IN-VIVO METAL SUBSTITUTIONS IN METAL SEQUESTERING SUBCELLULAR COMPARTMENTS - X-RAY MAPPING IN CRYOSECTIONS

Qualitative digital X-ray mapping techniques were employed to determin e the distributions of essential and non-essential elements in three i nvertebrate ''models'': (1) Pb, Zn, Cd, Cu, Fe in thin cryosections of the hepatopancreas of the terrestrial isopod, Oniscus asellus; (2) Pb , Zn, Cd, Ca in thin cryosections of the chloragogenous tissue of the earthworm, Lumbricus rubellus; and (3) As in air-dried smears and thin cryosections of chloragogen in L. rubellus. Four general conclusions were drawn from the results of these studies: (a) non-essential elemen ts can accumulate, distribute and be compartmentalized because they, o r the organo-complexes that they form, act as ''mimics'' of essential elements with which they share to a greater or lesser extent certain c hemical affinities; (b) thermodynamic considerations notwithstanding, the influence of biological factors on the sequestration and fates of certain elements (e.g., arsenic) is profound through modifications of redox states and organo-compound formation; (c) X-ray mapping, combine d with anhydrous preparative procedures, yields unbiased information c oncerning the relative spatial distributions of several elements in st ructurally heterogeneous sampling ''fields'', although the morphologic al characterization of (occasionally unsuspected) subcellular compartm ents may be constrained by the intrinsic quality of the preparation; a nd (d) X-ray microanalysis yields codistribution data, when integrated with biochemical information from other sources, which give strong po inters to the identity of binding ligands and of the valence state of sequestered cations.