterostomes. However, the new studies have shown that the nematodes belong to the Ecdysozoa and the flatworms (as Wilson had predicted) to the Lophotrochozoa. Each of these two groups is monophyletic, meaning that the phyla in each of them share a common ancestor.
The search for the Urbilaterian ancestor It is doubtful that we will find a fossilized representative of the ancestral phylum that gave rise to both the deuterostomes and the protostomes. Such a hypothetical animal is sometimes called the Urbilaterian ancestor or the PDA (protostome-deuterostome ancestor). Since such an animal probably had neither a bony endoskeleton (a deuterostome trait) nor a hard exoskeleton (characteristic of ecdysozoans), it would not fossilize well. However, we can undertake what Sean Carroll (quoted in DiSilvestro 1997) has called "paleontology without fossils." The logic of this approach is to find homologous genes that are performing the same functions in both a deuterostome (usually a chick or a mouse) and a protostome (generally an arthropod such as Drosophila). Many such genes have been found (Table 22.1), and their similarities of structure and function in protostomes and deuterostomes make it likely that these genes emerged in an animal that is ancestral to both groups. Table 22.1. Developmental regulatory genes conserved between protostomes and deuterostomes Gene Function Distribution achaete-scute group Cell fate specification Cnidarians, Drosophila, vertebrates Bcl2/Drob-1/ced9 Programmed cell death Drosophila, nematodes, vertebrates Caudal Posterior differentiation Drosophila, vertebrates delta/Xdelta-1 Primary neurogenesis Drosophila, Xenopus Distal-less/DLX Appendage formation (proximal-distal axis) Numerous phyla of protostomes and deuterostomes Dorsal/NFB Immune response Drosophila, vertebrates forkhead/Fox Terminal differentiation Drosophila, vertebrates Fringe/radical fringe Formation of limb margin (apical Drosophila, chick ectodermal ridge in vertebrates) Hac-1/Apaf/ced 4 Programmed cell death Drosophila, nematodes, vertebrates Hox complex Anterior-posterior patterning Widespread among metazoans lin-12/Notch Cell fate specification C. elegans, Drosophila, vertebrates Otx-1, Otx-2/Otd, Emx-1, Anterior patterning, cephalization Drosophila, vertebrates Emx-2/ems Pax6/eyeless; Eyes Anterior CNS/eye regulation Drosophila, vertebrates absent/eya Polycomb group Controls Hox expression/ cell Drosophila, vertebrates differentiation Netrins, Split proteins, and Axon guidance Drosophila, vertebrates their receptors RAS Signal transduction Drosophila, vertebrates sine occulus/Six3 Anterior CNS/eye pattern formation Drosophila, vertebrates sog/chordin, dpp/BMP4 Dorsal-ventral patterning, neurogenesis Drosophila, Xenopus tinman/Nkx 2-5 Heart/blood vascular system Drosophila, mouse vnd, msh Neural tube patterning Drosophila, vertebrates
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PART 1. Principles of development i
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PART 2: Early embryonic development
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15. Lateral plate mesoderm and endo
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Hox Genes: Descent with Modificatio
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The question of growth. How do our
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Embryonic homologies One of the mos
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absent (Figure 1.16A). Over 7000 af
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Such growth, in which the shape is
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One way to search for the chemicals
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2 3 hours as adults, and they must
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for the following spring's breeding
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VADE MECUM Amphibian development. T
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The formation of a cap is a complex
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In Chlamydomonas, recognition occur
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organism with two distinct and inte
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Aggregation is initiated as each of
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The mathematics of such oscillation
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gonidia to undergo a modified patte
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Sponges develop in a manner so diff
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Embryology provides an endless asso
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Environmental sex determination Sex
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Protection of the egg by sunscreens
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The Developmental Mechanics of Cell
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Autonomous specification was first
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In postulating this model, Weismann
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Insofar as it contains a nucleus, e
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Other tissues may use the same grad
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will give rise to its particular or
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Sydney Brenner (quoted in Wilkins 1
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The results of their experiments we
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This observation led Steinberg to p
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into a single layer of cells (Takei
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6. In conditional specification, th
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3. Geneticists had to explain pheno
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These events are not the normal rou
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differentiated; each of them contai
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federal proscription of human cloni
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ecombinase enzymes are active only
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In situ hybridization But where was
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damaged.) The second primer is adde
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By using the appropriate restrictio
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These homozygous mutant mice lacked
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Phenotype Manipulation This technol
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The second approach is candidate ge
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developmental genetics is different
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This gene consists of the following
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In addition to these basal transcri
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Enhancers are critical in the regul
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The third functional region of MITF
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A fourth enhancer sequence, located
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The discovery of the human globin L
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The results of this procedure are o
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In vertebrates, the presence of met
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chromatin that remains condensed th
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Second, knocking out one Xist locus
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types of cells (Kleene and Humphrey
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(Sxl) gene,* while the autosomes pr
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Table 5.3. Some mRNAs stored in ooc
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determine the anterior-posterior ax
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6. Cell-cell communication in devel
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The inducing tissue does not need i
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Instructive and permissive interact
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mouth, whereas the frog tadpole pro
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are utilized throughout the animal
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includes the TGF-β family, the act
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The RTK spans the cell membrane, an
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members of this family: the C. eleg
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The Wnt pathway Members of the Wnt
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The Hedgehog pathway is extremely i
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A series of mutations has been foun
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The Nature of Human Syndromes As we
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vertebral column deformities, myopi
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The mammalian homologue of CED-4 is
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In the absence of Notch gene transc
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extracellular matrix (Figure 6.32).
