Tissue stem cells (SCs) divide infrequently as a protective mechanism against internal and external stresses associated with aging.Here, we demonstrate that slow-and fast-cycling SCs in the mouse skin epidermis undergo distinct aging processes. Two years of lineage tracing reveals that Dlx1+ slow-cycling clones expand into the fast-cycling SC territory, while the number of Slc1a3+ fastcycling clones gradually declines. Transcriptome analysis further indicate that the molecular properties of each SC population are altered with age. Mice lacking fibulin 7, an extracellular matrix (ECM) protein, show early impairments resembling epidermal SC aging, such as the loss of fast-cycling clones, delayed wound healing, and increased expression of inflammation-and differentiationrelated genes. Fibulin 7 interacts with structural ECM and matricellular proteins, and the overexpression of fibulin 7 in primary keratinocytes results in slower proliferation and suppresses differentiation. These results suggest that fibulin 7 plays a crucial role in maintaining tissue resilience and epidermal SC heterogeneity during skin aging.

Adult tissues contain label-retaining cells (LRCs) which are relatively slow-cycling and considered to represent a property of tissue stem cells (SCs). In the ocular surface epithelium, LRCs are present in the limbus and conjunctival fornix; however, the character of these LRCs remains unclear due to lack of appropriate molecular markers. Using three CreER transgenic mouse lines, we demonstrate that the ocular surface epithelium accommodates spatially-distinct populations with different cell division dynamics. In the limbus, long-lived Slc1a3CreER-labeled SCs either migrate centripetally toward the central cornea or slowly expand their clones laterally within the limbal region. In the central cornea, non-LRCs labeled with Dlx1CreER and K14CreER behave as short-lived progenitor cells. The conjunctival epithelium in the bulbar, fornix, and palpebral compartment is regenerated by regionally-unique SC populations. Severe damage to the cornea leads to the cancellation of SC compartments and conjunctivalization, whereas milder limbal injury induces a rapid increase of laterally-expanding clones in the limbus. Taken together, our work defines compartmentalized, multiple SC/progenitor populations of the mouse eye in homeostasis and their behavioral changes in response to injury.

Tissue stem cells divide infrequently as a protective mechanism against internal and external stresses associated with aging. Here, we demonstrate that slow- and fast-cycling interfollicular epidermal stem cells in mouse skin undergo distinct aging processes. Two years of lineage tracing reveals that Dlx1+ slow-cycling clones expanded into the fast-cycling stem cell territory, while the number of Slc1a3+ fast-cycling clones gradually declined. Transcriptome analysis further indicated that the molecular properties of each stem cell population are altered with age. Mice lacking fibulin 7, an extracellular matrix (ECM), show early impairments resembling epidermal stem cell aging, such as the loss of fast-cycling clones, delayed wound healing, and increased expression of inflammation- and differentiation-related genes. Fibulin 7 interacts with structural ECM and matricellular proteins, and the overexpression of fibulin 7 in primary keratinocytes results in slower proliferation in the absence or presence of inflammatory cytokine IL-6. Thus, these results suggest that fibulin 7 plays a crucial role in maintaining tissue resilience and epidermal stem cell heterogeneity during skin aging.

Adult tissues contain label-retaining cell (LRC)s, which are relatively slow-cycling and considered to represent a unique property of tissue stem cell (SC)s. In the ocular surface epithelium, LRCs are detected in the limbus, a boundary between cornea and conjunctiva, and the fornix of the conjunctiva; however, the character of LRCs and identity of SCs remain unclear due to lack of appropriate molecular markers. Here we show that the ocular surface epithelium accommodates spatially distinct stem/progenitor populations with different cell division frequency. By combining EdU pulse-chase analysis and lineage tracing with three CreER transgenic mouse lines: Slc1a3 CreER , Dlx1 CreER and K14 CreER , we detect distinct dynamics of epithelial SCs in the cornea and conjunctiva. In the limbus, long-lived SCs are labeled with Slc1a3 CreER and they either migrate centripetally toward the central cornea or laterally expand their clones within the limbal region. In the central cornea, cells are mostly non-LRCs, labeled by Dlx1 CreER and K14 CreER , and the number of clones declines after a short period of time with rare long-lasting clones, suggesting their properties as short-lived progenitor cells. In the conjunctival epithelium, which consists of bulbar, fornix and palpebral conjunctiva, each territory is regenerated by compartmentalized, distinct SC populations without migrating one region to another. The severe damage of the cornea leads to the cancellation of SC compartments, causing conjunctivalization of the eye, whereas milder limbal injury induces a rapid increase of laterally-expanding clones in the limbus. Taken together, our work provides lineage tracing tools of the eye and defines compartmentalized, multiple SC/progenitor populations in homeostasis and their behavioral changes in response to injury.