Processing Human Thymic Tissue for Single Cell RNA-Seq.

Single cell RNA sequencing of human thymic cells is dependent on isolation of highly pure and viable cell populations. This protocol describes the isolation of CD34+ progenitor and more differentiated CD34- fractions from post-natal thymic tissue to study thymopoiesis. CD34+ cells represent <1% of thymic cells, so this protocol uses magnetic- followed by fluorescence-activated cell separation to isolate highly enriched CD34+ cells. For complete details on the use and execution of this protocol, please refer to Le et al. (2020).

Effects of human umbilical cord blood CD34+ cell transplantation in neonatal hypoxic-ischemia rat model.

Perinatal brain injury can cause death in the neonatal period and lifelong neurodevelopmental deficits. Stem cell transplantation had been proved to be effective approach to ameliorate neurological deficits after brain damage. In this study we examine the effect of human umbilical cord blood CD34+ cells on model of neonatal rat hypoxic-ischemic brain damage and compared the neuroprotection of transplantation of CD34+ cells to mononuclear cells from which CD34+ cells isolated on neonatal hypoxic-ischemia rat model. Seven-day-old Sprague-Dawley rats were subjected to hypoxic-ischemic (HI) injury, CD34+ cells (1.5 × 104 cells) or mononuclear cells (1.0 × 106 cells) were transplanted into mice by tail vein on the 7 day after HI. The transplantation of CD34+ cells significantly improved motor function of rat, and reduced cerebral atrophy, inhibited the expression of glial fibrillary acidic protein (GFAP) and apoptosis-related genes: TNF-α, TNFR1, TNFR2, CD40, Fas, and decreased the activation of Nuclear factor kappa B (NF-κB) in damaged brain. CD34+ cells treatment increased the expression of DCX and lectin in ipsilateral brain. Moreover, the transplantation of CD34+ cells and MNCs which were obtained from the same amount of human umbilical cord blood had similar effects on HI. Our data demonstrated that transplantation of human umbilical cord blood CD34+ cells can ameliorate the neural functional defect and reduce apoptosis and promote nerve and vascular regeneration in rat brain after HI injury and the effects of transplantation of CD34+ cells were comparable to that of MNCs in neonatal hypoxic-ischemia rat model.

Immunohistochemical evaluation of lymphovascular invasion in carcinoma breast with CD34 and D2-40 and its correlation with other prognostic markers.

BACKGROUND: Carcinoma breast is ever-evolving and becoming increasingly prevalent in India. Numerous prognostic factors based on morphology and immunohistochemistry (IHC) have been established which need to be interconnected to give patients best possible treatment. AIMS: This study aims to confirm and analyze lymphovascular invasion (LVI) detected by hematoxylin and eosin (H and E) using IHC with CD34 and D2-40 and its correlation with other biologic and morphologic prognostic markers. SETTINGS AND DESIGN: This was a prospective study. MATERIALS AND METHODS: Fifty mastectomy specimens diagnosed as infiltrating ductal carcinoma breast on histopathology selected for the study. Evaluation of formalin-fixed paraffin-embedded sections was done using H and E and IHC for estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 HER2/neu receptors, CD34, and D2-40 endothelial markers. Correlation of LVI done with prognostic markers of Carcinoma Breast, namely, age of the patient, tumor size, Nottingham grade, lymph node ratio (LNR), Nottingham prognostic index (NPI), ER/PR status, and HER2/neu status. CD34 and D2-40 utilized to distinguish blood vessel, lymph vessel, and retraction artifacts and to calculate lymphatic microvessel density (LMVD) and blood microvessel density (BMVD). STATISTICAL ANALYSIS USED: SPSS Software Package. RESULTS: LVI was associated with younger age (P = 0.001), greater tumor size (P = 0.007), higher Nottingham grade (P = 0.001), higher LNR (P = 0.001), higher NPI (P = 0.001), Negative ER Status (P = 0.001), Negative PR Status (P = 0.002), Positive HER2/neu status (P = 0.021), Higher Intratumoral BMVD (P = 0.016), Peritumoral BMVD (P = 0.001), and Intratumoral LMVD (P = 0.009). Blood vessels more commonly invaded than lymph vessels. Retraction artifacts can be mistaken for LVI without IHC. CONCLUSIONS: D2-40 is a promising marker for lymphatic endothelium. LVI is a poor prognostic marker hence should be evaluated imperatively in all cases of carcinoma breast.

