Multipotent/pluripotent stem cell populations in stromal tissues and peripheral blood: exploring diversity, potential, and therapeutic applications.

The concept of "stemness" incorporates the molecular mechanisms that regulate the unlimited self-regenerative potential typical of undifferentiated primitive cells. These cells possess the unique ability to navigate the cell cycle, transitioning in and out of the quiescent G0 phase, and hold the capacity to generate diverse cell phenotypes. Stem cells, as undifferentiated precursors endow with extraordinary regenerative capabilities, exhibit a heterogeneous and tissue-specific distribution throughout the human body. The identification and characterization of distinct stem cell populations across various tissues have revolutionized our understanding of tissue homeostasis and regeneration. From the hematopoietic to the nervous and musculoskeletal systems, the presence of tissue-specific stem cells underlines the complex adaptability of multicellular organisms. Recent investigations have revealed a diverse cohort of non-hematopoietic stem cells (non-HSC), primarily within bone marrow and other stromal tissue, alongside established hematopoietic stem cells (HSC). Among these non-HSC, a rare subset exhibits pluripotent characteristics. In vitro and in vivo studies have demonstrated the remarkable differentiation potential of these putative stem cells, known by various names including multipotent adult progenitor cells (MAPC), marrow-isolated adult multilineage inducible cells (MIAMI), small blood stem cells (SBSC), very small embryonic-like stem cells (VSELs), and multilineage differentiating stress enduring cells (MUSE). The diverse nomenclatures assigned to these primitive stem cell populations may arise from different origins or varied experimental methodologies. This review aims to present a comprehensive comparison of various subpopulations of multipotent/pluripotent stem cells derived from stromal tissues. By analysing isolation techniques and surface marker expression associated with these populations, we aim to delineate the similarities and distinctions among stromal tissue-derived stem cells. Understanding the nuances of these tissue-specific stem cells is critical for unlocking their therapeutic potential and advancing regenerative medicine. The future of stem cells research should prioritize the standardization of methodologies and collaborative investigations in shared laboratory environments. This approach could mitigate variability in research outcomes and foster scientific partnerships to fully exploit the therapeutic potential of pluripotent stem cells.

Role of Hedgehog Signaling Pathways in Multipotent Mesenchymal Stem Cells Differentiation.

Multipotent mesenchymal stem cells (MSCs) have high self-renewal and multi-lineage differentiation potentials and low immunogenicity, so they have attracted much attention in the field of regenerative medicine and have a promising clinical application. MSCs originate from the mesoderm and can differentiate not only into osteoblasts, cartilage, adipocytes, and muscle cells but also into ectodermal and endodermal cell lineages across embryonic layers. To design cell therapy for replacement of damaged tissues, it is essential to understand the signaling pathways, which have a major impact on MSC differentiation, as this will help to integrate the signaling inputs to initiate a specific lineage. Hedgehog (Hh) signaling plays a vital role in the development of various tissues and organs in the embryo. As a morphogen, Hh not only regulates the survival and proliferation of tissue progenitor and stem populations but also is a critical moderator of MSC differentiation, involving tri-lineage and across embryonic layer differentiation of MSCs. This review summarizes the role of Hh signaling pathway in the differentiation of MSCs to mesodermal, endodermal, and ectodermal cells.

Enhanced osteogenic differentiation potential of Arnica montana and Bellis perennis in C3H10T1/2 multipotent mesenchymal stem cells.

BACKGROUND: Arnica montana and Bellis perennis are two medicinal plants that are thought to accelerate bone repair in homoeopathic literature. Mesenchymal stem cells (MSCs) are multipotent stem cells with the ability to differentiate and regenerate bone or osteogenesis. Hence, we aimed to determine the role of Arnica montana and Bellis perennis on the osteogenic differentiation of the C3H10T1/2 stem cell line. METHODS AND RESULTS: The cell proliferation of Arnica montana and Bellis perennis was evaluated by MTT assay. Osteogenic differentiation of C3H10T1/2 was induced by the addition of ß-glycerophosphate, ascorbic acid and dexamethasone in the differentiation medium over 3 weeks. Cells were treated with Arnica montana and Bellis perennis individually as well as in combination. The osteogenic differentiation potential of Arnica montana and Bellis perennis to differentiate C3H10T1/2 into osteoblasts was measured by alkaline phosphatase activity, alizarin red staining and the expression of Osteocalcin using immunostaining and qRT-PCR. Arnica montana and Bellis perennis could enhance C3H10T1/2 cell proliferation at 1600 µg. Further, the compound showed the ability to augment osteogenesis as confirmed by increased expression of alkaline phosphatase and enhanced calcium accumulation as seen by the Alizarin Red staining and quantification. Enhanced osteogenesis was further supported by the increased expression of osteocalcin in the treated cells with individual and combined doses of Arnica montana and Bellis perennis. Therefore, the findings provide additional support for the positive impact of Arnica montana and Bellis perennis on bone formation. CONCLUSIONS: Our findings suggest that homoeopathic compounds Arnica montana and Bellis perennis can augment osteogenesis individually as well as in combination.

