Electron microscope study of the binding of Con A-gold to superficial and inner ectoderm layers ofXenopus laevis and its relation to the neural-inducing activity of this lectin

Extracellular matrix components prevent neural differentiation of disaggregated Xenopus ectoderm cells

Neuralization (archencephalic brain formation) takes place after dissociation and delayed reaggregation of animal caps of early gastrula without inducer (Grunz, H. and L. Tacke: Cell Differ. Dev. 28, 211-218 (1989)). This autoneuralization can be prevented by the cell supernatant from dissociated ectoderm of Xenopus laevis, which contains extracellular matrix components. After phenol extraction of the supernatant, the aqueous phase does no longer show inhibitory activity. It can be concluded from these results that glycoconjugates responsible for the prevention of neuralization represent glycoproteins or proteoglycans which are loosely attached to integral plasma membrane components. Single early gastrula ectoderm cells mixed with non-competent late gastrula ectoderm or endoderm, which primarily form common aggregates, do not differentiate into neural derivatives. In these reaggregates the ectoderm cells remain separated from each other by heterologous cells (non-competent ectoderm or endodermal cells) during the period of competence. These data indicate that the quick recovery of extracellular matrix components together with the restoration of the former organization of the plasma membrane is responsible for the prevention of neuralization.

Close juxtaposition between inducing chordamesoderm and reacting neuroectoderm is a prerequisite for neural induction in Xenopus laevis

The results of this study indicate that the induction of the central nervous system in Xenopus laevis depends on the close juxtaposition of inducing chordamesoderm and reacting ectoderm, which is necessary for the short distance migration of neural inducing factors. The examination of the neuroectoderm-chordamesoderm interface at intervals of 1 h up to 5 h showed that the onset of neural induction is correlated to the degree of contact formation between ectodermal and mesodermal cells. In the ectoderm cells the number of coated pits, a feature of receptor-mediated endocytosis, is increased. Furthermore there exist telophase bridges between some ectoderm cells, which are possibly correlated to secondary cell interactions.

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.

Cross-linking of membrane glycoconjugates is not a sufficient condition for neural induction by concanavalin A

Tetravalent native concanavalin A (Con A) has a neural inducing effect on amphibian presumptive ectoderm. The divalent dimeric form of this lectin, succinylated Con A (Succ-Con A), is devoid of neuralizing action on this target tissue in Pleurodeles waltlii. These results suggested that cross-linking of Con A receptors on the cell membrane (which is not provoked by the divalent lectin) might be required for neural induction. To test this possibility, Succ-Con A binding sites were experimentally cross-linked after binding of Succ-Con A to the target cell surface, using anti-Con A antibodies. The combination of these two agents mimics the cross-linking of Con A. The results showed that cross-linking alone, either by treatment with Succ-Con A and anti-Con A antibodies, or with the lectins WGA and PHA, which also cross-link cell surface binding sites, was not able to induce neuralization. This suggested that the inductive action of Con A cannot be explained in terms of receptor cross-linking.

Structural alteration of the target plasma membrane affects reception but not expression of the neural inductive signal

We have previously reported that incubation of presumptive neuroectoderm in a solution of lectin (soybean agglutinin or garden pea agglutinin at 50 micrograms ml-1, 30 min) prior to its association with blastoporal lip inhibits neural induction (Duprat et al., 1982). We have also shown that incubation of presumptive ectoderm immediately after its association with blastoporal lip does not prevent neural induction. The same pattern of fluorescence on the ectodermal surface was observed when incubation with lectin was carried out before or after association of ectoderm with blastoporal lip. Although a particular molecular organization of the plasma membrane of the target ectoderm appears to be essential for reception or initiation of neural induction, the subsequent transmission and expression of this neural information does not appear to be affected by structural modifications of the target membrane. Similar studies performed using the inducer Con A indicated that the experimentally induced signal differed from the natural one.