The role of the Spemann organizer in anterior-posterior patterning of the trunk.

The formation of the vertebrate body axis during gastrulation strongly depends on a dorsal signaling centre, the Spemann organizer as it is called in amphibians. This organizer affects embryonic development by self-differentiation, regulation of morphogenesis and secretion of inducing signals. Whereas many molecular signals and mechanisms of the organizer have been clarified, its function in anterior-posterior pattern formation remains unclear. We dissected the organizer functions by generally blocking organizer formation and then restoring a single function. In experiments using a dominant inhibitory BMP receptor construct (tBr) we find evidence that neural activation by antagonism of the BMP pathway is the organizer function that enables the establishment of a detailed anterior-posterior pattern along the trunk. Conversely, the exclusive inhibition of neural activation by expressing a constitutive active BMP receptor (hAlk-6) in the ectoderm prohibits the establishment of an anterior-posterior pattern, even though the organizer itself is still intact. Thus, apart from the formerly described separation into a head and a trunk/tail organizer, the organizer does not deliver positional information for anterior-posterior patterning. Rather, by inducing neurectoderm, it makes ectodermal cells competent to receive patterning signals from the non-organizer mesoderm and thereby enable the formation of a complete and stable AP pattern along the trunk.

The superposition of a temporally incoherent magnetic field inhibits 60 Hz-induced changes in the ODC activity of developing chick embryos.

Previously, we have shown that the application of a weak (4 microT) 60 Hz magnetic field (MF) can alter the magnitudes of the ornithine decarboxylase (ODC) activity peaks which occur during gastrulation and neurulation of chick embryos. We report here the ODC activity of chick embryos which were exposed to the superposition of a weak noise MF over a 60 Hz MF of equal (rms strength). In contrast to the results we obtain with a 60 Hz field alone, the activity of ODC in embryos exposed to the superposition of the incoherent and 60 Hz fields was indistinguishable from the control activity during both gastrulation and neurulation. This result adds to the body of experimental evidence which demonstrates that the superposition of an incoherent field inhibits the response of biological systems to a coherent MF. The observation that a noise field inhibits ODC activity changes is consistent with our speculation that MF-induced ODC activity changes during early development may be related to MF-induced neural tube defects at slightly later stages (which are also inhibited by the superposition of a noise field).

The effect of pulsed and sinusoidal magnetic fields on the morphology of developing chick embryos.

Several investigators have reported robust, statistically significant results that indicate that weak (approximately 1 microT) magnetic fields (MFs) increase the rate of morphological abnormalities in chick embryos. However, other investigators have reported that weak MFs do not appear to affect embryo morphology at all. We present the results of experiments conducted over five years in five distinct campaigns spanning several months each. In four of the campaigns, exposure was to a pulsed magnetic field (PMF); and in the final campaign, exposure was to a 60 Hz sinusoidal magnetic field (MF). A total of over 2500 White Leghorn chick embryos were examined. When the results of the campaigns were analyzed separately, a range of responses was observed. Four campaigns (three PMF campaigns and one 60 Hz campaign) exhibited statistically significant increases (P > or = 0.01), ranging from 2-fold to 7-fold, in the abnormality rate in MF-exposed embryos. In the remaining PMF campaign, there was only a slight (roughly 50%), statistically insignificant (P = 0.2) increase in the abnormality rate due to MF exposure. When the morphological abnormality rate of all of the PMF-exposed embryos was compared to that of all of the corresponding control embryos, a statistically significant (P > or = .001) result was obtained, indicating that PMF exposure approximately doubled the abnormality rate. Like-wise, when the abnormality rate of the sinusoid-exposed embryos was compared to the corresponding control embryos, the abnormality rate was increased (approximately tripled). This robust result indicates that weak EMFs can induce morphological abnormalities in developing chick embryos. We have attempted to analyze some of the confounding factors that may have contributed to the lack of response in one of the campaigns. The genetic composition of the breeding stock was altered by the breeder before the start of the nonresponding campaign. We hypothesize that the genetic composition of the breeding stock determines the susceptibility of any given flock to EMF-induced abnormalities and therefore could represent a confounding factor in studies of EMF-induced bioeffects in chick embryos.

Cerberus is a head-inducing secreted factor expressed in the anterior endoderm of Spemann's organizer.

An abundant cDNA enriched in Spemann's organizer, cerberus, was isolated by differential screening. It encodes a secreted protein that is expressed in the anterior endomesoderm. Microinjection of cerberus mRNA into Xenopus embryos induces ectopic heads, and duplicated hearts and livers. The results suggest a role for a molecule expressed in the anterior endoderm in the induction of head structures in the vertebrate embryo.

Injected Wnt RNA induces a complete body axis in Xenopus embryos.

Studies in Xenopus have shown that growth factors of the TGF beta and Wnt oncogene families can mimic aspects of dorsal axis formation. Here we directly compare the inductive properties of two Wnt proteins by injecting synthetic mRNA into developing embryos. The results show that Wnt-1 and Xwnt-8 can induce a new and complete dorsal axis and can rescue the development of axis-deficient, UV-irradiated embryos. In contrast, activin mRNA injection induces only a partial dorsal axis that lacks anterior structures. These studies demonstrate that the mechanism of Wnt-induced axis duplication results from the creation of an independent Spemann organizer. The relationship between the properties of the endogenous dorsal inducer and the effects of Wnts and activins is discussed.

