The Questions of Developmental Biology

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21. Environmental regulation of animal development One of the features shared by all the animals normally used to study developmental biology is their ability to develop regularly in the laboratory. Given adequate nutrition and temperature, these "model systems" develop independently of their environment (Bolker 1995). These animals give one the erroneous impression that "DNA provides the programme which controls the development of the embryo" (Wolpert 1991), or that everything needed to form the embryo is within the fertilized egg. Today, with new concerns about the loss of organismal diversity and about the effects of environmental pollutants, there is renewed interest in the regulation of development by the environment (see van der Weele 1999). There are numerous examples (and Homo sapiens provides some of the best) wherein the environment plays a critical role in determining the organism's phenotype. We discussed such environmental regulation of development in Chapters 3 and 17, where we encountered environmentally regulated sex determination and morphologies. In these and many other cases, the environment can elicit different phenotypes from the same genotype. The genetic ability to respond to such environmental factors has to be inherited, of course, but in these cases it is the environment that directs the different phenotypes from the same nuclear genotype. In this chapter, we will discuss how organisms use environmental cues in the course of their their normal development, as well as how exogenous compounds found in the environment can divert development from its usual path and cause congenital abnormalities. Environmental Regulation of Normal Development Environmental Cues and Normal Development Larval settlement The inclusion of environmental cues into normal development occurs during the settling of marine larvae. These cues may not be constant, but they need to be part of the environment if further development is to occur (Pechenik et al. 1998). A free-swimming marine larva often needs to settle near a source of food or on a firm substrate on which it can metamorphose. Thus, if prey or substrates give off soluble molecules, these molecules can be used by the larvae as cues to settle and begin metamorphosis. Among the molluscs, there are often very specific cues for settlement (Table 21.1; Hadfield 1977). In some cases, the prey supply the cues, while in other cases the substrate gives off molecules used by the larvae to initiate settlement.* One of the best studied cases of larval settlement is that of the red abalone, Haliotis rufescens. Its larvae only settle when they physically contact coralline red algae. A brief contact is all that is required for the competent larvae to stop swimming and begin metamorphosis. The chemical agent responsible for this change has not yet been isolated, but a receptor that recognizes an algal peptide induces metamorphosis in competent larvae. Larvae that are not competent to begin metamorphosis do not appear to have this receptor. The receptor is thought to be linked to a G protein similar to those found in vertebrates, and the activation of this G protein may be necessary for inducing larval settlement and metamorphosis (Morse et al. 1984; Baxter and Morse 1992; Degnan and Morse 1995).
Table 21.1. Specific settlement substrates of molluscan larvae Molluscan species Substrate Gastropoda (snails, nudibranchs) Nassarius obsoletus Mud from adult habitat Philippia radiata Porites lobata (a cnidarian) Adalaria proxima Electra pilosa (a bryozoan) Doridella obscura Electra crustulenta (a bryozoan) Phestilla sibogae Porites compressa (a cnidarian) Rostanga pulchra Ophlitaspongia pennata (a sponge) Trinchesia aurantia Tubularia indivisa (a cnidarian) Elysia chlorotica Primary film of microorganisms from adult habitat Haminoea solitaria Primary film of microorganisms from adult habitat Aplysia californica Laurencia pacifica (a red alga) Aplysia juliana Ulva spp. (green algae) Aplysia parvula Chondrococcus hornemanni (a red alga) Stylocheilus longicauda Lyngbya majuscula (a cyanobacterium) Onchidoris bilamellata Living barnacles Amphineura (chitons) Tonicella lineata Lithophyllum sp. and Lithothamnion sp. (red algae) Lamellibranchia (bivalves) Teredo sp. Wood Bankia gouldi Wood Mercenaria mercenaria Clam liquor; sand Placopecten magellanicus Adult shell; sand; etc. Mytilus edulis Filamentous algae; other nonbiological silk material Crassostrea virginica Shell liquor; body extract; "shellfish glycogen" Blood meals In many mosquitoes, egg production is triggered by a blood meal. Only female mosquitoes bite, and prior to a blood meal they make no vitellogenin yolk protein. In Aedes aegypti, the digested products of the blood meal stimulate the brain to secrete egg development neurosecretory hormone (EDNH, also known as ovarian ecdysteroidogenic hormone, or OEH). This hormone stimulates the ovary to make ecdysteroids, which instruct the fat body cells to make vitellogenin for the oocytes (Fallon et al. 1974; Hagedorn 1983; Borovsk et al. 1990). Vitellogenin is critical for egg production. Thus, without the blood meal, there is no vitellogenin and no eggs. In the blood-sucking bug Rhodinus prolixus, adult females produce a new batch of eggs each time they drink blood. This blood meal serves two functions. Blood proteins from the mammalian host supply the amino acids needed for vitellogenin synthesis, and the physical stretching of the abdomen by the blood initiates the endocrine stimuli that activates juvenile hormone secretion by the corpora allata. JH stimulates vitellogenin synthesis in the ovary and fat body (see Nijhout 1994).
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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
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 and 546: When a specific 340-base sequence f
Page 547 and 548: Oocyte chromosomes can be incubated
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
Page 557 and 558: oth the embryo and the mature sporo
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: genes, class D genes are now being
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 and 608: One could emphasize the common desc
Page 609 and 610: The search for the Urbilaterian anc
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
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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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