A recent experimental and clinical observation on the use of bone marrow-derived stem/progenitor cells (BM-SC) to treat non-hematologic degenerative diseases has renovated the long-standing knowledge of their biology and function. However, the paradoxical lack of histological studies limits the identification of SC within the human BM, constraining their purification and further obscuring the sites where SC fate is decided. In an attempt to identify human hematopoietic BM-SC and their relationships with the tissue environment, bone marrow biopsies from 31 subjects without BM involvement by hemo-lymphoproliferative diseases were analysed with stringent morphometric criteria for the quantification of cells expressing CD117 (c-kit) and CD34, as candidate SC markers, and/or CD45, CD3, CD20, myeloperoxidase (MPOX), CD68, Glycophorin A (GlyA) and Factor VIII (vWF), as lineage commitment. Immunofluorescence combined with confocal examination was also performed to allow the simultaneous detection of multiple surface/cytoplasmic antigens together with transcription factors. Hematopoietic cells constituted 54% of the tissue among which MPOX, GlyA and vWF positive cells represented 43.8%, 13.5% and 0.06% of marrow cells, respectively. The frequency of CD34pos cells was 93.5±43.6 each 105 total nucleated cells; c-kit involved 2.7±2.6/105 cells and c-kit-CD34 co-expression was present in 1.14±0.98/105cells. When the incidence of CD45 was evaluated among these two cell populations only 15% of c-kitpos and 49% of CD34pos cells showed hematopoietic lineage commitment, indicating the presence in the human bone marrow of undifferentiated SC. The analysis of the expression of Terminal Deoxynucleotidyl Transferase (TdT) in hematopoietic cells demonstrated that 10% of c-kitpos, 20% of CD34pos and 0.62% of CD45pos cells possessed the nuclear transcription factor. The distribution of BM-SC was predominantly scattered throughout the tissue although interstitial and paratrabecular clusters were detected and are currently under intense scrutiny in search of BM-SC sites of activation. In a subset of patients, the histological analysis was complemented by the cytofluorimetric estimation of the same BM cells populations. The results of this study may contribute to the improvement in the isolation and characterization of BM-SC aimed to the identification of their niches and to an appropriate clinical application in the field of regenerative medicine.

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