Ultrastructure of the putative stem cell niche in rat mammary epithelium

There is now strong evidence that the stem cells of many tissues reside in specialized structures termed niches. The stem cell niche functions to house and regulate symmetric and asymmetric mitosis of stem cells in mammalian skin, mouse and human bone marrow, mouse brain, gut, and hair follicle, and Drosophila ovary and testis. This regulation is effected through the action of various signaling pathways such as Notch, Hedgehog, Wnt and others. The hormones of the estrous cycle, pregnancy and lactation that initiate growth in mouse mammary epithelium appear to act at a paracrine level to regulate mitosis through Notch receptors. Previous work has established that the putative stem cells of the mammary epithelium in several animal species reside near the basement membrane and never make contact with the ductal lumen. We show that these putative stem cells are found in anatomically specialized places created by the cytoplasmic extensions and modifications of neighboring differentiated cells. Such specializations may help to regulate stem cell activity by modulating molecular traffic to putative stem cells and contact with signaling molecules in the basement membrane. The histological characteristics of these putative niches vary as to the kinds of relationships the cells can have with the basement membrane and neighboring cells and as to how many stem or progenitor cells they may contain. This suggests a plasticity that may be relevant to the response of niches to tissue demands, such as wound healing, the periodic growth and regression of mammary epithelium, the process of mammary tumorigenesis therapeutic strategies for breast cancer.     

Regeneration of functional alveoli by adult human SOX9+ airway basal cell transplantation

Irreversible destruction of bronchi and alveoli can lead to multiple incurable lung diseases. Identifying lung stem/progenitor cells with regenerative capacity and utilizing them to reconstruct functional tissue is one of the biggest hopes to reverse the damage and cure such diseases. Here we showed that a rare population of SOX9⁺ basal cells (BCs) located at airway epithelium rugae can regenerate adult human lung. Human SOX9⁺ BCs can be readily isolated by bronchoscopic brushing and indefinitely expanded in feeder-free condition. Expanded human SOX9⁺ BCs can give rise to alveolar and bronchiolar epithelium after being transplanted into injured mouse lung, with air-blood exchange system reconstructed and recipient’s lung function improved. Manipulation of lung microenvironment with Pirfenidone to suppress TGF-β signaling could further boost the transplantation efficiency. Moreover, we conducted the first autologous SOX9⁺ BCs transplantation clinical trial in two bronchiectasis patients. Lung tissue repair and pulmonary function enhancement was observed in patients 3–12 months after cell transplantation. Altogether our current work indicated that functional adult human lung structure can be reconstituted by orthotopic transplantation of tissue-specific stem/progenitor cells, which could be translated into a mature regenerative therapeutic strategy in near future.     

Rap-GEF signaling controls stem cell anchoring to their niche through regulating DE-cadherin-mediated cell adhesion in the Drosophila testis

Stem cells will undergo self-renewal to produce new stem cells if they are maintained in their niches. The regulatory mechanisms that recruit and maintain stem cells in their niches are not well understood. In Drosophila testes, a group of 12 nondividing somatic cells, called the hub, identifies the stem cell niche by producing the growth factor Unpaired (Upd). Here, we show that Rap-GEF/Rap signaling controls stem cell anchoring to the niche through regulating DE-cadherin-mediated cell adhesion. Loss of function of a Drosophila Rap-GEF (Gef26) results in loss of both germline and somatic stem cells. The Gef26 mutation specifically impairs adherens junctions at the hub-stem cell interface, which results in the stem cells "drifting away" from the niche and losing stem cell identity. Thus, the Rap signaling/E-cadherin pathway may represent one mechanism that regulates polarized niche formation and stem cell anchoring.     

Reprogramming cell fates in the mammary microenvironment

The capacity of any portion of the murine mammary gland to produce a complete functional mammary outgrowth upon transplantation to an epithelium-divested fat pad is unaffected by the age or reproductive history of the donor. Likewise, through serial transplantations, no loss of potency is detected when compared to similar transplantations of the youngest mammary tissue tested. This demonstrates that stem cell activity is maintained intact throughout the lifetime of the animal despite aging and the repeated expansion and depletion of the mammary epithelium through multiple rounds of pregnancy, lactation and involution. These facts support the contention that mammary stem cells reside in protected tissue locales (niches), where their reproductive potency remains essentially unchanged through life. Disruption of the tissue, to produce dispersed cells results in the desecration of the protection afforded by the “niche” and leads to a reduced capacity of dispersed epithelial cells (in terms of the number transplanted) to recapitulate complete functional mammary structures. Our studies demonstrate that during the reformation of mammary stem cell niches by dispersed epithelial cells in the context of the intact epithelium-free mammary stroma, non-mammary cells may be sequestered and reprogrammed to perform mammary epithelial cell functions including those ascribed to mammary stem/progenitor cells.     

