In and out of the niche: perspectives in mobilization of hematopoietic stem cells.

Several stem cell mobilization strategies have been employed in the past 2 decades, including chemotherapy, hematopoietic growth factors, and chemotherapy plus growth factors. Granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage CSF are standard agents approved for peripheral blood stem cell mobilization since the early 1990s. Between 5% and 20% of patients, however, fail to mobilize a sufficient numbers of peripheral blood stem cells in response to G-CSF with or without chemotherapy. Recent advances in defining the basic mechanisms regulating the interactions between hematopoietic stem cells and their marrow niche had led to the discovery that CXCR4 and stromal-cell-derived factor 1α axis play a significant role. Plerixafor, an antagonist of the CXCR4-stromal-cell-derived factor 1α axis has been shown to result in a significant mobilization of hematopoietic stem cells. Numerous clinical trials have demonstrated that the combination of G-CSF and AMD3100 (G+A) resulted in a significant increase in CD34(+) cell yield as compared to the administration of G-CSF alone. In particular, the progenitors mobilized have been shown to comprise a significantly higher proportion of primitive and possibly more potent CD34(+)/CD38(-) subpopulation. Transplantation of PBSC mobilized by G+A administration have led to a rapid and sustained neutrophil and platelet engraftment. Another prospective role of this new class of agents might lie in the mobilization of dormant leukemia stem cells that are well protected by the niche. The future role of CXCR4 antagonists in treatment of hematologic malignancies includes mobilization of hematopoietic stem cells for transplantation and mobilization of leukemia-initiating cells for long-term cure.

The endoplasmic reticulum chaperone protein GRP94 is required for maintaining hematopoietic stem cell interactions with the adult bone marrow niche.

Hematopoietic stem cell (HSC) homeostasis in the adult bone marrow (BM) is regulated by both intrinsic gene expression products and interactions with extrinsic factors in the HSC niche. GRP94, an endoplasmic reticulum chaperone, has been reported to be essential for the expression of specific integrins and to selectively regulate early T and B lymphopoiesis. In GRP94 deficient BM chimeras, multipotent hematopoietic progenitors persisted and even increased, however, the mechanism is not well understood. Here we employed a conditional knockout (KO) strategy to acutely eliminate GRP94 in the hematopoietic system. We observed an increase in HSCs and granulocyte-monocyte progenitors in the Grp94 KO BM, correlating with an increased number of colony forming units. Cell cycle analysis revealed that a loss of quiescence and an increase in proliferation led to an increase in Grp94 KO HSCs. This expansion of the HSC pool can be attributed to the impaired interaction of HSCs with the niche, evidenced by enhanced HSC mobilization and severely compromised homing and lodging ability of primitive hematopoietic cells. Transplanting wild-type (WT) hematopoietic cells into a GRP94 null microenvironment yielded a normal hematology profile and comparable numbers of HSCs as compared to WT control, suggesting that GRP94 in HSCs, but not niche cells, is required for maintaining HSC homeostasis. Investigating this, we further determined that there was a near complete loss of integrin α4 expression on the cell surface of Grp94 KO HSCs, which showed impaired binding with fibronectin, an extracellular matrix molecule known to play a role in mediating HSC-niche interactions. Furthermore, the Grp94 KO mice displayed altered myeloid and lymphoid differentiation. Collectively, our studies establish GRP94 as a novel cell intrinsic factor required to maintain the interaction of HSCs with their niche, and thus regulate their physiology.

Hypoxic induction of vascular endothelial growth factor regulates murine hematopoietic stem cell function in the low-oxygenic niche.

Hypoxia is emerging as an important characteristic of the hematopoietic stem cell (HSC) niche, but the molecular mechanisms contributing to quiescence, self-renewal, and survival remain elusive. Vascular endothelial growth factor A (VEGFA) is a key regulator of angiogenesis and hematopoiesis. Its expression is commonly regulated by hypoxia-inducible factors (HIF) that are functionally induced in low-oxygen conditions and that activate transcription by binding to hypoxia-response elements (HRE). Vegfa is indispensable for HSC survival, mediated by a cell-intrinsic, autocrine mechanism. We hypothesized that a hypoxic HSC microenvironment is required for maintenance or up-regulation of Vegfa expression in HSCs and therefore crucial for HSC survival. We have tested this hypothesis in the mouse model Vegfa(δ/δ), where the HRE in the Vegfa promoter is mutated, preventing HIF binding. Vegfa expression was reduced in highly purified HSCs from Vegfa(δ/δ) mice, showing that HSCs reside in hypoxic areas. Loss of hypoxia-regulated Vegfa expression increases the numbers of phenotypically defined hematopoietic stem and progenitor cells. However, HSC function was clearly impaired when assessed in competitive transplantation assays. Our data provide further evidence that HSCs reside in a hypoxic microenvironment and demonstrate a novel way in which the hypoxic niche affects HSC fate, via the hypoxia-VEGFA axis.

