Ultrastructure of Fanconi anemia fibroblasts

Employing indirect immunofluorescence and conventional electron microscopy, gross nuclear aberrations were observed in cultured interphase fibroblasts derived from a patient suffering from Fanconi's anemia (FA). Such aberrations were predominantly expressed in cells at high passages between 28 and 34. The structure of the nuclei appeared compound in nature, often consisting of two to three nuclear fragments connected to each other by thin nuclear bridges containing chromatin and nuclear lamin material. In other cases, the nuclei appeared lobed or budded but the cells did not contain distinct nuclear fragments. Chromatin was conspicuously absent from some nuclear lobes, revealing empty, cage-like structures comprising nuclear lamin material. Micronuclei were often abundant in the perinuclear cytoplasm but in some instances they appeared to be composed of chromatin lacking a delineating nuclear lamin matrix. Residual cytoskeletons examined by whole-mount electron microscopy revealed a network of intermediate filaments (IFs) within FA fibroblasts forming a bridge between the plasma membrane and the nucleus or its major fragments. In addition, there were thinner, 3-4 nm filaments connecting individual IFs with the surface of the nucleus. Micronuclei that were not connected to the main nuclear body, but which were delineated by a distinct lamina and possessed nuclear pores, did not appear to be anchored to the IF network. Multinuclearity, nuclear fragmentation, irregular chromatin distribution and inter-nuclear chromatin/lamin bridges might result from a failure in the redistribution of chromatin to sister nuclei, incomplete cytokinesis and proliferation of nuclear envelope material. These phenomena point to precocious aging of FA fibroblasts and may occur as a consequence of spontaneous damage to the sister chromatids or through the action of DNA-toxic agents.