The Drosophila ovarian and testis stem cell niches: similar somatic stem cells and signals

The stem cell niches at the apex of Drosophila ovaries and testes have been viewed as distinct in two major respects. While both contain germline stem cells, the testis niche also contains "cyst progenitor" stem cells, which divide to produce somatic cells that encase developing germ cells. Moreover, while both niches utilize BMP signaling, the testis niche requires a key JAK/STAT signal. We now show, by lineage marking, that the ovarian niche also contains a second type of stem cell. These "escort stem cells" morphologically resemble testis cyst progenitor cells and their daughters encase developing cysts before undergoing apoptosis at the time of follicle formation. In addition, we show that JAK/STAT signaling also plays a critical role in ovarian niche function, and acts within escort cells. These observations reveal striking similarities in the stem cell niches of male and female gonads, and suggest that they are largely governed by common mechanisms.     

Glutamate transporter Slc1a3 mediates inter‐niche stem cell activation during skin growth

Tissues contain distinct stem cell niches, but whether cell turnover is coordinated between niches during growth is unknown. Here, we report that in mouse skin, hair growth is accompanied by sebaceous gland and interfollicular epidermis expansion. During hair growth, cells in the bulge and outer root sheath temporarily upregulate the glutamate transporter SLC1A3, and the number of SLC1A3⁺ basal cells in interfollicular epidermis and sebaceous gland increases. Fate mapping of SLC1A3⁺ cells in mice revealed transient expression in proliferating stem/progenitor cells in all three niches. Deletion of slc1a3 delays hair follicle anagen entry, uncouples interfollicular epidermis and sebaceous gland expansion from the hair cycle, and leads to reduced fur density in aged mice, indicating a role of SLC1A3 in stem/progenitor cell activation. Modulation of metabotropic glutamate receptor 5 activity mimics the effects of SLC1A3 deletion or inhibition. These data reveal that stem/progenitor cell activation is synchronized over distinct niches during growth and identify SLC1A3 as a general marker and effector of activated epithelial stem/progenitor cells throughout the skin.     

Peculiarities of the extracellular matrix in the interstitium of the renal stem/progenitor cell niche

The development of the nephron is piloted by interactions between epithelial and surrounding mesenchymal stem/progenitor cells. Data show that an astonishingly wide interstitial space separates both kinds of stem/progenitor cells. A simple contrasting procedure was applied to visualize features that keep renal epithelial and mesenchymal stem/progenitor cells in distance. The kidney of neonatal rabbits was fixed in solutions containing glutaraldehyde (GA) in combination with alcian blue, lanthanum, ruthenium red, or tannic acid. To obtain a comparable view to the renal stem/progenitor cell niche, the tissue was exactly orientated along the axis of collecting ducts. Fixation with GA or in combination with alcian blue or lanthanum revealed an inconspicuous interstitial space. In contrast, fixation with GA containing ruthenium red exhibits strands of extracellular matrix lining from epithelial stem/progenitor cells through the interstitium up to the surface of mesenchymal stem/progenitor cells. Fixation with GA containing tannic acid shows that the basal lamina of epithelial stem/progenitor cells, the adjacent interstitial space and also the surface of mesenchymal stem/progenitor cells are connected over a net of extracellular matrix. The applied technique appears to be a suitable method to illuminate the interstitium in stem/progenitor cell niches of specialized tissues, the microenvironment of tumors and extension of degeneration.     

Putative epithelial stem cell loss corresponds with mammary growth senescence

Since the advent of transmission electron microscopy of tissues capable of growth and regeneration, cell and developmental biologists have postulated that the undifferentiated cells observed within these tissues represent tissue-specific stem or progenitor cells. However, no studies have addressed the issue of whether these undifferentiated, putative stem cells persist in growth senescent tissues. Serially transplanted mammary epithelium consistently displays growth senescence beginning at the third transplant generation. This process is not uniform throughout the transplanted population and complete growth quiescence for all portions of a given outgrowth is reached subsequent to the 6th transplant generation. Mammary epithelial cells bearing the morphological characteristics of undifferentiated stem cells likewise disappear from senescent populations simultaneous with growth cessation. In premalignant mammary epithelial populations, which exhibit indefinitely prolonged growth potential, both of these cell types are maintained. This observation provides further support for the conclusion that these ultrastructurally distinct mammary cells represent the mammary stem/progenitor cell population.     

