Bone marrow stem cells repopulate thyroid in X-ray regeneration in mice

► Bone marrow stem cells contribute into thyroid regeneration after X-ray exposure. ► Chronic experiment reveals both follicular and intrafollicular thyrocytes involved. ► Parabiosis method provides data correlated to BMSC injection method in mice The aim of this study was to clarify the regeneration mechanisms of thyroid in post-irradiated C57Bl/6 mice chimeric with transgenic green fluorescent protein (GFP) positive C57Bl/6 mice after 5 and 7.5Gy X-ray exposures with the aid of morphological and immunocytochemical research of GFP-positive cell distribution. Cryostat slides of larynxes with thyroid glands were fixed by mixture of cold methanol and ethanol, cell nuclei were stained with propidium iodide. After immunocytochemical staining the slides of larynx with thyroid gland were investigated by means of confocal LSM 5 PASCAL microscope. True GFP nature of green signals in tissue slides was confirmed via additional treatment by anti-GFP antibody and Texas Red labeled second antibody. Separate GFP-positive cells were observed in the walls of follicles and between follicles of chimeric mice 9–10 months after X-ray exposure. GFP signal was viewed as cytoplasmic droplets and within the colloid of follicles. The share of GFP-positive follicles reached 6.1±1.8%. There was also co-localization of GFP signals and positive staining for thyroglobulin by monoclonal antibody. As many as 20.8±1.8% among all propidium iodide positive blood cells and 52.3±8.3% among propidium iodide positive bone marrow cells were at the same time GFP-positive. In conclusion, the results show that the bone marrow stem cells participate in the thyroid gland regeneration after 5Gy X-ray exposure.