Cell types are fundamental units of metazoans, however their definition remains a long-standing challenge. We here use high throughput assays allowing for unprecedented resolution and systematic analyses. The comparison of the transcriptional landscapes of related and unrelated Drosophila differentiated cell types at larval and embryonic stages allowed us to identify features that are stable and cell-specific, revealing a unique signature for each cell type. Unexpectedly, all cell types share a stage-specific signature that is even stronger than the cell-specific one. Typically, embryonic hemocytes resemble more embryonic neurons than larval hemocytes. The stage-specific signature is associated to changes in chromatin accessibility between the two analyzed stages. We identified Rad50 as a chromatin boundary associated protein crucial for the temporal regulation of gene expression and chromatin organization, revealing a function of Rad50 beyond DNA repair. The identification of cell-specific and stage-specific signatures as equally important highlights another dimension of cell identity.