Combination of imaging flow cytometry and time-lapse microscopy for the study of label-free morphology dynamics of hematopoietic cells.

Cell differentiation is a longitudinal and dynamic process. Studying and quantifying such a process require tools combining precise time resolution and statistical power. Imaging flow cytometry (IFC) provides statistically significant number of microscopy images of individual cells in a sample at a given time point. Time-lapse microscopy (TLM) is the method of choice for studying the dynamics of cell processes at a high temporal, but low statistical resolution. In this work, we show that the dynamic changes of cord-blood derived CD34+ cells in response to cytokine stimulation can be successfully studied, in a label-free way, by the combination of the IFCs statistical power and the TLM's high time resolution. Cell morphology phenotypes were quantified through roundness and surface area, measured both in IFC and with a homemade segmentation algorithm in TLM. Two distinct morphologies-polarized and round-were observed in cord-blood derived CD34+. We show that some cells have the ability to fluctuate between these morphologies, suggesting that the apparent stable composition of round and polarized cells may actually represent a dynamic equilibrium. This example demonstrates that the different resolutions and modalities of IFC and TLM are complementary and allow the study of complex dynamic biological processes. © 2017 International Society for Advancement of Cytometry.

How do I perform hematopoietic progenitor cell selection?

Graft-versus-host disease remains the most important source of morbidity and mortality associated with allogeneic stem cell transplantation. The implementation of hematopoietic progenitor cell (HPC) selection is employed by some stem cell processing facilities to mitigate this complication. Current cell selection methods include reducing the number of unwanted T cells (negative selection) and/or enriching CD34+ hematopoietic stem/progenitors (positive selection) using immunomagnetic beads subjected to magnetic fields within columns to separate out targeted cells. Unwanted side effects of cell selection as a result of T-cell reduction are primary graft failure, increased infection rates, delayed immune reconstitution, possible disease relapse, and posttransplant lymphoproliferative disease. The Miltenyi CliniMACS cell isolation system is the only device currently approved for clinical use by the Food and Drug Administration. It uses magnetic microbeads conjugated with a high-affinity anti-CD34 monoclonal antibody capable of binding to HPCs in marrow, peripheral blood, or umbilical cord blood products. The system results in significantly improved CD34+ cell recoveries (50%-100%) and consistent 3-log CD3+ T-cell reductions compared to previous generations of CD34+ cell selection procedures. In this article, the CliniMACS procedure is described in greater detail and the authors provide useful insight into modifications of the system. Successful implementation of cell selection procedures can have a significant positive clinical effect by greatly increasing the pool of donors for recipients requiring transplants. However, before a program implements cell selection techniques, it is important to consider the time and financial resources required to properly and safely perform these procedures.

Dermatofibrosarcoma protuberans metastásico: una causa poco frecuente de masa pulmonar / No disponible

El dermatofibrosarcoma protuberans (DFSP) es una neoplasia de partes blandas de malignidad intermedia localizada inicialmente en la piel, desde donde invade tejidos más profundos. Clínicamente se caracteriza por su presentación como un nódulo o placa indurada en tronco o extremidades. El tratamiento inicial es la extirpación quirúrgica local, siendo el factor de mal pronóstico más reconocido una extirpación quirúrgica inadecuada. Hay pocos casos de metástasis descritos en la literatura siendo la localización pulmonar la más frecuente. El estudio inmunohistoquímico se caracteriza por la expresión de vimentina y CD34, y el estudio genético-molecular por la presencia de translocación de cromosomas 17 y 22. El tratamiento de elección es la extirpación quirúrgica completa, seguida de radioterapia y quimioterapia en los casos avanzados. Por tanto, aunque las metástasis son extremadamente raras, la presencia de una masa pulmonar debe incluirse en el diagnóstico diferencial de un paciente con antecedentes de un DFSP

Dermatofibrosarcoma protuberans (DFSP) is a neoplasm of soft tissue of intermediate malignancy, initially localized in the skin developing in the deep layers of skin. Clinically it is characterized by its presentation as a nodule or indurated plaque on the trunk and proximal extremities.Initial treatment is local surgical excision, and an inadequate surgical removal is themayor risk for recurrence. There are few reports of metastases described in the literature; the lung is the most common site of metastasis. Immunohistochemical study is characterized by the expression of vimentin and CD34, and by a reciprocal chromosomal translocation, t(17;22). The treatment is a complete surgical excision, followed by radiotherapy and chemotherapy in advanced cases. Therefore, although metastases are extremely rare, the presence of a lung mass should be included in the differential diagnosis of a patient with a history of DFSP

Red blood cells derived from peripheral blood and bone marrow CD34⁺ human haematopoietic stem cells are permissive to Plasmodium parasites infection.