Resilient anatomy and local plasticity of naive and stress haematopoiesis.

The bone marrow adjusts blood cell production to meet physiological demands in response to insults. The spatial organization of normal and stress responses are unknown owing to the lack of methods to visualize most steps of blood production. Here we develop strategies to image multipotent haematopoiesis, erythropoiesis and lymphopoiesis in mice. We combine these with imaging of myelopoiesis1 to define the anatomy of normal and stress haematopoiesis. In the steady state, across the skeleton, single stem cells and multipotent progenitors distribute through the marrow enriched near megakaryocytes. Lineage-committed progenitors are recruited to blood vessels, where they contribute to lineage-specific microanatomical structures composed of progenitors and immature cells, which function as the production sites for each major blood lineage. This overall anatomy is resilient to insults, as it was maintained after haemorrhage, systemic bacterial infection and granulocyte colony-stimulating factor (G-CSF) treatment, and during ageing. Production sites enable haematopoietic plasticity as they differentially and selectively modulate their numbers and output in response to insults. We found that stress responses are variable across the skeleton: the tibia and the sternum respond in opposite ways to G-CSF, and the skull does not increase erythropoiesis after haemorrhage. Our studies enable in situ analyses of haematopoiesis, define the anatomy of normal and stress responses, identify discrete microanatomical production sites that confer plasticity to haematopoiesis, and uncover unprecedented heterogeneity of stress responses across the skeleton.

Clonal dynamics of haematopoiesis across the human lifespan.

Age-related change in human haematopoiesis causes reduced regenerative capacity1, cytopenias2, immune dysfunction3 and increased risk of blood cancer4-6, but the reason for such abrupt functional decline after 70 years of age remains unclear. Here we sequenced 3,579 genomes from single cell-derived colonies of haematopoietic cells across 10 human subjects from 0 to 81 years of age. Haematopoietic stem cells or multipotent progenitors (HSC/MPPs) accumulated a mean of 17 mutations per year after birth and lost 30 base pairs per year of telomere length. Haematopoiesis in adults less than 65 years of age was massively polyclonal, with high clonal diversity and a stable population of 20,000-200,000 HSC/MPPs contributing evenly to blood production. By contrast, haematopoiesis in individuals aged over 75 showed profoundly decreased clonal diversity. In each of the older subjects, 30-60% of haematopoiesis was accounted for by 12-18 independent clones, each contributing 1-34% of blood production. Most clones had begun their expansion before the subject was 40 years old, but only 22% had known driver mutations. Genome-wide selection analysis estimated that between 1 in 34 and 1 in 12 non-synonymous mutations were drivers, accruing at constant rates throughout life, affecting more genes than identified in blood cancers. Loss of the Y chromosome conferred selective benefits in males. Simulations of haematopoiesis, with constant stem cell population size and constant acquisition of driver mutations conferring moderate fitness benefits, entirely explained the abrupt change in clonal structure in the elderly. Rapidly decreasing clonal diversity is a universal feature of haematopoiesis in aged humans, underpinned by pervasive positive selection acting on many more genes than currently identified.

Lifelong multilineage contribution by embryonic-born blood progenitors.

