Accumulation of senescent cells in tissues is the main cause of aging and age-related diseases. Senescence, under normal conditions, is primarily a consequence of telomere shortening, which signals a double-stranded DNA break and permanent cessation of cell division. Senescent cells change in morphology and physiology, lose their primary function and profile of gene expression and secretion, and as such negatively affect the surrounding healthy cells in which they induce secondary senescence. Mitochondria play a key role in creating the senescent phenotype, including secretion. Cells with eliminated mitochondria enter senescence, but do not develop a senescent secretory phenotype. The process of spreading senescence to the surrounding cells was almost entirely attributed to the influence of the soluble components of the senescent secretion. The latest research suggests that senescent cells in culture, after accumulating dysfunctional mitochondria, become very aggressive in transferring their mitochondria to the surrounding healthy cells where transmitophagy takes place. While mitochondrial transfer in necrotic and tumor cells is well known, it is completely unexplored in aging and senescence, and especially not in 3D tissues cultures and in vivo. Our preliminary results indicate that mitochondrial transfer between senescent and young/proliferating cells is frequent. In this project, we will investigate the influence of intercellular transfer of mitochondria in spreading of senescence among cells in few systems; combinations of skin cells in culture, 3D culture of full skin profile and in vivo in the Fischer 344 rat model. As it has been observed that senescent cells in tissues in vivo appear in groups and not uniform, the project will test the hypothesis that the most influential mechanism of spreading senescence in tissues in the aging process is the transfer of dysfunctional mitochondria from senescent cells to the surrounding young cells, including stem cells.