Antler development was inhibited or stimulated by cryosurgery to periosteum or skin in a central antlerogenic region respectively.

Antler development is triggered by interactions between antler stem cells resident in the antlerogenic periosteum (AP) and the niche cells in the upper portion of overlying skin mediated by diffusible molecules. These interactive cell populations are interposed by the lower portion of the skin and the subcutaneous loose connective tissue (SLCT). It is known that mechanical deletion of just the central AP (having an area equivalent to the size of a pedicle base) by cutting through the skin and SLCT effectively stimulates the marginal AP to initiate antler development. This study was designed to investigate whether the SLCT layer plays a role in antler development by acting as a physical barrier. The results showed that the marginal AP failed to give rise to an antler after the central AP was cryosurgically destroyed with the preservation of the collagen structure of the SLCT. Furthermore, antler development was significantly advanced when the collagen structures of the skin and SLCT layers were substantially attenuated by repeated sprays with liquid nitrogen while keeping the central AP intact. Therefore, we conclude that the interposing SLCT layer acts as a physical barrier between antler stem cells and the niche cell types, and that timing of antler development is primarily controlled by the permeability of the SLCT layer to the putative interactive diffusible molecules.

Galectin-1-expressing stromal cells constitute a specific niche for pre-BII cell development in mouse bone marrow.

In the bone marrow (BM), stromal cells constitute a supportive tissue indispensable for the generation of pro-B/pre-BI, pre-BII, and immature B lymphocytes. IL-7-producing stromal cells constitute a cellular niche for pro-B/pre-BI cells, but no specific stromal cell microenvironment was identified for pre-BII cells expressing a functional pre-B cell receptor (pre-BCR). However expression of the pre-BCR represents a crucial checkpoint during B-cell development. We recently demonstrated that the stromal cell derived-galectin1 (GAL1) is a ligand for the pre-BCR, involved in the proliferation and differentiation of normal mouse pre-BII cells. Here we show that nonhematopoietic osteoblasts and reticular cells in the BM express GAL1. We observed that pre-BII cells, unlike the other B-cell subsets, were specifically localized in close contact with GAL1(+) reticular cells. We also determined that IL-7(+) and GAL1(+) cells represent 2 distinct mesenchymal populations with different BM localization. These results demonstrate the existence of a pre-BII specific stromal cell niche and indicate that early B cells move from IL-7(+) to GAL1(+) supportive BM niches during their development.

Niche contributions to oncogenesis: emerging concepts and implications for the hematopoietic system.

The field of hematopoietic oncology has traditionally focused on the study of hematopoietic cell autonomous genetic events in an effort to understand malignant transformation and develop therapeutics. Although highly rewarding in both aspects, this cell autonomous approach has failed to fully satisfy our need to understand tumor cell behavior and related clinical observations. In recent years, it has been increasingly recognized that the tumor microenvironment plays a pivotal role in cancer initiation and progression. This review will discuss recent experimental evidence in support of this view derived from investigations in both epithelial and hematopoietic systems. Based on this, conceptual views and therapeutic implications will be provided on the emerging role of the bone marrow microenvironment in leukemogenesis.

Osteoblasts on rod shaped hydroxyapatite nanoparticles incorporated PCL film provide an optimal osteogenic niche for stem cell differentiation.