Haematopoietic stem cells (HSCs) arise in the embryo from the arterial endothelium through a process known as the endothelial-to-haematopoietic transition (EHT)1-4. This process generates hundreds of blood progenitors, of which a fraction go on to become definitive HSCs. It is generally thought that most adult blood is derived from those HSCs, but to what extent other progenitors contribute to adult haematopoiesis is not known. Here we use in situ barcoding and classical fate mapping to assess the developmental and clonal origins of adult blood in mice. Our analysis uncovers an early wave of progenitor specification-independent of traditional HSCs-that begins soon after EHT. These embryonic multipotent progenitors (eMPPs) predominantly drive haematopoiesis in the young adult, have a decreasing yet lifelong contribution over time and are the predominant source of lymphoid output. Putative eMPPs are specified within intra-arterial haematopoietic clusters and represent one fate of the earliest haematopoietic progenitors. Altogether, our results reveal functional heterogeneity during the definitive wave that leads to distinct sources of adult blood.

Human distal airways contain a multipotent secretory cell that can regenerate alveoli.

The human lung differs substantially from its mouse counterpart, resulting in a distinct distal airway architecture affected by disease pathology in chronic obstructive pulmonary disease. In humans, the distal branches of the airway interweave with the alveolar gas-exchange niche, forming an anatomical structure known as the respiratory bronchioles. Owing to the lack of a counterpart in mouse, the cellular and molecular mechanisms that govern respiratory bronchioles in the human lung remain uncharacterized. Here we show that human respiratory bronchioles contain a unique secretory cell population that is distinct from cells in larger proximal airways. Organoid modelling reveals that these respiratory airway secretory (RAS) cells act as unidirectional progenitors for alveolar type 2 cells, which are essential for maintaining and regenerating the alveolar niche. RAS cell lineage differentiation into alveolar type 2 cells is regulated by Notch and Wnt signalling. In chronic obstructive pulmonary disease, RAS cells are altered transcriptionally, corresponding to abnormal alveolar type 2 cell states, which are associated with smoking exposure in both humans and ferrets. These data identify a distinct progenitor in a region of the human lung that is not found in mouse that has a critical role in maintaining the gas-exchange compartment and is altered in chronic lung disease.

A single factor elicits multilineage reprogramming of astrocytes in the adult mouse striatum.

SignificanceOutside the neurogenic niches, the adult brain lacks multipotent progenitor cells. In this study, we performed a series of in vivo screens and reveal that a single factor can induce resident brain astrocytes to become induced neural progenitor cells (iNPCs), which then generate neurons, astrocytes, and oligodendrocytes. Such a conclusion is supported by single-cell RNA sequencing and multiple lineage-tracing experiments. Our discovery of iNPCs is fundamentally important for regenerative medicine since neural injuries or degeneration often lead to loss/dysfunction of all three neural lineages. Our findings also provide insights into cell plasticity in the adult mammalian brain, which has largely lost the regenerative capacity.

Downregulated Calcium-Binding Protein S100A16 and HSP27 in Placenta-Derived Multipotent Cells Induce Functional Astrocyte Differentiation.

Little is known about genes that induce stem cells differentiation into astrocytes. We previously described that heat shock protein 27 (HSP27) downregulation is directly related to neural differentiation under chemical induction in placenta-derived multipotent stem cells (PDMCs). Using this neural differentiation cell model, we cross-compared transcriptomic and proteomic data and selected 26 candidate genes with the same expression trends in both omics analyses. Those genes were further compared with a transcriptomic database derived from Alzheimer's disease (AD). Eighteen out of 26 candidates showed opposite expression trends between our data and the AD database. The mRNA and protein expression levels of those candidates showed downregulation of HSP27, S100 calcium-binding protein A16 (S100A16) and two other genes in our neural differentiation cell model. Silencing these four genes with various combinations showed that co-silencing HSP27 and S100A16 has stronger effects than other combinations for astrocyte differentiation. The induced astrocyte showed typical astrocytic star-shape and developed with ramified, stringy and filamentous processes as well as differentiated endfoot structures. Also, some of them connected with each other and formed continuous network. Immunofluorescence quantification of various neural markers indicated that HSP27 and S100A16 downregulation mainly drive PDMCs differentiation into astrocytes. Immunofluorescence and confocal microscopic images showed the classical star-like shape morphology and co-expression of crucial astrocyte markers in induced astrocytes, while electrophysiology and Ca2+ influx examination further confirmed their functional characteristics. In conclusion, co-silencing of S100A16 and HSP27 without chemical induction leads to PDMCs differentiation into functional astrocytes.

Data-driven modeling predicts gene regulatory network dynamics during the differentiation of multipotential hematopoietic progenitors.

