Gradual differentiation uncoupled from cell cycle exit generates heterogeneity in the epidermal stem cell layer

Abstract

Highly regenerative tissues continuously produce terminally differentiated cells to replace those that are lost. How they orchestrate the complex transition from undifferentiated stem cells towards post-mitotic, molecularly distinct and often spatially segregated differentiated populations is not well understood. In the adult skin epidermis, the stem cell compartment contains molecularly heterogeneous subpopulations1–4 whose relationship to the complete trajectory of differentiation remains unknown. Here we show that differentiation, from commitment to exit from the stem cell layer, is a multi-day process wherein cells transit through a continuum of transcriptional changes with upregulation of differentiation genes preceding downregulation of typical stemness genes. Differentiation-committed cells remain capable of dividing to produce daughter cells fated to further differentiate, demonstrating that differentiation is uncoupled from cell cycle exit. These cell divisions are not required as part of an obligate transit-amplifying programme but help to buffer the differentiating cell pool during heightened demand. Thus, instead of distinct contributions from multiple progenitors, a continuous gradual differentiation process fuels homeostatic epidermal turnover.

Apical size anddeltaAexpression predict adult neural stem cell decisions along lineage progression

“…Although these observations await confirmation by monitoring DeltaA protein -we currently lack an antibody detecting DeltaA in adult zebrafish NSCs-, they suggest that sister deltaA pos cells are generally not equivalent, and that fate acquisition is a progressive process along the division sequence of each deltaA pos NSC. This is highly reminiscent of the progressive transition from commitment to differentiation described in mouse skin stem cells 53 . There was a trend but no clear-cut AA-related rule associated with the assignment of deltaA :GFP differences between daughters (Figure S4D).…”

Live imaging reveals chromatin compaction transitions and dynamic transcriptional bursting during stem cell differentiation in vivo

“…The epidermis is a highly regenerative organ involving the continual differentiation of cells from the basal stem cell layer to form the functional barrier of our skin. Cells within the stem cell layer will continually alter their transcriptome, delaminate from the stem cell compartment, and move apically/outward to form the overlaying differentiated layer (called spinous) over the course of 3–4 days ( Cockburn et al, 2022 ; Mesa et al, 2018 ; Figure 2A ). Previous studies have demonstrated that basal stem cells and differentiated spinous cells have different nuclear volumes and numbers of pericentromeric clusters ( Gdula et al, 2013 ).…”

“…We next wondered whether the transition in chromatin compaction occurs slightly later during differentiation in cells actively delaminating, located in between the basal and spinous layer ( Cockburn et al, 2022 ; Figure 3—figure supplement 1B ). Strikingly, we noticed the chromatin of these cells spin over the course of the timelapse ( Figure 3B , Video 2 ).…”

“…Recently, single-cell RNA-sequencing data has shown that the cell identity transition through epidermal differentiation is progressive and takes place over several days. Specifically, cells within the basal stem cell layer show global transcriptional changes associated with differentiation preceding exit from the basal layer ( Aragona et al, 2020 ; Cockburn et al, 2022 ). Chromatin accessibility and the architecture of individual loci have been investigated in embryonic skin ( Fan et al, 2018 ; Gdula et al, 2013 ; Mardaryev et al, 2014 ; Shue et al, 2020 ), but how and when chromatin changes relative to the transcriptional and morphological transitions of adult epidermal stem cell differentiation remains unknown.…”

A Kaleidoscope of Keratin Gene Expression and the Mosaic of Its Regulatory Mechanisms

“…The importance of type II keratins is confirmed by the fact that keratin 5 is often present in the basal layer of pseudostratified epithelium, while keratin-14 expression is sparse [ 43 , 50 , 52 , 59 , 64 ]. However, recently published details about mouse epidermal keratinocytes differentiation revealed the expression of keratin 10 belonging to type I of keratins prior to type II keratin 1 at a protein level [ 181 ]. These novel results indicate the importance of further investigation of the transcription activation of paired keratins within the keratin loci for the identification of the basic principles of their coordinated expression.…”