After the clinical insertion of a bone biomaterial, the surrounding osteoblasts would migrate and attach to the implant surface and foster a microenvironment that largely determines the differentiation fate of the comigrated mesenchymal stem cells. Whether the fostered microenvironment is suitable for osteogenic differentiation of mesenchymal stem cells is critical for the subsequent osseointegration. In this study, we determined (1) how the spherical or rod-shaped hydroxyapatite nanoparticles (nHA) incorporated poly(ɛ-caprolactone) (PCL) films (PCL-spherical nHA, PCL-rod nHA) interact with primary human osteoblasts (HOBs); (2) how the microenvironment rendered by their interaction affects osteogenic differentiation of adipose tissue-derived mesenchymal stem cells (ASCs). HOBs were seeded on PCL, PCL-spherical nHA, and PCL-rod nHA films, respectively. When cultured alone, the HOBs on PCL-rod nHA films showed most efficient osteoblastic differentiation compared with those on PCL or PCL-spherical nHA films. When cocultured with ASCs in an indirect coculture system, only the HOBs on PCL-rod nHA films up-regulated the gene expression of Runx2, bone sialoprotein, and osteocalcin of ASCs. Additionally, the HOBs on PCL-rod nHA films showed significant up-regulation of bone morphogenic protein 2 gene and protein expression and induced highest phosphorylated Smad1/5 protein level in ASCs. Treatment of the coculture medium with bone morphogenic protein 2 inhibitor (Noggin) largely abolished the osteogenic differentiation of the ASCs induced by the HOBs on PCL-rod nHA films. In conclusion, HOBs can not only best display their osteoblastic phenotype by culturing on PCL-rod nHA films but also render an optimal osteogenic niche for the differentiation of stem cells.

A novel role for platelet secretion in angiogenesis: mediating bone marrow-derived cell mobilization and homing.

Angiogenesis alleviates hypoxic stress in ischemic tissues or during tumor progression. In addition to endothelial cell proliferation and migration, the angiogenic process requires bone marrow-derived cell (BMDC) recruitment to sites of neovascularization. However, the mechanism of communication between hypoxic tissues and the BM remains unknown. Using 2 models of hypoxia-induced angiogenesis (ischemic hindlimb surgery and subcutaneous tumor growth), we show that platelet infusion promotes BMDC mobilization into the circulation, BMDC recruitment into growing neovasculature, tumor vascularization, and blood flow restoration in ischemic limbs, whereas platelet depletion inhibits these effects. Thus, platelets are required for BMDC recruitment into ischemia-induced vasculature. Secretion of platelet α-granules, but neither dense granules nor platelet aggregation is crucial for BMDC homing and subsequent angiogenesis, as determined using VAMP-8(-/-), Pearl, and integrin Beta 3(-/-) platelets. Finally, platelets sequester tumor-derived promoters of angiogenesis and BMDC mobilization, which are counterbalanced by the antiangiogenic factor thrombospondin-1. A lack of thrombospondin-1 in platelets leads to an imbalance in proangiogenic and antiangiogenic factors and accelerates tumor growth and vascularization. Our data demonstrate that platelets stimulate BMDC homing in a VAMP-8-dependent manner, revealing a previously unknown role for platelets as key mediators between hypoxic tissues and the bone marrow during angiogenesis.

The porcine limbal epithelial stem cell niche as a new model for the study of transplanted tissue-engineered human limbal epithelial cells.

Transparency of the human cornea is dependent upon the integrity of its epithelium and hence a population of limbal epithelial stem cells (LESCs). We have previously shown that LESCs reside in limbal epithelial crypts at the periphery of the human cornea. In this study the anatomy and functionality of the porcine limbus was evaluated for the first time as a novel model of the human limbus. Scanning electron microscopy, confocal microscopy, and histology revealed common structures in the porcine and human limbus in terms of the location and topography of palisades of Vogt and limbal epithelial crypts. Epithelial cells harvested from crypt regions achieved higher colony forming efficiency than cultures established from the noncrypt regions and central cornea. Also, expression of the putative SC markers p63α and integrin ß1 brightness was higher in the basal layer of the crypt regions, as shown by immunocytochemistry. De-epithelialized porcine corneas were used as an in vitro organ culture model to study the fate of transplanted human epithelium cultured from the limbus. Multilayered epithelium was observed after ∼1 week. Subsequently, wounds were inflicted on the central corneal epithelium. The wounded tissue healed within 5-7 days, and multilayering of the central corneal epithelium was re-established. The transplanted epithelia were repeatedly wounded at least four times and the wounds healed by 1 week. Putative SC marker expression of the transplanted epithelia was confirmed using immunohistochemistry. These results demonstrate that the porcine limbus shares features with the human limbus and as such provides a suitable model for the study of cultured limbal epithelial cell transplantation. These data have significant clinical value as this model can provide information on LESC fate post-transplantation and their ability to respond to injury, which is not possible to study in patients.

Expression regulation and function of heparan sulfate 6-O-endosulfatases in the spermatogonial stem cell niche.