Cellular differentiation during hematopoiesis is guided by gene regulatory networks (GRNs) comprising transcription factors (TFs) and the effectors of cytokine signaling. Based largely on analyses conducted at steady state, these GRNs are thought to be organized as a hierarchy of bistable switches, with antagonism between Gata1 and PU.1 driving red- and white-blood cell differentiation. Here, we utilize transient gene expression patterns to infer the genetic architecture-the type and strength of regulatory interconnections-and dynamics of a twelve-gene GRN including key TFs and cytokine receptors. We trained gene circuits, dynamical models that learn genetic architecture, on high temporal-resolution gene-expression data from the differentiation of an inducible cell line into erythrocytes and neutrophils. The model is able to predict the consequences of gene knockout, knockdown, and overexpression experiments and the inferred interconnections are largely consistent with prior empirical evidence. The inferred genetic architecture is densely interconnected rather than hierarchical, featuring extensive cross-antagonism between genes from alternative lineages and positive feedback from cytokine receptors. The analysis of the dynamics of gene regulation in the model reveals that PU.1 is one of the last genes to be upregulated in neutrophil conditions and that the upregulation of PU.1 and other neutrophil genes is driven by Cebpa and Gfi1 instead. This model inference is confirmed in an independent single-cell RNA-Seq dataset from mouse bone marrow in which Cebpa and Gfi1 expression precedes the neutrophil-specific upregulation of PU.1 during differentiation. These results demonstrate that full PU.1 upregulation during neutrophil development involves regulatory influences extrinsic to the Gata1-PU.1 bistable switch. Furthermore, although there is extensive cross-antagonism between erythroid and neutrophil genes, it does not have a hierarchical structure. More generally, we show that the combination of high-resolution time series data and data-driven dynamical modeling can uncover the dynamics and causality of developmental events that might otherwise be obscured.

Simultaneous Administration of NOD-2 (MDP) and TLP-4 (LPS) Ligands to Bone Marrow Donors 24 h before Transplantation Increases the Content of Multipotent Stromal Cells (MSCs) in Bone Marrow Grafts in CBA Mice Compared to the Total Result of Their Isolated Administration.

In 3-month bone marrow transplants of CBA mice from bone marrow donors receiving single injections of TLR-4 ligand (LPS) or NOD-2 ligand (muramyl dipeptide, MDP) 24 h before transplantation, an increase in the total number of MSCs (by 2.6 and 1.9 times, respectively), as well as a slight increase in the number of nuclear cells and the mass of bone capsules (by 1.3 and 1.2 times) were observed. After combined administration of MDР and LPS to donors, the total content of MSCs in the grafts was higher by 1.6 times in comparison with the total result of their isolated administration (and by 7.2 times in comparison with the control). At the same time, the concentration of osteogenic MSCs in the grafts of all groups was almost the same and corresponded to the control level. The number of nuclear cells and the mass of bone capsules of the grafts after combined administration of LPS and MDP were close (~80%) to the sum of the results of their isolated administration. These findings suggest that activation of the stromal tissue and the success of bone marrow transplantation depend on the intensity of innate immune responses. These data can be useful for the development of optimal methods of tissue transplantation.

It takes all kinds: heterogeneity among satellite cells and fibro-adipogenic progenitors during skeletal muscle regeneration.

Vertebrate skeletal muscle is composed of multinucleate myofibers that are surrounded by muscle connective tissue. Following injury, muscle is able to robustly regenerate because of tissue-resident muscle stem cells, called satellite cells. In addition, efficient and complete regeneration depends on other cells resident in muscle - including fibro-adipogenic progenitors (FAPs). Increasing evidence from single-cell analyses and genetic and transplantation experiments suggests that satellite cells and FAPs are heterogeneous cell populations. Here, we review our current understanding of the heterogeneity of satellite cells, their myogenic derivatives and FAPs in terms of gene expression, anatomical location, age and timing during the regenerative process - each of which have potentially important functional consequences.

Isolation of multipotent progenitor cells from pleura and pericardium for tracheal tissue engineering purposes.

