Tissue homeostasis is achieved through a balance of cell production (growth) and elimination (regression)1,2. In contrast to tissue growth, the cells and molecular signals required for tissue regression remain unknown. To investigate physiological tissue regression, we use the mouse hair follicle, which cycles stereotypically between phases of growth and regression while maintaining a pool of stem cells to perpetuate tissue regeneration3. Here we show by intravital microscopy in live mice4,5,6 that the regression phase eliminates the majority of the epithelial cells by two distinct mechanisms: terminal differentiation of suprabasal cells and a spatial gradient of apoptosis of basal cells. Furthermore, we demonstrate that basal epithelial cells collectively act as phagocytes to clear dying epithelial neighbours. Through cellular and genetic ablation we show that epithelial cell death is extrinsically induced through transforming growth factor (TGF)-β activation and mesenchymal crosstalk. Strikingly, our data show that regression acts to reduce the stem cell pool, as inhibition of regression results in excess basal epithelial cells with regenerative abilities. This study identifies the cellular behaviours and molecular mechanisms of regression that counterbalance growth to maintain tissue homeostasis.
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We thank E. Fuchs for the K14-H2BGFP, Lef1-RFP and K14-GFPActin mice; M. Taketo for the Cttnb1fl(Ex3)/+ mice; A. Horwich and A. Mesa for critical feedback; M. Rendl for technical advice; M. Graham and X. Liu for technical support with electron microscopy; D. Egli for the H2BmCherry construct; and T. Nottoli for generating the K14-H2BmCherry mouse line. K.R.M. and S.B. were supported by the National Institutes of Health (NIH) Predoctoral Program in Cellular and Molecular Biology, grant no. 5T32 GM007223. K.R.M. is currently a National Science Foundation (NSF) Graduate Research Fellow. This work is supported by the American Cancer Society, grant no. RSG-12-059-02; Yale Spore Grant National Cancer Institute, NIH, grant no. 2P50CA121974; the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), NIH, grant no. 1R01AR063663-01; and by The New York Stem Cell Foundation. V.G. is a New York Stem Cell Foundation–Robertson Investigator. P.R. is a New York Stem Cell Foundation–Druckenmiller Fellow. A.M.H. is supported by NIAMS Rheumatic Diseases Research Core Centers grant no. 5 P30 AR053495-07. K.B.B. was supported by the NSF. The NSF had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The views presented here are not those of the NSF and represent solely the views of the authors.
Extended data figures
Time-lapse recording of regressing hair follicles as seen using a K14-H2BGFP reporter in a live mouse by two-photon laser scanning microscopy (hh:mm).
Time-lapse recording of inner layers as seen using a K14-H2BGFP reporter. Note the relative upward displacement of the inner layers (centrally located) (hh:mm). Also see Fig 1b.
Time-lapse recording of basal epithelial cell death as seen by nuclear fragmentation using a K14-H2BGFP reporter (hh:mm). Note the relocation of nuclear fragments around neighboring epithelial nuclei. Also see Fig 1d.
Gallery view of a time-lapse recording highlights the complexity of the regression process and illustrates how several cellular behaviors and events occur in parallel using K14-H2BmCherry and K14-GFPActin reporters. Note that this view captures both an apoptotic event (in Row 3) as well as a phagocytic event (in Row 2).
Time-lapse recording of multiple apoptotic bodies from a single apoptotic basal cell being engulfed by neighboring basal epithelial cells using K14-GFPActin and K14-H2BmCherry reporters. Note that the apoptotic body originated from an apoptotic cell located 8µm below the site of engulfment. The apoptotic body is internalized by a neighboring epithelial cell as shown in both the (x,y) and (x,z) views. Also see Fig. 1g.
3D tracking of three apoptotic bodies formed from a single apoptotic basal epithelial cell as visualized by K14-GFPActin and K14-H2BmCherry reporters. Note the individual movement and divergent paths of the apoptotic bodies from the apoptotic cell (red) toward one of the phagocytic cells (green).
Time-lapse recording of a hair follicle in regression with surrounding LysMCre+ dermal populations (red) as seen using LysM-Cre;tdTomato reporter, in addition to the K14-H2BGFP reporter. Note that the nuclear fragments (green) are retained in the hair follicle. Also see Fig. 2a and Extended Data Fig. 4a.
Time-lapse recording of a hair follicle in regression with surrounding myeloid populations (green) as seen using CX3CR1-GFP, in addition to the K14-H2BmCherry reporter. Note that the epithelial nuclear fragments (red) are retained in the regressing hair follicle.
Time-lapse recording of a hair follicle in early regression as seen using the K14-H2BGFP and Lef1-RFP reporters. Note the contact between dying basal epithelial cells (green nuclei) and DP (red cells) during regression (hh:mm).
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Drug Discovery Today (2019)