Glial cell line-derived neurotrophic factor (GDNF) is a heparan sulfate (HS)-binding factor. GDNF is produced by somatic Sertoli cells, where it signals to maintain spermatogonial stem cells (SSCs) and reproduction. Here, we investigate the roles of extracellular HS 6-O-endosulfatases (Sulfs), Sulf1 and Sulf2, in the matrix transmission of GDNF from Sertoli cells to SSCs. Although Sulfs are not required for testis formation, Sulf deficiency leads to the accelerated depletion of SSCs, a testis phenotype similar to that of GDNF+/- mice. Mechanistically, we show that Sulfs are expressed in GDNF-producing Sertoli cells. In addition, reduced Sulf activity profoundly worsens haplo-deficient GDNF phenotypes in our genetic studies. These findings establish a critical role of Sulfs in promoting GDNF signaling and support a model in which Sulfs regulate the bioavailability of GDNF by enzymatically remodeling HS 6-O-desulfation to release GDNF from matrix sequestration. Further, Sertoli cell-specific transcriptional factor Wilm's tumor 1 (WT1) directly activates the transcription of both Sulf1 and Sulf2 genes. Together, our studies not only identify Sulfs as essential regulators of GDNF signaling in the SSC niche, but also as direct downstream targets of WT1, thus establishing a physiological role of WT1 in Sertoli cells.

Fetal stromal niches enhance human embryonic stem cell-derived hematopoietic differentiation and globin switch.

Hematopoiesis during mammalian embryonic development has been perceived as a migratory phenomenon, from the yolk sac blood island to the aorta-gonad-mesonephros (AGM) region, fetal liver (FL), and subsequently, the fetal bone marrow. In this study, we investigated the effects of primary stromal cells from fetal hematopoietic niches and their conditioned media (CM), applied singly or in sequential orders, on induction of human embryonic stem cells, H1, H9, and H14 lines, to hematopoietic cells. Our results demonstrated that stromal support of FL, AGM + FL, and AGM + FL + fetal bone marrow significantly increased the proliferation of embryoid bodies (EB) at day 18 of hematopoietic induction in the presence of thrombopoietin, stem cell factor, and Flt-3 ligand. AGM + FL also increased hematopoietic colony-forming unit (CFU) formation. CM did not enhance EB proliferation but CM of FL and AGM + FL significantly increased the density of total CFU and early erythroid (burst-forming unit) progenitors. Increased commitment to the hematopoietic lineage was demonstrated by enhanced expressions of CD45, alpha-, beta-, and gamma-globins in CFU at day 32, compared with EB at day 18. CM of FL significantly increased these globin expressions, indicating enhanced switches from embryonic to fetal and adult erythropoiesis. Over 50% and 10% of cells derived from CFU expressed CD45 and beta-globin proteins, respectively. Expressions of hematopoietic regulatory genes (Bmi-1, ß-Catenin, Hox B4, GATA-1) were increased in EB or CFU cultures supported by FL or sequential CM. Our study has provided a strategy for derivation of hematopoietic cells from embryonic stem cells under the influence of primary hematopoietic niches and CM, particularly the FL.

Angiopoietin-like protein 3 supports the activity of hematopoietic stem cells in the bone marrow niche.

The physiologic roles of angiopoietin-like proteins (Angptls) in the hematopoietic system remain unknown. Here we show that hematopoietic stem cells (HSCs) in Angptl3-null mice are decreased in number and quiescence. HSCs transplanted into Angptl3-null recipient mice exhibited impaired repopulation. Bone marrow sinusoidal endothelial cells express high levels of Angptl3 and are adjacent to HSCs. Importantly, bone marrow stromal cells or endothelium deficient in Angptl3 have a significantly decreased ability to support the expansion of repopulating HSCs. Angptl3 represses the expression of the transcription factor Ikaros, whose unregulated overexpression diminishes the repopulation activity of HSCs. Angptl3, as an extrinsic factor, thus supports the stemness of HSCs in the bone marrow niche.

Fes tyrosine kinase expression in the tumor niche correlates with enhanced tumor growth, angiogenesis, circulating tumor cells, metastasis, and infiltrating macrophages.

Fes is a protein tyrosine kinase with cell autonomous oncogenic activities that are well established in cell culture and animal models, but its involvement in human cancer has been unclear. Abundant expression of Fes in vascular endothelial cells and myeloid cell lineages prompted us to explore roles for Fes in the tumor microenvironment. In an orthotopic mouse model of breast cancer, we found that loss of Fes in the host correlated with reductions in engrafted tumor growth rates, metastasis, and circulating tumor cells. The tumor microenvironment in Fes-deficient mice also showed reduced vascularity and fewer macrophages. In co-culture with tumor cells, Fes-deficient macrophages also poorly promoted tumor cell invasive behavior. Taken together, our observations argue that Fes inhibition might provide therapeutic benefits in breast cancer, in part by attenuating tumor-associated angiogenesis and the metastasis-promoting functions of tumor-associated macrophages.