Tissue engineering (TE) of long tracheal segments is conceptually appealing for patients with inoperable tracheal pathology. In tracheal TE, stem cells isolated from bone marrow or adipose tissue have been employed, but the ideal cell source has yet to be determined. When considering the origin of stem cells, cells isolated from a source embryonically related to the trachea may be more similar. In this study, we investigated the feasibility of isolating progenitor cells from pleura and pericard as an alternative cells source for tracheal tissue engineering. Porcine progenitor cells were isolated from pleura, pericard, trachea and adipose tissue and expanded in culture. Isolated cells were characterized by PCR, RNA sequencing, differentiation assays and cell survival assays and were compared to trachea and adipose-derived progenitor cells. Progenitor-like cells were successfully isolated and expanded from pericard and pleura as indicated by gene expression and functional analyses. Gene expression analysis and RNA sequencing showed a stem cell signature indicating multipotency, albeit that subtle differences between different cell sources were visible. Functional analysis revealed that these cells were able to differentiate towards chondrogenic, osteogenic and adipogenic lineages. Isolation of progenitor cells from pericard and pleura with stem cell features is feasible. Although functional differences with adipose-derived stem cells were limited, based on their gene expression, pericard- and pleura-derived stem cells may represent a superior autologous cell source for cell seeding in tracheal tissue engineering.

Simplified murine multipotent progenitor isolation scheme: Establishing a consensus approach for multipotent progenitor identification.

The mouse hematopoietic system has served as a paradigm for analysis of developmental fate decisions in tissue homeostasis and regeneration. However, multiple immunophenotypic definitions of, and sometimes divergent nomenclatures used to classify, murine multipotent progenitors (MPPs) have emerged in the field over time. This has created significant confusion and inconsistency in the hematology field. To facilitate easier comparison of murine MPP phenotypes between research laboratories, a working group of four International Society for Experimental Hematology (ISEH) members with extensive experience studying the functional activities associated with different MPP phenotypic definitions reviewed the current state of the field with the goal of developing a position statement toward a simplified and unified immunophenotypic definition of MPP populations. In November of 2020, this position statement was presented as a webinar to the ISEH community for discussion and feedback. Hence, the Simplified MPP Identification Scheme presented here is the result of curation of existing literature, consultation with leaders in the field, and crowdsourcing from the wider experimental hematology community. Adoption of a unified definition and nomenclature, while still leaving room for individual investigator customization, will benefit scientists at all levels trying to compare these populations between experimental settings.

Multipotent progenitors and hematopoietic stem cells arise independently from hemogenic endothelium in the mouse embryo.

During embryogenesis, waves of hematopoietic progenitors develop from hemogenic endothelium (HE) prior to the emergence of self-renewing hematopoietic stem cells (HSCs). Although previous studies have shown that yolk-sac-derived erythromyeloid progenitors and HSCs emerge from distinct populations of HE, it remains unknown whether the earliest lymphoid-competent progenitors, multipotent progenitors, and HSCs originate from common HE. In this study, we demonstrate by clonal assays and single-cell transcriptomics that rare HE with functional HSC potential in the early murine embryo are distinct from more abundant HE with multilineage hematopoietic potential that fail to generate HSCs. Specifically, HSC-competent HE are characterized by expression of CXCR4 surface marker and by higher expression of genes tied to arterial programs regulating HSC dormancy and self-renewal. Taken together, these findings suggest a revised model of developmental hematopoiesis in which the initial populations of multipotent progenitors and HSCs arise independently from HE with distinct phenotypic and transcriptional properties.

Direct differentiation of tonsillar biopsy-derived stem cells to the neuronal lineage.

BACKGROUND: Neurological disorders are considered one of the greatest burdens to global public health and a leading cause of death. Stem cell therapies hold great promise for the cure of neurological disorders, as stem cells can serve as cell replacement, while also secreting factors to enhance endogenous tissue regeneration. Adult human multipotent stem cells (MSCs) reside on blood vessels, and therefore can be found in many tissues throughout the body, including palatine tonsils. Several studies have reported the capacity of MSCs to differentiate into, among other cell types, the neuronal lineage. However, unlike the case with embryonic stem cells, it is unclear whether MSCs can develop into mature neurons. METHODS: Human tonsillar MSCs (T-MSCs) were isolated from a small, 0.6-g sample, of tonsillar biopsies with high viability and yield as we recently reported. Then, these cells were differentiated by a rapid, multi-stage procedure, into committed, post-mitotic, neuron-like cells using defined conditions. RESULTS: Here we describe for the first time the derivation and differentiation of tonsillar biopsy-derived MSCs (T-MSCs), by a rapid, multi-step protocol, into post-mitotic, neuron-like cells using defined conditions without genetic manipulation. We characterized our T-MSC-derived neuronal cells and demonstrate their robust differentiation in vitro. CONCLUSIONS: Our procedure leads to a rapid neuronal lineage commitment and loss of stemness markers, as early as three days following neurogenic differentiation. Our studies identify biopsy-derived T-MSCs as a potential source for generating neuron-like cells which may have potential use for in vitro modeling of neurodegenerative diseases or cell replacement therapies.