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mammary gland tissue. The binding o
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In some cells, gene transcription r
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PARTE 2. Early embryonic developmen
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The means by which sperm are propel
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The sperm released during ejaculati
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Lying immediately beneath the plasm
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The mechanisms of chemotaxis differ
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Once the sea urchin sperm has penet
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Removal of these threonine- or seri
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undergo the acrosomal reaction with
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Gamete Fusion and the Prevention of
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The fast block to polyspermy. The f
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envelope to expand and become the f
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The calcium ions responsible for th
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Thus, the conversion of NAD + to NA
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eticulum (McPherson et al. 1992). T
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cAMP-dependent protein kinase activ
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These cells divide to form embryos
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(Figure 7.34; Elinson and Rowning 1
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6. In many species, eggs secrete di
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One consequence of this rapid cell
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Table 8.1. Karyokinesis and cytokin
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provides a classification of cleava
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This occurred at precisely the time
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These rapid and invariant cell clea
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The identities of the signaling mol
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Ingression of primary mesenchyme Fu
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But these guidance cues cannot be s
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lastocoel wall (Dan and Okazaki 195
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The orientation of the cleavage pla
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stage, the remaining cells divide n
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embryos can produce these cells, bu
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Early Development in Tunicates Tuni
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(or inactivating) specific genes. T
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occur under laboratory conditions.
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the germ cells. Using fluorescent a
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Conditional specification As we saw
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eggs. Coda This chapter has describ
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9. The genetics of axis specificati
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Following cycle 13, the oocyte plas
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Thus, at the end of germ band forma
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Classic embryological experiments d
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posterior region of the unfertilize
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Further studies have strengthened t
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The Hunchback protein also works wi
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transcription factor to activate an
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The gap genes The gap genes were or
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Table 9.2. Major genes affecting se
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The development of the normal patte
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However, the mechanism by which the
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As we saw above, the gap gene and p
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can substitute for Ultrabithorax an
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The Translocation of Dorsal Protein
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The Gurken signal is received by th
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This fate map is generated by the g
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first glimpses of the multiple leve
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10. Early development and axis form
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While these cells are dividing, num
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The next phase of gastrulation invo
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Black and Gerhart (1985, 1986) let
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The convergent extension of the dor
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During early gastrulation, three ro
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Spemann concluded from this experim
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Hans Spemann and Hilde Mangold: Pri
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We will take up each of these quest
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Moreover, the injection of exogenou
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These Nodal-related proteins will a
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The diffusible proteins of the orga
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Follistatin. The fourth organizer-s
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Frzb and Dickkopf. Shortly after th
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Moreover, when dorsal blastopore li
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The relationship between these thre
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Nodal protein activates expression
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11. The early development of verteb
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Gastrulation in Fish Embryos The fi
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If one conceptually opens a Xenopus
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Left-right axis formation Little is
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thereby forming the secondary hypob
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This region, the germinal crescent,
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Axis Formation in the Chick Embryo
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The mechanisms for neural induction
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*Arendt and Nübler-Jung (1999) hav
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Compaction The fifth, and perhaps t
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Modifications for development withi
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The ectodermal precursors end up an
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These include the genes for transcr
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Experimental analysis of the hox co
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Kessel and Gruss (1991) found this
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Mice homozygous for an insertion mu
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7. In birds, gravity is critical in
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This tissue forms the amnionic sac
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12. The central nervous system and
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surrounding them. As much as 50% of
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Cell wedging is intimately linked t
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Fig 12.6 Human neural tube closure
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The anterior-posterior axis The ear
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Ventral patterning of the neural tu
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The time of this vertical division
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layer (Wallace 1999). Each Purkinje
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However, if these cells are transpl
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Neuronal Types The human brain cons
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Neurons transmit electrical impulse
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Development of the Vertebrate Eye A
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In addition, there are numerous Mü
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are shed and are replaced by new ce
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Just as there is a pluripotent neur
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13. Neural crest cells and axonal s
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The Trunk Neural Crest Migration pa
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Recognition of surrounding extracel
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However, D. J. Anderson's laborator
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Neural crest cells from rhombomeres
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the aortic arch arteries and the se
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Soon afterward, the expression patt
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Ericson and colleagues (1996) have
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That it must be a sort of a surface
