The researchers mapped changes in chromatin (DNA packaging) during aging in 21 mouse tissues from males and females in three age groups[1]. They found that about a quarter of 536 organ-specific cell types and 1,828 more subtle subtypes show significant changes in cell number associated with aging[1]. Cells from different tissues show coordinated changes with age, suggesting systemic signals that drive these changes[1]. At the molecular level, researchers have identified widespread changes in DNA availability and activity of transcription factors, as well as changes in the genes that regulate these factors[1]. About 40 percent of the changes in cell number associated with aging are gender-dependent, with tens of thousands of DNA segments changing in just one gender[1]. Research has revealed coordinated changes including remodeling of the immune system, loss of functional cells, and the development of conditions associated with inflammation[1]. These findings provide a framework for understanding how aging alters chromatin and cell composition in various tissues[1].