呼吸道上皮中的短暂扩充细胞——clara细胞

clara细胞为一类无纤毛、无粘液,而有着丰富分泌颗粒的呼吸道上皮细胞。clara细胞的功能为分泌蛋白、表达细胞色素氧化酶、对外源物的生物转换作用,以及作为呼吸道中的短暂扩充细胞来修复受损的呼吸道上皮。随着对干细胞、肿瘤干细胞及所处壁龛的深入研究,其在呼吸道上皮中的更新、修复及肿瘤发生中的作用也愈来愈受到重视,并为肿瘤的治疗研究带来了前景。     

Reprogramming non-mammary and cancer cells in the developing mouse mammary gland

The capacity of any portion of the murine mammary gland to produce a complete functional mammary outgrowth upon transplantation to an epithelium-divested fat pad is unaffected by the age or reproductive history of the donor. Likewise, through serial transplantations, no loss of potency is detected when compared to similar transplantations of the youngest mammary tissue tested. This demonstrates that stem cell activity is maintained intact throughout the lifetime of the animal despite aging and the repeated expansion and depletion of the mammary epithelium through multiple rounds of pregnancy, lactation and involution. These facts support the contention that mammary stem cells reside in protected tissue locales (niches), where their reproductive potency remains essentially unchanged through life. Disruption of the tissue, to produce dispersed cells results in the desecration of the protection afforded by the "niche" and leads to a reduced capacity of dispersed epithelial cells (in terms of the number transplanted) to recapitulate complete functional mammary structures. Our studies demonstrate that during the reformation of mammary stem cell niches by dispersed epithelial cells in the context of the intact epithelium-free mammary stroma, non-mammary cells, including mouse and human cancer cells, may be sequestered and reprogrammed to perform mammary epithelial cell functions including those ascribed to mammary stem/progenitor cells.     

Cell differentiation lineage in the prostate

Prostatic epithelium consists mainly of luminal and basal cells, which are presumed to differentiate from common progenitor/stem cells. We hypothesize that progenitor/stem cells are highly concentrated in the embryonic urogenital sinus epithelium from which prostatic epithelial buds develop. We further hypothesize that these epithelial progenitor/stem cells are also present within the basal compartment of adult prostatic epithelium and that the spectrum of differentiation markers of embryonic and adult progenitor/stem cells will be similar. The present study demonstrates that the majority of cells in embryonic urogenital sinus epithelium and developing prostatic epithelium (rat, mouse, and human) co-expressed luminal cytokeratins 8 and 18 (CK8, CK18), the basal cell cytokeratins (CK14, CK5), p63, and the so-called transitional or intermediate cell markers, cytokeratin 19 (CK19) and glutathione-S-transferase-pi (GSTpi). The majority of luminal cells in adult rodent and human prostates only expressed luminal markers (CK8, CK18), while the basal epithelial cell compartment contained several distinct subpopulations. In the adult prostate, the predominant basal epithelial subpopulation expressed the classical basal cell markers (CK5, CK14, p63) as well as CK19 and GSTpi. However, a small fraction of adult prostatic basal epithelial cells co-expressed the full spectrum of basal and luminal epithelial cell markers (CK5, CK14, CK8, CK18, CK19, p63, GSTpi). This adult prostatic basal epithelial cell subpopulation, thus, exhibited a cell differentiation marker profile similar to that expressed in embryonic urogenital sinus epithelium. These rare adult prostatic basal epithelial cells are proposed to be the progenitor/stem cell population. Thus, we propose that at all stages (embryonic to adult) prostatic epithelial progenitor/stem cells maintain a differentiation marker profile similar to that of the original embryonic progenitor of the prostate, namely urogenital sinus epithelium. Adult progenitor/stem cells co-express both luminal cell, basal cell, and intermediate cell markers. These progenitor/stem cells differentiate into mature luminal cells by maintaining CK8 and CK18, and losing all other makers. Progenitor/stem cells also give rise to mature basal cells by maintaining CK5, CK14, p63, CK19, and GSTpi and losing K8 and K18. Thus, adult prostate basal and luminal cells are proposed to be derived from a common pleuripotent progenitor/stem cell in the basal compartment that maintains its embryonic profile of differentiation markers from embryonic to adult stages.