Cell Therapy for Anal Sphincter Incontinence: Where Do We Stand?

Anal sphincter incontinence is a chronic disease, which dramatically impairs quality of life and induces high costs for the society. Surgery, considered as the best curative option, shows a disappointing success rate. Stem/progenitor cell therapy is pledging, for anal sphincter incontinence, a substitute to surgery with higher efficacy. However, the published literature is disparate. Our aim was to perform a review on the development of cell therapy for anal sphincter incontinence with critical analyses of its pitfalls. Animal models for anal sphincter incontinence were varied and tried to reproduce distinct clinical situations (acute injury or healed injury with or without surgical reconstruction) but were limited by anatomical considerations. Cell preparations used for treatment, originated, in order of frequency, from skeletal muscle, bone marrow or fat tissue. The characterization of these preparations was often incomplete and stemness not always addressed. Despite a lack of understanding of sphincter healing processes and the exact mechanism of action of cell preparations, this treatment was evaluated in 83 incontinent patients, reporting encouraging results. However, further development is necessary to establish the correct indications, to determine the most-suited cell type, to standardize the cell preparation method and to validate the route and number of cell delivery.

Erythropoietin (EPO) as a Key Regulator of Erythropoiesis, Bone Remodeling and Endothelial Transdifferentiation of Multipotent Mesenchymal Stem Cells (MSCs): Implications in Regenerative Medicine.

Human erythropoietin (EPO) is an N-linked glycoprotein consisting of 166 aa that is produced in the kidney during the adult life and acts both as a peptide hormone and hematopoietic growth factor (HGF), stimulating bone marrow erythropoiesis. EPO production is activated by hypoxia and is regulated via an oxygen-sensitive feedback loop. EPO acts via its homodimeric erythropoietin receptor (EPO-R) that increases cell survival and drives the terminal erythroid maturation of progenitors BFU-Es and CFU-Es to billions of mature RBCs. This pathway involves the activation of multiple erythroid transcription factors, such as GATA1, FOG1, TAL-1, EKLF and BCL11A, and leads to the overexpression of genes encoding enzymes involved in heme biosynthesis and the production of hemoglobin. The detection of a heterodimeric complex of EPO-R (consisting of one EPO-R chain and the CSF2RB ß-chain, CD131) in several tissues (brain, heart, skeletal muscle) explains the EPO pleotropic action as a protection factor for several cells, including the multipotent MSCs as well as cells modulating the innate and adaptive immunity arms. EPO induces the osteogenic and endothelial transdifferentiation of the multipotent MSCs via the activation of EPO-R signaling pathways, leading to bone remodeling, induction of angiogenesis and secretion of a large number of trophic factors (secretome). These diversely unique properties of EPO, taken together with its clinical use to treat anemias associated with chronic renal failure and other blood disorders, make it a valuable biologic agent in regenerative medicine for the treatment/cure of tissue de-regeneration disorders.

Isolation of human fibroadipogenic progenitors and satellite cells from frozen muscle biopsies.

Skeletal muscle contains multiple cell types that work together to maintain tissue homeostasis. Among these, satellite cells (SC) and fibroadipogenic progenitors cells (FAPs) are the two main stem cell pools. Studies of these cells using animal models have shown the importance of interactions between these cells in repair of healthy muscle, and degeneration of dystrophic muscle. Due to the unavailability of fresh patient muscle biopsies, similar analysis of interactions between human FAPs and SCs is limited especially among the muscular dystrophy patients. To address this issue here we describe a method that allows the use of frozen human skeletal muscle biopsies to simultaneously isolate and grow SCs and FAPs from healthy or dystrophic patients. We show that while the purified SCs differentiate into mature myotubes, purified FAPs can differentiate into adipocytes or fibroblasts demonstrating their multipotency. We find that these FAPs can be immortalized and the immortalized FAPs (iFAPs) retain their multipotency. These approaches open the door for carrying out personalized analysis of patient FAPs and interactions with the SCs that lead to muscle loss.