The production of fully functional human red cells in vitro from haematopoietic stem cells (hHSCs) has been successfully achieved. Recently, the use of hHSCs from cord blood represented a major improvement to develop the continuous culture system for Plasmodium vivax. Here, we demonstrated that CD34⁺ hHSCs from peripheral blood and bone marrow can be expanded and differentiated to reticulocytes using a novel stromal cell. Moreover, these reticulocytes and mature red blood cells express surface markers for entrance of malaria parasites contain adult haemoglobin and are also permissive to invasion by P. vivax and Plasmodium falciparum parasites.

Determination of optimal replicate number for validation of imprecision using fluorescence cell-based assays: proposed practical method.

BACKGROUND: Assay validation includes determination of inherent imprecision across the reportable range. However, specific practical guidelines for determinations of precision for cell-based fluorescence assays performed on flow cytometers are currently lacking. METHODS: Replicates of 10 or 20 measurements were obtained for flow cytometric assays developed for clinical in vitro diagnostic use, including neutrophil CD64 expression for infection/sepsis detection, fetal red cell enumeration for fetomaternal hemorrhage detection, human equilibrative nucleoside transporter 1 quantitation in leukocytes for possible correlation with drug responsiveness, and CD34+ hematopoietic stem cell enumeration of apheresis products, using up to three different instrument platforms for each assay. For each assay, the mean, 95% confidence intervals (95% CIs) of the mean, standard deviation, and coefficient of variation (CV) of sequential replicates were determined. RESULTS: For all assays and most instrument platforms, <5 replicates were found adequate to validate assay imprecision levels below the 5-10% CV for repeatability claimed by the manufacturers of these assays. Results plotted as a novel parameter derived from the 95% CI and the cumulative mean for replicates, termed variance factor (VF), provide a data-driven means for determining optimal replicate numbers. CONCLUSIONS: The novel VF can provide information to guide the practical selection of optimal replicate numbers for validation of imprecision in flow cytometric assays. The optimal number of replicates was assay and instrument platform dependent. Our findings indicate that three to four replicates are sufficient for most flow cytometric assays and instrument combinations, rather than the higher numbers suggested by CLSI guidelines for soluble analytes.

Diagnostic application and clinical significance of FCM progress scoring system based on immunophenotyping in CD34+ blasts in myelodysplastic syndromes.

BACKGROUND: Abnormal proliferation and differentiation of hematopoietic stem/progenitor cells, which reflect the malignant nature of clonal cells in myelodysplastic syndromes (MDS), can be detected by flow cytometry (FCM) and potentially applied to assist diagnosis and evaluate prognosis in MDS. METHODS: In this study, a series of immunophenotypes such as CD34, CD19, CD38, CD117, and CD7, which are related to proliferation and differentiation of HSCs, were determined by FCM in the patients with nonclonal cytopenias diseases and MDS. Based on the expression pattern of these immunophenotypes, a FCM progress scoring (FPS) system was constructed and evaluated. RESULTS: The FPS system showed good sensitivity and specificity (63.6% and 100.0%) in distinguishing MDS from nonclonal cytopenias diseases. Validation analysis of FPS system indicated comparable sensitivity and specificity (73.7% and 97.1%) and high agreement rate (82.6%) of FCM diagnosis with morphological diagnosis. The high-grade MDS had higher FPS score compared to low-grade MDS (P < 0.001). Noticeably, hypocellular MDS had lower FPS score (P < 0.001), most of which could not be diagnosed by FPS system. Besides, FPS score showed obvious positive correlation with WHO classification, IPSS score, percentage of marrow blasts, and cytogenetic prognosis scoring. Elevated FPS score predicted higher disease progression and shorter survival in MDS. CONCLUSION: The FPS system based on immunophenotyping in CD34+ blasts is a useful and simple tool for diagnosis and prognosis evaluation in MDS.

MicroRNAs as markers for neurally committed CD133+/CD34+ stem cells derived from human umbilical cord blood.

