Fig. 1. Microchimeric fetal cells in female thyroid express cytokeratin. Photomicrographs show FISH studies using Cy3-labeled X (orange) and FITC-labeled Y (green) chromosome probes, and immunofluorescence staining with anti-cytokeratin antibody, Texas Red (red). Nuclei are counterstained with DAPI (blue). (A) Male microchimeric cell in maternal thyroid with one Y chromosome (small arrow) and one X chromosome; 1000x magnification. (B) Same cell stained with anti-cytokeratin antibody (large arrow, red), indicating an epithelial cell; 1000x magnification. (C) Interphase FISH study of female thyroid tissue showing a group of male microchimeric cells; 400x magnification. Arrow shows one X and one Y chromosome. This group of cells did not stain positively for cytokeratin; therefore, they are not epithelial cells. (D) Combined FISH and immunofluorescence staining of a group of male microchimeric cells with one X and one Y (small arrow) chromosome. Note that in this plane of focus not all of the X chromosomes can be seen. This group of cells expresses cytokeratin (large arrow); 400x magnification.