Injected Xwnt-8 RNA acts early in Xenopus embryos to promote formation of a vegetal dorsalizing center.

Expression cloning from a pool of gastrula cDNAs identified the Wnt family member Xwnt-8 as having dorsal axis-inducing activity in Xenopus embryos. Microinjected Xwnt-8 mRNA was able to rescue the development of a dorsally complete anterior-posterior axis in embryos ventralized by exposure to UV light. Axis induction was observed in embryos injected in either marginal or vegetal blastomeres at the 32-cell stage. Vegetal blastomeres receiving Xwnt-8 mRNA contributed progeny not to the induced dorsal axis, but to the endoderm, a result consistent with Xwnt-8 causing cells to act as a Nieuwkoop center (the vegetal-inducing component of normal dorsal axis formation), rather than as a Spemann organizer (the induced dorsal marginal zone component that directly forms the dorsal mesoderm). Xwnt-8, which is normally expressed ventrally in midgastrula and neurula embryos, appears to mimic, when injected, maternally encoded dorsal mesoderm-inducing factors that act early in development.

Weak low frequency electromagnetic fields and chick embryogenesis: failure to reproduce positive findings.

Fertilised chicken eggs were incubated for 48 hours while exposed to pulsed trains of magnetic fields having a duration of 0-5 ms, a rise time of 42 microsecond, and a pulse repetition rate of 100 at a magnetic field flux density of 1 microT. Some eggs were exposed to 1,552 rad X-rays as a positive control. After exposure the embryos were scored blind for a variety of morphological features. X-irradiated eggs displayed highly significant and repeatable anatomical alterations. There were no differences between magnetic field-exposed, sham-exposed and control eggs.

Arrest of intravitelline mitoses in Drosophila embryos by u.v. irradiation of the egg surface.

The intravitelline mitosis in Drosophila was arrested at the anaphase within the span of a single cell cycle after irradiation with 300 nm u.v. Embryos at and before the 8-nucleus stage were influenced by the u.v. only when irradiated anteriorly, while at and after the 16-nucleus stage, embryos are sensitive to either anterior or posterior irradiation. In embryos anteriorly irradiated at or before the 8-nucleus stage all nuclei in the embryo were prevented from performing mitosis. When irradiated at or after the 16-nucleus stage, inhibition of the intravitelline mitosis is limited to the nuclei in approximately anterior-half region of embryos in anterior irradiation, and to those in approximately posterior-half region in posterior irradiation, resulting in incomplete blastoderm formation. Sites sensitive to 300 nm u.v. are postulated to be present in the peripheral cytoplasmic region of the embryo and not in the nucleus, because the half-attenuation thickness of 300 nm u.v. light for the Drosophila egg cytoplasm is 3 microns and nuclei are at least 50 microns away from the periphery at the stage of irradiation. In addition lateral irradiation of a portion of an egg where there is no nucleus underneath was also effective in arresting division of nuclei in the same egg. It is suggested that the effects of 300 nm u.v. may not be conveyed to the nuclei from the periphery by simple diffusion of a substance, and a hypothesis is proposed for the involvement of cytoskeletal elements associated with the u.v. sensitive sites on the surface to the control mechanism of the intravitelline mitosis of the Drosophila embryo.

[Tumors in animals irradiated during embryogenesis]. / Ob opukholiakh u zhivotnykh, obluchennykh v émbriogeneze.

In experiments on 465 male and female white non-lineal rats irradiated in their embryonal (7th day--240 rats), organogenesis (14th day--105 rats) and foetal periods of embryogenesis (19th day--120 rats) with 60Co--rays in a dose of 100 r, its actual power 168 r/m, and on 73 control rats the authors have studied the frequency and terms of tumors appearance. A group comprising 219 experimental rats and 53 control animals, died during a period of 200--600 days following their birth, was analysed. Irradiation at different stages of embryogenesis would provide a statistically significant increase in the frequency of tumors and shorten the terms of their appearance. In control animals tumors arose in 16,9% (in 9 of 53 animals). Embryonal and foetal periods of embryogenesis proved to be mostly susceptible as evidenced by the yield of radiation tumors. In animals exposed to irradiation in their embryonal period tumors arose in 64,5% (in 69 of 107 rats), in the foetal period--in 62,8% (in 44 of 70 rats), and during the period of organogenesis--in 42,8% (in 18 of 42 rats). Rats exposed to irradiation in the embryonal and foetal period of embryogenesis developed tumors to the 120--180th day of the experiment, while in the organogenesis period--to the 400th day. Maximum tumor appearance was observed in the last third of the lifetime. Male rats irradiated in the period of embryogenesis developed tumors of the mammary gland and bones, females- ovarian and bone tumors, which were absent in control animals.

Repair of ultraviolet irradiation damage to a cytoplasmic component required for neural induction in the amphibian egg.

Localized ultraviolet irradiation of the amphibian egg destroys a cytoplasmic component that is required for neural induction. Destruction of that component severely diminishes the inducing capacity of the dorsal lip at gastrulation, as determined by embryological assays. Repair of the ultraviolet lesion can be achieved by replacing the dorsal lip of the irradiated embyro with a lip from an unirradiated embryo.