Neural differentiation of the CD133+/CD34+ subpopulation of human umbilical cord blood stem cells was investigated, and neuro-miR (mir-9 and mir-124) expression was examined. An efficient induction protocol for neural differentiation of hematopoietic stem cells together with the exclusion of retinoic acid in this process was also studied. Transcription of some neural markers such as microtubule-associated protein-2, beta-tubulin III, and neuron-specific enolase was evaluated by real-time PCR, immunocytochemistry, and western blotting. Increased expression of neural indicators in the treated cells confirmed the appropriate neural differentiation, which supported the high efficiency of our defined neuronal induction protocol. Verified high expression of neuro-miRNAs along with neuronal specific proteins not only strengthens the regulatory role of miRNAs in determining stem cell fate but also introduces these miRNAs as novel indicators of neural differentiation. These data highlight the prominent therapeutic potential of hematopoietic stem cells for use in cell therapy of neurodegenerative diseases.

Sarcoma de Kaposi a tipo linfangioma / Lymphangioma-Like Kaposi Sarcoma

No disponible

Repertoire of endothelial progenitor cells mobilized by femoral artery ligation: a nonhuman primate study.

To determine in the baboon model the identities and functional characteristics of endothelial progenitor cells (EPCs) mobilized in response to artery ligation, we collected peripheral blood mononuclear cells (PBMNCs) before and 3 days after a segment of femoral artery was removed. Our goal was to find EPC subpopulations with highly regenerative capacity. We identified 12 subpopulations of putative EPCs that were altered >1.75-fold; two subpopulations (CD146+/CD54-/CD45- at 6.63-fold, and CD146+/UEA-1-/CD45- at 12.21-fold) were dramatically elevated. To investigate the regenerative capacity of putative EPCs, we devised a new assay that maximally resembled their in vivo scenario, we purified CD34+ and CD146+ cells and co-cultured them with basal and mobilized PBMNCs; both cell types took up Dil-LDL, but purified CD146+ cells exhibited accelerated differentiation by increasing expression of CD31 and CD144, and by exhibiting more active cord-like structure formation by comparison to the CD34+ subpopulation in a co-culture with mobilized PBMNCs. We demonstrate that ischaemia due to vascular ligation mobilizes multiple types of cells with distinct roles. Baboon CD146+ cells exhibit higher reparative capacity than CD34+ cells, and thus are a potential source for therapeutic application.

ISHAGE-based single-platform flowcytometric analysis for measurement of absolute viable T cells in fresh or cryopreserved products: CD34/CD133 selected or CD3/CD19 depleted stem cells, DLI and purified CD56+CD3- NK cells.

Considering the growing use of immunotherapeutic strategies in paediatric stem cell transplantation associated with risk of graft-versus-host disease, an accurate method for the enumeration of residual T cells/kg recipient's body weight is of paramount importance. Therefore, we propose a multi colour-flow cytometric strategy for correct absolute vital T cell enumeration in manipulated cell preparations for clinical use. The gating strategy is based on the ISHAGE single-platform stem cell enumeration method in combination with experiences from lymphocyte subtyping, using low scatter, high expression of CD3 and CD45 antigens and 7-AAD staining in a no-wash-preparation with counting beads. In spiking experiments, the detection limit was determined to be at 0.7 +/- 0.5 CD3(+) cells/microl with a minimum of 50 T cell events acquired. The cell preparations analysed contained a median absolute CD3(+) T cell number of 221 x 10(3) (0.09%, CD34 selected grafts, n = 187), 900 x 10(3) (0.004%, CD3/CD19 depleted grafts, n = 15) and 283 x 10(3) (0.012%, CD3 depleted/CD56 enriched NK-cells, n = 14), respectively. The results differed of those from conventional T cell measurement in cell products after extensive manipulation. Our method provides reliable residual T cell enumeration even at extremely low concentrations.

CD34(+) hematopoietic progenitor cell selection of bone marrow grafts for autologous transplantation in pediatric patients.