A new isolation method of human limbal progenitor cells by maintaining close association with their niche cells.

In human corneal epithelium, self-renewal and fate decision of stem cells are highly regulated in a niche microenvironment called palisades of Vogt in the limbus. Herein, we discovered that digestion with dispase, which cleaves off the basement membrane, did not remove the entire basal epithelial progenitor cells. In contrast, digestion with collagenase isolated on cluster consisting of not only entire epithelial progenitor cells but also their closely associated mesenchymal cells because of better preservation of some basement membrane matrix. Collagenase isolated more basal epithelial progenitor cells, which were p63α+ and small in the size (8 µm in diameter), and generated significantly more holoclones and meroclones on 3T3 fibroblast feeder layers than dispase. Further, collagenase isolated more small pan-cytokeratin-/p63α-/vimentin+ cells with the size as small as 5 µm in diameter and heterogeneously expressing vimentin, Oct4, Sox2, Nanog, Rex1, Nestin, N-cadherin, SSEA4, and CD34. Maintenance of close association between them led to clonal growth in a serum-free, low-calcium medium, whereas disruption of such association by trypsin/EDTA resulted in no clonal growth unless cocultured with 3T3 fibroblast feeder layers. Similarly, on epithelially denuded amniotic membrane, maintenance of such association led to consistent and robust epithelial outgrowth, which was also abolished by trypsin/EDTA. Epithelial outgrowth generated by collagenase-isolated clusters was significantly larger in diameter and its single cells yielded more holoclones on 3T3 fibroblast feeder layers than that from dispase-isolated sheets. This new isolation method can be used for exploring how limbal epithelial stem cells are regulated by their native niche cells.

Annexin-2 is a regulator of stromal cell-derived factor-1/CXCL12 function in the hematopoietic stem cell endosteal niche.

OBJECTIVE: Previously, we reported that annexin-2 (anxa2) plays an important role in hematopoietic stem cell (HSC) localization to the endosteal/osteoblastic marrow niche. This study explored the role that annexin-2 plays in presenting stromal cell-derived factor-1 (or CXCL12) to HSCs. MATERIALS AND METHODS: Competitive long-term bone marrow transplant assays were used to determine if HSC engraftment is altered in annexin-2-deficient animals. Colony-forming cell assays, CXCL12 enzyme-linked immunosorbent assay, and real-time reverse transcription polymerase chain reaction analyses were used to determine stem or progenitor cell mobilization by granulocyte colony-stimulating factor. Immunohistochemistry, immunoprecipitation, binding assays, and chemotactic assays were employed to determine if annexin-2 is associated with CXCL12. Degradation assays were also used to determine if annexin-2 and CXCL12 protect each other from proteolytic degradation. RESULTS: Anxa2(-/-) animals had fewer HSCs in their marrow, and the HSCs in anxa2(-/-) animals express less CXCR4 and CXCR7, suggesting a cell intrinsic defect. Transplantation studies of wild-type marrow into anxa2(-/-) animals demonstrated a cell-extrinsic defect in the anxa2(-/-) animals. CXCL12 binds directly to annexin-2, and this interaction facilitates presentation of CXCL12 to HSCs. Yet the binding of CXCL12 to annexin-2 did not protect CXCL12 from proteolytic cleavage after stem or progenitor cell mobilization by granulocyte colony-stimulating factor. CONCLUSIONS: These results suggest that annexin-2 serves as an anchor for CXCL12 to help in the localization of HSCs to the niche.

Dynamic expression of the p53 family members p63 and p73 in the mouse and human telencephalon during development and in adulthood.

p63 and p73, family members of the tumor suppressor p53, are critically involved in the life and death of mammalian cells. They display high homology and may act in concert. The p73 gene is relevant for brain development, and p73-deficient mice display important malformations of the telencephalon. In turn, p63 is essential for the development of stratified epithelia and may also play a part in neuronal survival and aging. We show here that p63 and p73 are dynamically expressed in the embryonic and adult mouse and human telencephalon. During embryonic stages, Cajal-Retzius cells derived from the cortical hem co-express p73 and p63. Comparison of the brain phenotypes of p63- and p73- deficient mice shows that only the loss of p73 function leads to the loss of Cajal-Retzius cells, whereas p63 is apparently not essential for brain development and Cajal-Retzius cell formation. In postnatal mice, p53, p63, and p73 are present in cells of the subventricular zone (SVZ) of the lateral ventricle, a site of continued neurogenesis. The neurogenetic niche is reduced in size in p73-deficient mice, and the numbers of young neurons near the ventricular wall, marked with doublecortin, Tbr1 and calretinin, are dramatically decreased, suggesting that p73 is important for SVZ proliferation. In contrast to their restricted expression during brain development, p73 and p63 are widely detected in pyramidal neurons of the adult human cortex and hippocampus at protein and mRNA levels, pointing to a role of both genes in neuronal maintenance in adulthood.

