Quantitative Imaging of Femoral Bone Marrow Microenvironments Reveals a Heterogenous Distribution of Hematopoietic Stem and Progenitor Cells.

Abstract 1455

Poster Board I-478

Sustained production of all mature blood cell types relies on the continuous proliferation and differentiation of a rare population of self-renewing, multipotent hematopoietic stem cells (HSCs). HSC maintenance and lineage differentiation are thought to be regulated by spatially confined niches, defined by cellular components, soluble regulators, and the extracellular matrix immediately surrounding stem cells. Identification of these microenvironments in which endogenous and transferred HSCs reside within the BM is a major challenge in stem cell biology with relevant clinical implications. Yet the extreme rarity of HSCs, their dynamic nature, and the lack of specific markers to identify them, have precluded an accurate definition of HSC niches to date. Quantitative imaging technologies such as Laser Scanning Cytometry (LSC) are designed for the automated analysis of large cell numbers at a single cell level with high resolution while preserving the morphological information lost in flow cytometry, therefore providing data of statistical significance even for rare cell populations such as HSCs. We have employed LSC to analyze the localization of both adoptively transferred and endogenous hematopoietic stem and progenitor cell (HSPC) populations inside whole longitudinal sections of murine femoral BM cavities.

Our results indicate that, as previously suggested, purified HSPC (Lin⁻c-kit⁺Sca-1⁺) significantly accumulate in endosteal regions (ER) of BM cavities (within 100μm of inner bone surface) upon transplantation. Nevertheless, analysis of sufficient numbers of more differentiated cell subsets (Lin⁻c-kit⁺Sca-1⁻ progenitors, pro B cells and mature B cells) indicated that these areas serve as homing sites for most hematopoietic cells, highlighting the limitations of any conclusions drawn on HSC niche identity from studies performed with transferred HSPC populations. Immunofluorescent staining of endogenous cell populations revealed a gradient in distribution of early hematopoietic progenitors (c-kit⁺), which accumulated in but were not restricted to ER regions. Of note, a vast majority (>80%) of HSPC (Bmi-GFP^hic-kit⁺, or Lin⁻c-kit⁺Sca-1⁺),were found inside ER, although not directly adjacent to endosteal surfaces. Our studies define endosteal areas as tissue regions where HSPC reside in close proximity, but not necessarily in direct contact with a dense vascular network, osteoblastic cells and other potential niche cell types and growth factors currently under investigation.