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the G growth cone acts abnormally,
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Guidance by diffusible molecules Ne
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There is some reciprocity in scienc
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The activity of the synapse release
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Growth of the retinal ganglion axon
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The complicated nerve fiber circuit
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Snapshot Summary: Neural Crest Cell
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Fetal Neurons in Adult Hosts In 197
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Somites give rise to the cells that
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Eph signaling is thought to mediate
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(The dermis of other areas of the b
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Muscle cell fusion The myotome cell
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The mechanism of intramembranous os
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mitochondrial energy potential (Sha
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Mutations in FGFR1 can cause Pfeiff
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These buds eventually separate from
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Step 2: The metanephrogenic mesench
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Step 5: Conversion of the aggregate
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6. The two myotome regions are spec
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The Heart The circulatory system is
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This fusion occurs at about 29 hour
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Formation of Blood Vessels Although
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networks of each organ arise within
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In some instances, angiogenesis may
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This became apparent when experimen
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Blood and lymphocyte lineages. Figu
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Chick cells are readily distinguish
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In mammalian embryos, food and oxyg
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*In some infants, the septa fail to
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As Figure 15.27 shows, the stomach
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The surfactant enables the alveolar
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In mammals, the size of the allanto
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inhibits liver formation (Figure 15
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Moreover, as will be detailed in Ch
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These cells secrete the paracrine f
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Induction of the apical ectodermal
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The progress zone: The mesodermal c
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The mechanism by which Hox genes co
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Thus, while the Hox gene expression
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Charité and colleagues (1994) have
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Between the time when the cell's de
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Snapshot Summary: The Tetrapod Limb
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This environmental view of sex dete
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The cells of the seminiferous tubul
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into the mesenchyme, but stay near
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There was a strict correlation betw
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Wnt4: a potential ovary-determining
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Anti-müllerian duct hormone Anti-M
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in another (see Jacklin 1981; Bleie
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Chromosomal Sex Determination in Dr
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The sex-lethal gene as the pivot fo
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Doublesex: The switch gene of sex d
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It is not known whether the tempera
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18. Metamorphosis, regeneration, an
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There are no ipsilateral (same-side
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Organ-specific response to thyroid
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The more T 3 receptors a tissue has
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Other species, such as A. tigrinum,
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Metamorphosis in Insects Types of i
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On one side of the sac, the epithel
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During the first larval instar, the
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central region producing the Wingle
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The molecular biology of hydroxyecd
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Since its discovery in 1954, when B
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How do they regain the ability to d
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It is probable that during normal r
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The head activator gradient. The ab
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*The tradition of leaving one's boo
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However, recent studies have indica
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Moreover, if a mutation occured in
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Snapshot Summary: Metamorphosis, Re
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19. The saga of the germ line We be
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In the nematode C. elegans, the ger
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When ultraviolet light is applied t
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Fibronectin is likely to be an impo
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Germ cell migration in birds and re
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EG Cells, ES Cells, and Teratocarci
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come together and are then separate
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The distal tip cell extends long fi
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The initiation of spermatogenesis d
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ecause the flagellum is beginning t
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A partial catalogue of the material
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When a specific 340-base sequence f
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Oocyte chromosomes can be incubated
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The structure of the meroistic ovar
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Periodically, a group of primordial
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Following ovulation, the luteal pha
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20. An overview of plant developmen
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Page 559 and 560: *A small weed in the mustard family
Page 561 and 562: should provide information on the e
Page 563 and 564: of hormones. In scarlet runner bean
Page 565 and 566: embryos demonstrate that isolated p
Page 567 and 568: When the shoot emerges from the soi
Page 569 and 570: etween nodes is called an internode
Page 571 and 572: flowering patterns among the over 3
Page 573 and 574: genes, class D genes are now being
Page 575 and 576: Table 21.1. Specific settlement sub
Page 577 and 578: Predictable Environmental Differenc
Page 579 and 580: Diapause Many species of insects ha
Page 581 and 582: The hindwing pigments of the short-
Page 583 and 584: Ferguson and Joanen (1982) speculat
Page 585 and 586: Predator-Induced Defenses One survi
Page 587 and 588: 3. Antigens are presented (usually
Page 589 and 590: the ipsilateral eye. The situation
Page 591 and 592: *The importance of substrates for l
Page 593 and 594: Thus, most abnormal embryos are spo
Page 595 and 596: Retinoic acid as a teratogen In som
Page 597 and 598: Another pathway to apoptosis involv
Page 599 and 600: Whether heavy maternal alcohol cons
Page 601 and 602: Some scientists, however, say that
Page 603 and 604: Chains of causation Whether in law
Page 605 and 606: Snapshot Summary: The Environmental
Page 607: One could emphasize the common desc
Page 611 and 612: Hox Genes: Descent with Modificatio
Page 613 and 614: Changes in Hox gene transcription p
Page 615 and 616: But within the crustacean lineages,
Page 617 and 618: Changes in Hox gene number The numb
Page 619 and 620: Homologous Pathways of Development
Page 621 and 622: The gradient of Dpp concentration f
Page 623 and 624: Modularity: The Prerequisite for Ev
Page 625 and 626: Such a trait would be selected for
Page 627 and 628: *The lack of such transitional form
Page 629 and 630: Coevolution of ligand and receptor
Page 631 and 632: Evidence for this mathematical mode
Page 633 and 634: It is difficult for a mutation to a
Page 635 and 636: The population genetics model conta
Page 637 and 638: However, once one adds development
Page 639 and 640: A partial list of genes active in h
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The Questions of Developmental Biology

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