Consequences of the Spemann-Mangold organizer concept for embryological research in Russia: personal impressions.

The impact of the organizer concept on Russian experimental embryology is shortly reviewed. Attempts to study embryonic induction in Russia may be grouped into embryological and biochemical approaches. This paper provides a framework for, and overvalue of, the contributions of Russian biologists to the problem of embryonic induction. Two model systems--lens and neural inductions--are of special significance to modern developmental biologists. Moreover, the study of eye lens induction actually gave rise to research on developmental mechanics in Russia. This was one of the reasons why we limited this article to these two model systems. After retrospective consideration of the results of the search for possible lens-inducing factor candidates, the discussion turns towards some of the examples of neural-inducing agents detected in embryonic tissues and the new questions raised by the progress that has been made in the analysis of the Spemann-Mangold organizer.

Developmental patterns of crystallin expression during lens fiber differentiation in amphibians.

Data on activation of crystallin synthesis during lens fiber (LF) formation in amphibians are summarized to point out the questions particularly interesting in the context of lens cell lineage-specific expression programming under different developmental conditions. LFs are known to differentiate throughout life along the same pathway that includes at least five compartments. Using the amphibian eye lens as a model, we have studied how crystallins are expressed in the course of: (1) embryonic LF formation, (2) LF differentiation in adults, and (3) LF transdifferentiation from other (non-lens) eye tissues. Our experiments showed that synthesis of crystallins during morphologically similar LF differentiation in embryonic and adult amphibian lens has different spatial-temporal patterns (i.e., is apparently activated according to different programs). Certain results obtained in our studies suggest the absence of any direct relationship between the capacity of adult newt iris cells to transdifferentiate into LFs and crystallin synthesis (<> to such transdifferentiation) in them. Crystallins appear at the advanced stages of iris transdifferentiation into the lens and dynamics of their synthesis in the <> lens resembles that in the embryonic lens, although a new lens rudiment develops from the adult iris epithelium. Data on alternative patterns of the crystallin gene activation are summarized and compared with recent observations on spatial-temporal expression of Pax genes, which play an essential role in lens cell commitment and crystallin synthesis. On this basis, it is suggested that ontogenetic and tissue- or cell-specific changes in Pax gene expression may result in altered programs for activation of crystallin genes in embryonic, adult, and regenerating lens.

Partially purified factor from embryonic chick brain can provoke neuralization of Rana temporaria and Triturus alpestris but not Xenopus laevis early gastrula ectoderm.

A high neuralizing activity has been determined in forebrain of 7.5-day old chick embryos using Rana temporaria early gastrula ectoderm as reacting tissue (Mikhailov and Gorgolyuk, Soviet Scientific Reviews, Section of Physiology and General Biology, Vol. 1: 267-306, 1987). The corresponding protease-sensitive agent was extracted, partially purified by chromatography on DEAE-Toyopearl and Heparin-Ultragel columns, and its neuralizing activity was tested in vitro on ectoderm isolated from early gastrulae of R. temporaria, Triturus alpestris, and Xenopus laevis at different concentrations and for different periods of time (animal cap assay). Induction of neural structures was found in R. temporaria and T. alpestris explants (up to 100 and 60%, respectively), but not in cultures of X. laevis ectoderm. Under our experimental conditions, so-called "autoneuralization" of the ectoderm explants can safely be excluded. The results are discussed in relation to the neural competence of amphibian ectoderm and the mechanisms of neuralizing actions of different factors which might be involved in neural induction and patterning.

Concanavalin A induces neural tissue and cartilage in amphibian early gastrula ectoderm.

We have studied in vitro differentiation of explants of the amphibian (Rana temporaria) early gastrula ectoderm after treatment with various concentrations (50-300 micrograms/ml) of 'free' and Sepharose-bound concanavalin A (Con A). The explants were incubated with Con A for 3 h at 20 degrees C; the rolling up of the explants was prevented by using special weights. We have demonstrated that: (1) free Con A has an inducing action on the explants in the concentration range 100-300 micrograms/ml medium; (2) when treated with Con A the explants produce neural tissue (50-70%), cartilage (20-40%) and, rarely, lentoids (5-10%); (3) the frequency of neural and cartilage inductions was similar at various Con A concentrations; (4) alpha-methyl-D-mannoside pyranoside inhibited the Con A effects; (5) Sepharose-bound Con A had no effect on the explants, although it was bound to the cell surface of the ectoderm inner layer. Possible mechanisms of the neutralizing and chondrogenic effects of Con A on ectodermal explants are discussed.

Retrospective immunofluorescence of specific antigens in stained and balsam embedded sections of the developing amphibian lens.

A modification of the indirect immunofluorescent method is proposed for the detection of specific proteins in sections of the developing amphibian lens that were attached using egg albumin, stained and embedded into Canada balsam.