RESUMO
We previously demonstrated that 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment of Xenopus laevis during the early stages of life induces apoptosis in larval erythrocytes (Sakamoto et al., 1997). In the present study, an examination of these cells at the ultrastructural level was undertaken to elucidate the sequential morphological changes that occur during apoptosis. Xenopus embryos were exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin for 5 days shortly after fertilization. The circulating erythrocytes in larvae 12 days after fertilization were examined. Ultramicroscopic studies revealed four roughly defined stages of apoptosis. During the first stage, many small roundish vacuoles begin to appear in the cytoplasm. No noticeable changes can be found in the nucleus. In the second stage, the perinuclear cisterna become dilated, and huge cisternae can be seen in some erythrocytes. The roundish cytoplasmic vacuoles also become larger. Condensation of nuclear chromatin is not yet evident and the erythrocytes still maintain their elliptical shape. During the third stage, chromatin condensation and margination along the nuclear membrane becomes apparent. The nuclear pores gather in the diffuse chromatin region where the perinuclear cisterna is not dilated. The cytoplasm of some erythrocytes also becomes condensed and electron-dense. The normal arrangement of microtubules is disorderly and the erythrocytes deform into a roundish shape. Also, macrophages usually contact some part of the cell. In the final stage, those erythrocytes which show typical nuclear condensation, where neither nuclear or cytoplasmic fragmentation have occurred, are almost or completely phagocytosed by macrophages.