CD34(+)-selection of hematopoietic grafts for patients undergoing autologous hematopoietic stem cell transplantation (HSCT) is frequently used to obtain a tumor-free graft. The majority of published experience is with peripheral blood stem cell (PBSC) products; only scant information has been published on bone marrow (BM) grafts. We reviewed our experience using CD34(+) selection of BM grafts in children undergoing autologous BM transplantation. After obtaining institutional approval, we performed a retrospective review of the medical records of patients who underwent autologous stem cell collection at St. Jude. From January 1, 1999, to December 31, 2003, 373 patients underwent autologous HSCT; 131 received marrow grafts, 237 received PBSC grafts, and 5 received a combination. Seventeen patients underwent BM harvests for CD34(+) selection of their stem cell grafts. Sixteen patients received 19 CD34 purified grafts processed on the Isolex 300i Magnetic Cell Selection System device. Four patients were not included in the engraftment analysis as 1 did not receive the collected product, 1 received a tandem product, and 2 received products that were composed of 2 or 3 combined purified products. Following selection, marrow grafts contained a median of 1.4 x 10(6) CD34(+) cells/kg (range: 0.09-8.3 x 10(6)/kg) and a median of 0.014 x10(8) total nucleated cell cells/kg (range: 0.001-0.09 x 10(8)/kg). The median CD34% recovery was 30.9% (range: 9.3%-57.1%), with the median CD34 purity being 95.5% (range: 62.2%-98.8%). All patients engrafted. The median time to absolute neutrophil count > or = 500/mm(3) was 19 days (range: 12-35 days), and to platelet recovery was 28 days (range 18-37 days). No patient died from transplant-related complications. Our study demonstrates that CD34(+)-selection of marrow grafts is feasible, and these grafts are able to successfully reconstitute hematopoiesis in patients undergoing autologous BMT.

Mobilization, harvesting and selection of peripheral blood stem cells in patients with autoimmune diseases undergoing autologous hematopoietic stem cell transplantation.

Peripheral blood stem cells (PBSC) were mobilized in 130 patients with autoimmune diseases undergoing autologous hematopoietic stem cell transplantation using cyclophosphamide 2 g/m(2) and either granulocyte colony-stimulating factor (G-CSF) 5 mcg/kg/day (for systemic lupus erythematosus (SLE) and secondary progressive multiple sclerosis, SPMS) or G-CSF 10 mcg/kg/day (for relapsing remitting multiple sclerosis (RRMS), Crohn's disease (CD), systemic sclerosis (SSc), and other immune-mediated disorders). Mobilization-related mortality was 0.8% (one of 130) secondary to infection. Circulating peripheral blood (PB) CD34(+) cells/microl differed significantly by disease. Collected CD34(+) cells/kg/apheresis and overall collection efficiency was significantly better using Spectra apheresis device compared to the Fenwall CS3000 instrument. Patients with SLE and RRMS achieved the lowest and the highest CD34(+) cell yields, respectively. Ex vivo CD34(+) cell selection employing Isolex 300iv2.5 apparatus was significantly more efficient compared to CEPRATE CS device. Circulating PB CD34(+) cells/microl correlated positively with initial CD34(+) cells/kg/apheresis and enriched product CD34(+) cells/kg. Mean WBC and platelet engraftment (ANC>0.5 x 10(9)/l and platelet count >20 x 10(9)/l) occurred on days 9 and 11, respectively. Infused CD34(+) cell/kg dose showed significant direct correlation with faster white blood cell (WBC) and platelet engraftment. When adjusted for CD34(+) cell/kg dose, patients treated with a myeloablative regimen had significantly slower WBC and platelet recovery compared to non-myeloablative regimens.

Megadose transplantation of highly purified haploidentical stem cells: current results and future prospects.

The transplantation of megadoses of haploidentical hematopoietic stem cells is for a number of children with malignant or nonmalignant diseases the only curative approach. In order to prevent severe GvHD, the removal of T lymphocytes from the stem cell graft either by positive selection of CD34+ stem cells or by negative depletion of CD3+ T lymphocytes is necessary. We present the results obtained so far by CD34+ positive selection and discuss new techniques of graft engineering which might hopefully further improve the outcome of haploidentical stem cell transplantation.

CXCR3 expression on CD34(+) hemopoietic progenitors induced by granulocyte-macrophage colony-stimulating factor: II. Signaling pathways involved.

