The effects of nabam and nabam's degradative products on developing Xenopus laevis embryos
Birch, William Xaxier
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The effects of nabam and nabam's degradative products on Xenopus 1aevis embryos were determined. Three groups of experiments were performed using the fungicide disodium ethylenebisdithiocarbamate (nabam); 1) determination of minimum effective concentration, 2) effects of nabam on different stages of development, and 3) determination of the biological stability of nabam. Yolk plug embryos were also exposed to various concentrations and combinations of nabam's degradative products ethylene thiourea, ethylene diamine, and methyl isothiocyanate. Tetraethylthiuram disulfide and sodium thiocyanate were also tested. The parameters examined were mortality, gross morphology, histology, and ultrastructure. The total copper concentration of nabam-treated animals was also determined. Nabam induced malformations of the notochord at concentrations as low as 40 ppb (ug/1). The diameter of the notochord was increased in cross section. Ultrastructurally the collagen fibres of the outer connective tissue cells were reduced or absent. The changes induced by nabam were dose and stage of development dependent. Embryos exposed to nabam at neural a were the most severely affected. These animals developed wavy notochords and the pigmentation of the eye was delayed. Embryos treated with 100 ppb nabam stored 1-7 weeks were retarded in growth and a transient absence or reduction in melanogenesis in the eye was observed. The tail was shortened and the notochord had a distinct waviness. Histological examination showed a general decrease in optic cup size, a diminished pigmented retina, and a general increase in the diameter of the notochord. Ultrastructural examination revealed that the collagen fibres of the notochordal sheath were in complete disarray. Furthermore, the elastica externa was greatly diminished and fragmented. Embryos treated with various concentrations of tetraethylthiuram disulfide developed completely wavy notochords and pigmentation of the eye was reduced. The histological and ultrastructural changes were similar to those induced by nabam stored 1-7 weeks. However, the elastica externa of these animals was completely absent. Methyl isothiocyanate was shown to be extremely toxic. Embryos treated with 100 - 500 ppb methyl isothiocyanate developed a variety of severe malformations. Embryos treated with combinations of ethylene thiourea and methyl isothiocyanate developed abnormalities of the notochord similar to embryos treated with 40 ppb nabam. The diameter of the notochord as seen in cross section increased significantly. However, ultrastructurally the elastica externa of these animals was diminished and fragmented. These results indicate that minute doses of nabam and/or its decomposition products deleteriously alter cell and tissue organization, and thus, affect normal morphogenesis.