The Questions of Developmental Biology

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It appears that the specification of cell fates in sea urchins involves a cascade initiated by the micromeres. It has been hypothesized (Davidson 1989) that the signal from the micromeres induces a post-translational modification in some factor in the veg 2 layer. Similarly, the signal from the veg 2 layer probably modifies some protein (possibly a transcription factor) in the veg 1 layer. In this way, the different tiers are assigned different fates. Axis specification In the sea urchin blastula, the cell fates line up along the animal-vegetal axis established in the egg cytoplasm prior to fertilization. The animal-vegetal axis also appears to structure the future anterior-posterior axis, with the vegetal region sequestering those maternal components necessary for posterior development (Boveri 1901; Maruyama et al. 1985). In most sea urchins, the dorsal-ventral and left-right axes are specified after fertilization, but the manner of their specification is not well understood. Since the first cleavage plane can be either parallel, perpendicular, or oblique with respect to the eventual dorsal-ventral axis, it is probable that the dorsal-ventral axis is not specified until the 8-cell stage, when there are cell boundaries that correspond to these positions (Kominami 1983; Henry et al. 1992). Interestingly, in those sea urchins that bypass the larval stage to develop directly into juveniles, the dorsalventral axis is specified maternally in the egg cytoplasm (Henry and Raff 1990). Sea Urchin Gastrulation The late sea urchin blastula consists of single layer of about 1000 cells that form a hollow ball, somewhat flattened at the vegetal end. The blastomeres, derived from different regions of the zygote, have different sizes and properties. Figures 8.16 and 8.17 show the fates of the various regions of the blastula as it develops through gastrulation to the pluteus larva stage characteristic of sea urchins. The fate of each cell layer can be seen through its movements during gastrulation.
Ingression of primary mesenchyme Function of primary mesenchyme cells Shortly after the blastula hatches from its fertilization envelope, the vegetal side of the spherical blastula begins to thicken and flatten (Figure 8.17, 9 hours). At the center of this flat vegetal plate, a cluster of small cells begins to change. These cells begin extending and contracting long, thin (30 × 5 μm) processes called filopodia from their inner surfaces. The cells then dissociate from the epithelial monolayer and ingress into the blastocoel (Figure 8.17, 9 10 hours). These cells, derived from the micromeres, are called the primary mesenchyme. They will form the larval skeleton, so they are sometimes called the skeletogenic mesenchyme. At first the cells appear to move randomly along the inner blastocoel surface, actively making and breaking filopodial connections to the wall of the blastocoel. Eventually, however, they become localized within the prospective ventrolateral region of the blastocoel. Here they fuse into syncytial cables, which will form the axis of the calcium carbonate spicules of the larval skeleton (Figure 8.18). Importance of extracellular lamina inside the blastocoel The ingression of the micromere descendants into the blastocoel is caused by these cells losing their affinity for their neighbors and for the hyaline membrane and acquiring a strong affinity for a group of proteins that line the blastocoel. This model was first proposed by Gustafson and Wolpert (1967) and was confirmed in 1985, when Rachel Fink and David McClay measured the strengths of sea urchin blastomere adhesion to the hyaline layer, to the basal lamina lining the blastocoel, and to other blastomeres. Originally, all the cells of the blastula are connected on their outer surface to the hyaline layer and on their inner surface to a basal lamina secreted by the cells.
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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).
Page 151 and 152: mammary gland tissue. The binding o
Page 153 and 154: In some cells, gene transcription r
Page 155 and 156: PARTE 2. Early embryonic developmen
Page 157 and 158: The means by which sperm are propel
Page 159 and 160: The sperm released during ejaculati
Page 161 and 162: Lying immediately beneath the plasm
Page 163 and 164: The mechanisms of chemotaxis differ
Page 165 and 166: Once the sea urchin sperm has penet
Page 167 and 168: Removal of these threonine- or seri
Page 169 and 170: undergo the acrosomal reaction with
Page 171 and 172: Gamete Fusion and the Prevention of
Page 173 and 174: The fast block to polyspermy. The f
Page 175 and 176: envelope to expand and become the f
Page 177 and 178: The calcium ions responsible for th
Page 179 and 180: Thus, the conversion of NAD + to NA
Page 181 and 182: eticulum (McPherson et al. 1992). T
Page 183 and 184: cAMP-dependent protein kinase activ
Page 185 and 186: These cells divide to form embryos
Page 187 and 188: (Figure 7.34; Elinson and Rowning 1
Page 189 and 190: 6. In many species, eggs secrete di
Page 191 and 192: One consequence of this rapid cell
Page 193 and 194: Table 8.1. Karyokinesis and cytokin
Page 195 and 196: provides a classification of cleava
Page 197 and 198: This occurred at precisely the time
Page 199 and 200: These rapid and invariant cell clea
Page 201: The identities of the signaling mol
Page 205 and 206: But these guidance cues cannot be s
Page 207 and 208: lastocoel wall (Dan and Okazaki 195
Page 209 and 210: The orientation of the cleavage pla
Page 211 and 212: stage, the remaining cells divide n
Page 213 and 214: embryos can produce these cells, bu
Page 215 and 216: Early Development in Tunicates Tuni
Page 217 and 218: (or inactivating) specific genes. T
Page 219 and 220: occur under laboratory conditions.
Page 221 and 222: the germ cells. Using fluorescent a
Page 223 and 224: Conditional specification As we saw
Page 225 and 226: eggs. Coda This chapter has describ
Page 227 and 228: 9. The genetics of axis specificati
Page 229 and 230: Following cycle 13, the oocyte plas
Page 231 and 232: Thus, at the end of germ band forma
Page 233 and 234: Classic embryological experiments d
Page 235 and 236: posterior region of the unfertilize
Page 237 and 238: Further studies have strengthened t
Page 239 and 240: The Hunchback protein also works wi
Page 241 and 242: transcription factor to activate an
Page 243 and 244: The gap genes The gap genes were or
Page 245 and 246: Table 9.2. Major genes affecting se
Page 247 and 248: The development of the normal patte
Page 249 and 250: However, the mechanism by which the
Page 251 and 252: 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
Page 521 and 522:
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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oth the embryo and the mature sporo
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*A small weed in the mustard family
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should provide information on the e
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of hormones. In scarlet runner bean
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embryos demonstrate that isolated p
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When the shoot emerges from the soi
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etween nodes is called an internode
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flowering patterns among the over 3
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genes, class D genes are now being
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Table 21.1. Specific settlement sub
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Predictable Environmental Differenc
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Diapause Many species of insects ha
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The hindwing pigments of the short-
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Ferguson and Joanen (1982) speculat
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Predator-Induced Defenses One survi
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3. Antigens are presented (usually
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the ipsilateral eye. The situation
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*The importance of substrates for l
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Thus, most abnormal embryos are spo
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Retinoic acid as a teratogen In som
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Another pathway to apoptosis involv
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Whether heavy maternal alcohol cons
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Some scientists, however, say that
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Chains of causation Whether in law
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Snapshot Summary: The Environmental
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One could emphasize the common desc
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The search for the Urbilaterian anc
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Hox Genes: Descent with Modificatio
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Changes in Hox gene transcription p
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But within the crustacean lineages,
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Changes in Hox gene number The numb
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Homologous Pathways of Development
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The gradient of Dpp concentration f
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Modularity: The Prerequisite for Ev
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Such a trait would be selected for
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*The lack of such transitional form
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Coevolution of ligand and receptor
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Evidence for this mathematical mode
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It is difficult for a mutation to a
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The population genetics model conta
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However, once one adds development
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A partial list of genes active in h
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The Questions of Developmental Biology

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