CXCR3, known to have four ligands (IFN-gamma inducible protein 10 (gamma IP-10), monokine induced by IFN-gamma (Mig), I-TAC, and 6Ckine), is predominantly expressed on memory/activated T lymphocytes. We recently reported that GM-CSF induces CXCR3 expression on CD34(+) hemopoietic progenitors, in which gamma IP-10 and Mig induce chemotaxis and adhesion. Here we further report that stimulation with GM-CSF causes phosphorylation of Syk protein kinase, but neither Casitas B-lineage lymphoma (Cbl) nor Cbl-b in CD34(+) hemopoietic progenitors can be blocked by anti-CD116 mAb. Specific Syk blocking generated by PNA antisense completely inhibits GM-CSF-induced CXCR3 expression in CD34(+) progenitors at both mRNA and protein as well as at functional levels (chemotaxis and adhesion). Cbl and Cbl-b blocking have no such effects. Thus, GM-CSF binds to its receptor CD116, and consequently activates Syk phosphorylation, which leads to induce CXCR3 expression. gamma IP-10 and Mig can induce Syk, Cbl, and Cbl-b phosphorylation in CD34(+) progenitors by means of CXCR3. gamma IP-10 or Mig has induced neither chemotaxis nor adhesion in GM-CSF-stimulated Cbl-b-blocked CD34(+) hemopoietic progenitors, whereas SDF-1alpha induces both chemotaxis and adhesion in these cells. Interestingly, gamma IP-10 and Mig can induce chemotaxis and adhesion in GM-CSF-stimulated Syk- or Cbl-blocked CD34(+) hemopoietic progenitors. Thus, Cbl-b, but not Syk and Cbl phosphorylation, is essential for gamma IP-10- and Mig-induced chemotaxis and adhesion in CD34(+) hemopoietic progenitors. This study provides a useful insight into novel signaling transduction pathways of the functions of CXCR3/gamma IP-10 and Mig, which may be especially important in the cytokine/chemokine environment for mobilization, homing, and recruitment during proliferation, differentiation, and maturation of hemopoietic progenitor cells.

Retrovirally transduced human dendritic cells can generate T cells recognizing multiple MHC class I and class II epitopes from the melanoma antigen glycoprotein 100.

Involvement of tumor-Ag specific CD4(+) and CD8(+) T cells could be critical in the generation of an effective immunotherapy for cancer. In an attempt to optimize the T cell response against defined tumor Ags, we previously developed a method allowing transgene expression in human dendritic cells (DCs) using retroviral vectors. One advantage of using gene-modified DCs is the potential ability to generate CD8(+) T cells against multiple class I-restricted epitopes within the Ag, thereby eliciting a broad antitumor immune response. To test this, we generated tumor-reactive CD8(+) T cells with DCs transduced with the melanoma Ag gp100, for which a number of HLA-A2-restricted epitopes have been described. Using gp100-transduced DCs, we were indeed able to raise T cells recognizing three distinct HLA-A2 epitopes within the Ag, gp100(154-162), gp100(209-217), and gp100(280-288). We next tested the ability of transduced DCs to raise class II-restricted CD4(+) T cells. Interestingly, stimulation with gp100-transduced DCs resulted in the generation of CD4(+) T cells specific for a novel HLA-DRbeta1*0701-restricted epitope of gp100. The minimal determinant of this epitope was defined as gp100(174-190) (TGRAMLGTHTMEVTVYH). These observations suggest that retrovirally transduced DCs have the capacity to present multiple MHC class I- and class II-restricted peptides derived from a tumor Ag, thereby eliciting a robust immune response against that Ag.

Expression of CD34 and Myf5 defines the majority of quiescent adult skeletal muscle satellite cells.

Skeletal muscle is one of a several adult post-mitotic tissues that retain the capacity to regenerate. This relies on a population of quiescent precursors, termed satellite cells. Here we describe two novel markers of quiescent satellite cells: CD34, an established marker of hematopoietic stem cells, and Myf5, the earliest marker of myogenic commitment. CD34(+ve) myoblasts can be detected in proliferating C2C12 cultures. In differentiating cultures, CD34(+ve) cells do not fuse into myotubes, nor express MyoD. Using isolated myofibers as a model of synchronous precursor cell activation, we show that quiescent satellite cells express CD34. An early feature of their activation is alternate splicing followed by complete transcriptional shutdown of CD34. This data implicates CD34 in the maintenance of satellite cell quiescence. In heterozygous Myf5(nlacZ/+) mice, all CD34(+ve) satellite cells also express beta-galactosidase, a marker of activation of Myf5, showing that quiescent satellite cells are committed to myogenesis. All such cells are positive for the accepted satellite cell marker, M-cadherin. We also show that satellite cells can be identified on isolated myofibers of the myosin light chain 3F-nlacZ-2E mouse as those that do not express the transgene. The numbers of satellite cells detected in this way are significantly greater than those identified by the other three markers. We conclude that the expression of CD34, Myf5, and M-cadherin defines quiescent, committed precursors and speculate that the CD34(-ve), Myf5(-ve) minority may be involved in maintaining the lineage-committed majority.