The Questions of Developmental Biology

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Table 21.2. Some agents thought to cause disruptions in human fetal development a DRUGS AND CHEMICALS IONIZING RADIATION (X-RAYS) Alcohol HYPERTHERMIA Aminoglycosides (Gentamycin) INFECTION MICROORGANISMS Aminopterin Coxsackie virus Antithyroid agents (PTU) Cytomegalovirus Bromine Herps simplex Cigarette smoke Parvovirus Cocaine Rubella (German measles) Cortisone Toxoplasma gondii (toxoplasmosis) Diethylstilbesterol (DES) Treponema Pallidum (syphilis) Diphenylhydantoin METABOLIC CONDITIONS IN THE MOTHER Heroin Autoimmune disease (including Rh incompatibility) Lead Diabetes Methylmercury Dietary deficiencies, malnutrition Penicillamine Phenylketonuria Retinoic acid (Isotretinoin, Accutane) Streptomycin Tetracycline Thalidomide Trimethadione Valproic acid Warfarin Source: Adapted from Opitz 1991. a This list includes known and possible teratogenic agents and is not exhaustive. Quinine and alcohol, two substances derived from plants, can also cause congenital malformations. Quinine can cause deafness, and alcohol (when more than 2 3 ounces per day are imbibed by the mother) can cause physical and mental retardation in the infant. Nicotine and caffeine have not been proved to cause congenital anomalies, but women who are heavy smokers (20 cigarettes a day or more) are likely to have infants that are smaller than those born to women who do not smoke. There is controvery over whether cigarette smoking enhances the risk of facial anomalies. Smoking also significantly lowers the number and motility of sperm in the semen of males who smoke at least four cigarettes a day (Kulikauskas et al. 1985). In addition, our industrial society produces hundreds of new artificial compounds that come into general use each year. Pesticides and organic mercury compounds have caused neurological and behavioral abnormalities in infants whose mothers have ingested them during pregnancy. Moreover, drugs that are used to control diseases in adults may have deleterious effects on fetuses. For example, valproic acid is an anticonvusant drug used to control epilepsy. It is known to be teratogenic in humans as it can cause major and minor bone defects. Barnes and colleagues (1996) have shown that valproic acid decreases the level of Pax1 transcription in chick somites. This causes the malformation of the somite and the corresponding malformations of vertebrae and ribs.
Retinoic acid as a teratogen In some instances, even a compound that is involved in normal development can have deleterious effects if present in large amounts at particular times. Retinoic acid is important in forming the anterior-posterior axis of the mammalian embryo and also in forming the limbs (see Chapters 11 and 16; Morriss-Kay and Ward 1999). In these instances, retinoic acid is secreted from discrete cells and works in a small area. However, if retinoic acid is present in large amounts, cells that normally would not receive such high concentrations of this molecule will respond to it. Inside the developing embryo, vitamin A and 13-cis-retinoic acid become isomerized to the developmentally active forms of retinoic acid, all-trans-retinoic acid and 9-cis-retinoic acid (Creech Kraft 1992). Retinoic acid cannot bind directly to genes. To effect gene regulation, RA must bind to a group of transcription factors called the retinoic acid receptors (RARs). These proteins have the same general structure as the steroid and thyroid hormone receptors, and they are active only when they have bound retinoic acid (Linney 1992). The RARs bind to specific enhancer elements in the DNA called retinoic acid response elements. Retinoic acid response elements contain at least two copies of the sequence GGTCA (Ruberte et al. 1990, 1991). Some of the Hox genes have retinoic acid response elements in their promoters (Yu et al. 1991; Pöpperl and Featherstone 1993; Studer et al. 1994). There are three major types of retinoic acid receptors: RAR-, RAR-β, and RAR-. They each bind both forms of retinoic acid, and they each bind to the same retinoic acid response element. Retinoic acid has been useful in treating severe cystic acne and has been available (under the name Accutane) since 1982. Because the deleterious effects of administering large amounts of vitamin A or its analogues to pregnant animals have been known since the 1950s (Cohlan 1953; Giroud and Martinet 1959; Kochhar et al. 1984), the drug carries a label warning that it should not be used by pregnant women. However, about 160,000 women of childbearing age (15 to 45 years) have taken this drug since it was introduced, and some of them have used it during pregnancy. Lammer and his co-workers (1985) studied a group of women who inadvertently exposed themselves to retinoic acid and who elected to remain pregnant. Of their 59 fetuses, 26 were born without any noticeable anomalies, 12 aborted spontaneously, and 21 were born with obvious anomalies. The malformed infants had a characteristic pattern of anomalies, including absent or defective ears, absent or small jaws, cleft palate, aortic arch abnormalities, thymic deficiencies, and abnormalities of the central nervous system. This pattern of multiple congenital anomalies is similar to that seen in rat and mouse embryos whose pregnant mothers have been given these drugs. Goulding and Pratt (1986) have placed 8-day mouse embryos in a solution containing 13-cis-retinoic acid at a very low concentration (2 × 10 -6 M). Even at this concentration, approximately one-third of the embryos developed a very specific pattern of anomalies, including dramatic reduction in the size of the first and second pharyngeal arches (see Figure 11.40). In normal mice, the first arch eventually forms the maxilla and mandible of the jaw and two ossicles of the middle ear, while the second arch forms the third ossicle of the middle ear as well as other facial bones. Retinoic acid probably works by several mechanisms. One basis for the teratogenicity of retinoic acid appears to reside in the drug's ability to alter the expression of the Hox genes and thereby respecify portions of the anterior-posterior axis and inhibit neural crest cell migration from the cranial region of the neural tube (Moroni et al. 1994; Studer et al. 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
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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
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 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: Thus, most abnormal embryos are spo
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
Page 625 and 626: Such a trait would be selected for
Page 627 and 628: *The lack of such transitional form
Page 629 and 630: Coevolution of ligand and receptor
Page 631 and 632: Evidence for this mathematical mode
Page 633 and 634: It is difficult for a mutation to a
Page 635 and 636: The population genetics model conta
Page 637 and 638: However, once one adds development
Page 639 and 640: A partial list of genes active in h
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