The Questions of Developmental Biology

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Entry into the mitotic (M) phase of the cell cycle (in both meiosis and mitosis) is regulated by mitosis-promoting factor, or MPF (originally called "maturation-promoting factor" after its meiotic function). MPF contains two subunits, cyclin B and the p34 cdc2 protein. The p34 protein is a cyclin-dependent-kinase its activity is dependent upon the presence of cyclin. Since all the components of MPF are present in the amphibian oocyte, it is generally thought that progesterone somehow converts a pre-MPF complex into active MPF. The mediator of the progesterone signal is the c-mos protein. Progesterone reinitiates meiosis by causing the egg to polyadenylate the maternal c-mos mRNA that has been stored in its cytoplasm (Sagata et al. 1988, 1989; Sheets et al. 1995). This message is translated into a 39-kDa phosphoprotein, known as c-mos. This protein is detectable only during oocyte maturation and is destroyed quickly upon fertilization. Yet during its brief lifetime, it plays a major role in releasing the egg from its dormancy. If the translation of c-mos is inhibited (by injecting c-mos antisense mRNA into the oocyte), germinal vesicle breakdown and the resumption of oocyte maturation do not occur. The c-mos protein activates a phosphorylation cascade that phosphorylates and activates the p34 subunit of MPF (Ferrell and Machleder 1998; Ferrell 1999). The active MPF allows the germinal vesicle to break down and the chromosomes to divide. However, the chromosomes then encounter a second block. MPF can take the chromosomes through only the first meiotic division and the prophase of the second meiotic division. The oocyte is arrested again in the metaphase of the second meiotic division. This metaphase block is caused by the combined actions of c-mos and another protein, cyclindependent kinase 2 (cdk2; Gabrielli et al. 1993). These two proteins are subunits of cytostatic factor (CSF), which is found in mature frog eggs, and which can block cell cycles in metaphase (Matsui 1974). It is thought that CSF prevents the degradation of cyclin (Figure 19.24). The metaphase block is broken by fertilization. Evidence suggests that the calcium ion flux attending fertilization enables the calcium-binding protein calmodulin to become active. Calmodulin, in turn, can activate two enzymes that inactivate CSF: calmodulin-dependent protein kinase II, which inactivates p34, and calpain II, a calcium-dependent protease that degrades c- mos (Watanabe et al. 1989; Lorca et al. 1993). Without CSF, cyclin can be degraded, and the meiotic division can be completed. Gene transcription in oocytes In most animals (insects being a major exception), the growing oocyte is active in transcribing genes whose products are (1) necessary for cell metabolism, (2) necessary for oocyte-specific processes, or (3) needed for early development before the zygote-derived nuclei begins to function. In mice, for instance, the growing diplotene oocyte is actively transcribing the genes for zona pellucida proteins ZP1, ZP2, and ZP3. Moreover, these genes are transcribed only in the oocyte and not in any other cell (Figure 19.25; Roller et al. 1989; Lira et al. 1990; Epifano et al. 1995). The amphibian oocyte has certain periods of very active RNA synthesis. During the diplotene stage, certain chromosomes stretch out large loops of DNA, causing the chromosome to resemble a lampbrush (a handy instrument for cleaning test tubes in the days before microfuges). These lamp brush chromosomes (Figure 19.26) can be revealed as the sites of RNA synthesis by in situ hybridization.
Oocyte chromosomes can be incubated with a radioactive RNA probe, and autoradiography used to visualize the precise location where the gene is being transcribed. Figure 19.26B shows diplotene chromosome I of the newt Triturus cristatus after incubation with radioactive histone mRNA.It is obvious that a histone gene (or set of histone genes) is located on one of these loops of the lampbrush chromosome (Old et al. 1977). Electron micrographs of gene transcripts from lampbrush chromosomes also enable one to see chains of mRNA coming off each gene as it is transcribed (Hill and MacGregor 1980). In addition to mRNA synthesis, the patterns of ribosomal RNA and transfer RNA transcription are also regulated during oogenesis. Figure 19.27A shows the course of rRNA and tRNA synthesis during Xenopus oogenesis. Transcription appears to begin in early (stage I, 25 40 μm) oocytes, during the diplotene stage of meiosis. At this time, all the rRNAs and tRNAs needed for protein synthesis until the mid-blastula stage are made, and all the maternal mRNAs needed for early development are transcribed. This stage lasts for months in Xenopus. The rate of ribosomal RNA production is prodigious.
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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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Page 497 and 498: Metamorphosis in Insects Types of i
Page 499 and 500: On one side of the sac, the epithel
Page 501 and 502: During the first larval instar, the
Page 503 and 504: central region producing the Wingle
Page 505 and 506: The molecular biology of hydroxyecd
Page 507 and 508: Since its discovery in 1954, when B
Page 509 and 510: How do they regain the ability to d
Page 511 and 512: It is probable that during normal r
Page 513 and 514: The head activator gradient. The ab
Page 515 and 516: *The tradition of leaving one's boo
Page 517 and 518: However, recent studies have indica
Page 519 and 520: Moreover, if a mutation occured in
Page 521 and 522: Snapshot Summary: Metamorphosis, Re
Page 523 and 524: 19. The saga of the germ line We be
Page 525 and 526: In the nematode C. elegans, the ger
Page 527 and 528: When ultraviolet light is applied t
Page 529 and 530: Fibronectin is likely to be an impo
Page 531 and 532: Germ cell migration in birds and re
Page 533 and 534: EG Cells, ES Cells, and Teratocarci
Page 535 and 536: come together and are then separate
Page 537 and 538: The distal tip cell extends long fi
Page 539 and 540: The initiation of spermatogenesis d
Page 541 and 542: ecause the flagellum is beginning t
Page 543 and 544: A partial catalogue of the material
Page 545: When a specific 340-base sequence f
Page 549 and 550: The structure of the meroistic ovar
Page 551 and 552: Periodically, a group of primordial
Page 553 and 554: Following ovulation, the luteal pha
Page 555 and 556: 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
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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
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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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