The stem cell niche should be a key issue for cell therapy in regenerative medicine.

Recent advances in stem cell research have highlighted the role played by such cells and their environment (the stem cell niche) in tissue renewal and homeostasis. The control and regulation of stem cells and their niche are remaining challenges for cell therapy and regenerative medicine on several tissues and organs. These advances are important for both, the basic knowledge of stem cell regulation, and their practical translational applications into clinical medicine. This article is primarily concerned with the mesenchymal stem cells (MSCs) and it reviews the current aspects of their own niche. We discuss on the need for a deeper understanding of the identity of this cell type and its microenvironment in order to improve the effectiveness of any cell therapy for regenerative medicine. Ex vivo reproduction of the conditions of the natural stem cell niche, when necessary, would provide success to tissue engineering. The first challenge of regenerative medicine is to find cells able to replace and/or repair the lost function of tissues and organs by disease or aging and the trophic and immunomodulatory effects recently found for MSCs open up for new opportunities. If MSCs are pericytes, as it has been proposed, perhaps it may explain the ubiquity of these cells and their possible role in miscellaneous repairs throughout the body opening for new chances for extensive tissue repair.

Wnt signaling regulates Snai1 expression and cellular localization in the mouse intestinal epithelial stem cell niche.

Snail genes are transcriptional repressors well known to play important roles in epithelial to mesenchymal transitions during both embryogenesis and cancer metastasis. Although they are generally regarded as markers of mesenchymal cells, Snail genes have also recently been implicated in regulating stem cell populations in both Drosophila and vertebrates. In this study we investigate Snai1, a member of the mouse Snail family, in the intestinal stem cell niche and examine the relationship between canonical Wnt signaling, a key regulatory pathway of intestinal stem cells, and expression and cellular localization of Snai1. Strong nuclear expression of Snai1 was detected in the crypt base columnar stem cells in the adult small intestine while Snai1 was mostly found in the cytoplasm of differentiated enterocytes and enteroendocrine cells. Expression and cellular localization of Snai1 in the intestinal epithelium appears to be regulated by the canonical Wnt signaling pathway as Snai1 expression was dramatically reduced after conditional deletion of ß-catenin. Conversely, significant nuclear Snai1 was detected in polyps derived from Apc(min) mice and in intestinal villi after conditional mutation of Apc in AhCre, Apc(f/f) mice, indicating that upregulation of the Wnt pathway in the intestinal epithelium induces both increased expression and nuclear localization of Snai1.

Biology of the adult hepatic progenitor cell: "ghosts in the machine".

This chapter reviews some of the basic biological principles governing adult progenitor cells of the liver and the mechanisms by which they operate. If scientists were better able to understand the conditions that govern stem cell mechanics in the liver, it may be possible to apply that understanding in a clinical setting for use in the treatment or cure of human pathologies. This chapter gives a basic introduction to hepatic progenitor cell biology and explores what is known about progenitor cell-mediated liver regeneration. We also discuss the putative stem cell niche in the liver, as well as the signaling pathways involved in stem cell regulation. Finally, the isolation and clinical application of stem cells to human diseases is reviewed, along with the current thoughts on the relationship between stem cells and cancer.

The intestinal stem cell.

The intestinal epithelium is one of the most rapidly proliferating organs in the body. A complete turnover of the epithelium occurs every 3-5 days in the mouse, a process that is maintained by a small population of intestinal stem cells (ISCs) that reside in the crypt bases. The signals that regulate the behavior of these ISCs are still unknown. This has been due, until recently, to the singular lack of definitive ISC markers. The recent identification of genes that mark functional stem cells has yielded insights into how ISCs are regulated and maintained by their surrounding niche. Herein, we examine the body of literature regarding the precise identity and location of the ISCs, the role of the surrounding niche in ISC maintenance and regulation, as well as the hypothesis that the ISCs are the cells of origin in colorectal cancer.