The observed reorganization of the egg cortex supports the initial hypothesis derived from the literature survey (cf. Table ) – the interplay between the JC and its underlying extra- and intracellular layers (the vitelline envelope and the plasma mambrane, respectively) mediate magnetic field effects in Xenopus laevis eggs.
The pigmentation – melanin granula closely linked to the cortex [16
] – served us as a visual indicator for this cortical reorganization. However, the melanin might well be responsable for the effect, since it resembles vertebrate pigments discussed in relation to magnetoreception at Earth's field strength [18
As for Xenopus
, the involvement of pigments in such rearrangements is also compatible with earlier reports of increased pigmentation anomalies in tadpoles subjected to static fields of 1 T [6
]. The occurence of TBE in all
fertilizable eggs without JC at higher field strengths points at a passive reaction to the magnetic field and suggests the involvement of structures or pathways in the oocyte that are not present before maturation and normally kept in place by the JC.
The cortical rearrangements leading to the TBE probably go along with a redistribution of sperm receptors, which might impede fertilization. However, this could not be tested, as fertilization requires the JC [12
], but experiments are under way to clarify whether embryos developing with or without JC show any difference due to magnetic field exposure.
The way in which the JC was removed could also influence the pronounciation of magnetic field effects. Five major approaches have been proposed to achieve it in a way useful for further biological studies of the eggs [13
]: Mechanial removal, UV irradiation, alkaline or enzymatic digestion or disulfid-reducing reagents. The first one is too time-consuming for the thousands of eggs necessary for our experiments, and the following three do not specifically act on the JC. This problem concerns the last group as well [6
] but we chose cysteine dejellying because it provides a relatively soft approach [13
]: It can reliably be stopped before attacking the vitelline envelope. Possibly, though, cysteine actions beyond JC lysis might contribute to the TBE, and further studies should seek to incorporate dejellying mechanisms in the assessment of magnetic field effects.
A detailed understanding of the mechanisms underlying such effects in model systems like Xenopus
can provide better estimates of possible biological limitations on the applicability of high magnetic fields to cells, tissues and organisms, including humans. In this respect, it is important to note that the minimum field strengths required for TBE I onset and for TBE II saturation, respectively, coincide with the current lower and upper limits of typical clinical magnetic resonance studies [19