Compartmentalized organization: a common and required feature of stem cell niches?

Abstract

A key question in the stem cell field is how to balance the slow cycling of stem cells with active organ growth. Recent studies of the hair follicle stem cell niche have shown that this can be achieved by organizing the stem cell niche into two compartments: one that engages in immediate, rapid new growth and one that contributes later to long-term growth that fuels hair regeneration. Based on these and other recent findings, we propose that several other adult stem cell niches, including those in the blood, intestine and brain, have a similar bi-compartmental organization and that stem cells might work cooperatively with their progeny to sustain tissue regeneration.

Fig. 1.

The hair follicle as a model system to study stem cells. (A) A schematic of the hair follicle stem cell niche, which resides in the lower part of the follicle and comprises epithelial cells (the bulge and the hair germ) and mesenchymal cells (dermal papilla). Many cells are in contact with this niche. A basement membrane (blue) surrounds the hair follicle, along with a dermal sheath made of fibroblasts, connective tissue and macrophages. Nerve endings (purple) surround the bulge (dark green) and the arrector pili muscle (orange) inserts into the bulge. The melanocyte stem cells (melanoblasts; black) are also located in the bulge. For simplicity, the immune cells have been excluded. (B) The lower part of the hair follicle is organized into two different epithelial compartments, the hair germ (red) and the bulge (green), and the mesenchymal dermal papilla. The bulge contains bona fide epithelial stem cells and is positive for epithelial markers, such as integrin alpha 6 (not shown) and the cell surface glycoprotein CD34 (green). The hair germ is enriched for the adhesion molecule P-cadherin (red) and contains the epithelial progenitors that initiate a new round of regeneration. The epithelial cells are delineated by a dashed line and the mesenchymal dermal papilla cells by a solid line. All nuclei are marked in blue with DAPI. Scale bar: 50 μm.

Fig. 2.

A bi-compartmental organization of different adult mammalian stem cell niches. A schematic of stem cells in many tissues. Because stem cells have a slow cell cycle, they can be identified by their ability to retain nucleotide analogs such as bromodeoxyuridine (BrdU) longer than other cells, resulting in their name as label retaining cells (LRCs). (A) The hair follicle stem cell niche is organized into two different epithelial compartments, the hair germ and the bulge. The hair germ (light green) is located close to the mesenchymal dermal papilla (red), which acts as a signaling center. The bulge (dark green) is located further away from the dermal papilla. The cells with the most label retention are found in the bulge. (B) In the hematopoietic system, dormant hematopoietic stem cells (d-HSCs, dark green) and activated (a-) HSCs (light green) have been identified. The d-HSCs retain label better than do the a-HSCs and are probably located closer to the endosteal niche of osteoblasts (red), whereas the a-HSCs are probably further away from the bone and closer to the vasculature (red vessel). (C) In the intestine, stem cells are positive for Bmi1 (dark green) and are located in position +4 in the intestinal crypt. Lgr5-labelled stem cells (light green) have also been identified at the base of the crypts. The differentiated Paneth cells (red) could possibly function as a niche for the Lgr5⁺ stem cells. (D) In the subventricular zone of the striatum, two groups of astrocytic stem cells have been identified: Gfap⁺ Egfr⁻ astrocytes (dark green) and Gfap⁺ Egfr⁺ astrocytes (light green). These cells presumably rely on the ventricle and the blood vessels as a niche (red). Bmi1, B lymphoma Mo-MLV insertion region 1; Egfr, epidermal growth factor receptor; Lgr5, leucine-rich repeat-containing G-protein-coupled receptor 5; LRC, label retaining cells.

Fig. 3.

Cre-recombinase-based lineage tracing system. Cre recombinase expression can be spatially restricted by expressing it under the control of a tissue-specific promoter. Temporal restriction is achieved by fusing it to the tamoxifen-responsive hormone-binding domain of the estrogen receptor (Cre-ER^TAM). The Cre enzyme is in an inactive state in the absence of the ligand tamoxifen. Once tamoxifen is added, the Cre is active and can translocate to the nucleus. When these Cre constructs are used in conjunction with reporter genes, such as green fluorescent protein (GFP) ubiquitously expressed under the control of the ROSA26 (R26) promoter for example, and placed downstream of a STOP codon flanked by Cre recombinase recognition (loxP) sites, reporter gene expression can be activated in specific cell types at defined time-points. (A) In the absence of tamoxifen, no expression of the reporter gene is observed because of the presence of the stop signal upstream of the reporter gene. (B) When tamoxifen is administered, the Cre is activated and mediates recombination between the loxP sites in cells. As a consequence, the STOP codon is excised and the cells are permanently marked by the reporter gene. ER, estrogen receptor; GFP, green fluorescent protein.

Fig. 4.

Testing cell potential with lineage tracing. The inducible Cre system can be utilized to specifically mark different subsets of cells within a tissue. The persistence of labeled cells within a tissue is influenced by the turnover rate of the tissue, as well as by the cell cycling properties of the labeled cells. If cell-specific promoters are available (for stem cells, transit-amplifying cells or differentiated cells), several different labeling scenarios can be predicted. (A) If the stem cell population is marked, the whole tissue will be labeled and this label will persist for a long time, as stem cells are multipotent and long-lived. (B) If stem cell progeny are marked, and stem cells directly contribute to the organ regeneration, the transit-amplifying and differentiated cells will have a mosaic pattern (labeled and non-labeled cells) in the short term. If the progeny is also long-lived, this mosaic pattern will be sustained long term as well. (C) If transit-amplifying cells are marked, their differentiated daughter cells will also be marked in the short-term. However, as the transit-amplifying cells are short-lived, both labeled transit and differentiated cells will be replaced by unlabelled cells in the long term. (D) Conversely, if differentiated cells are marked, the label will be quickly lost, as differentiated cells are, in most tissues, short-lived and replaced by new stem cell progeny (there are exceptions of long-lived differentiated cells, such as memory B and T cells). By using this scheme, it is possible to address the regenerative potential of each of the cellular compartments without disrupting normal tissue physiology.