The Hematopoietic Microenvironment: Stromal Cell Types: Characterization and Function In Situ and In Vitro

Area of Science:

• Hematopoietic stem cell biology
• Marrow microenvironment research
• Stromal cell interactions

Background:

• The murine hematopoietic stroma is vital for hematopoietic stem cell (HSC) homing, replication, and lineage control post-transplantation.
• The stroma comprises distinct cell types, including myofibroblasts, endothelial-like cells, and macrophages, each with unique characteristics.

Purpose of the Study:

• To characterize the cellular components of the hematopoietic stroma and their roles in HSC regulation.
• To investigate the interactions between stromal cells and HSCs, focusing on growth stimulation and suppression.
• To assess the impact of aging on stromal cell function and HSC support.

Main Methods:

• In vitro culture of murine bone marrow stromal cells to assess antigenic displays and growth factor responses.
• Analysis of cell adhesion molecules (CAMs) and cytokines associated with stromal cell types.
• Co-culture experiments with long-term repopulating stem cells (LTRCs) and stromal cell layers.
• Characterization of growth-inhibiting molecules produced by endothelial-like cells.
• In vivo tracking of transplanted HSCs in murine femurs.
• In vivo and in vitro assessment of stromal cell replicative capacity in young versus old mice.

Main Results:

• Murine hematopoietic stroma consists of three cell types: myofibroblasts, endothelial-like cells, and macrophages.
• Myofibroblasts support LTRC proliferation, while endothelial-like cells and macrophages suppress LTRC and myofibroblast growth.
• Endothelial-like cells produce a soluble factor (<3000 Da) that inhibits cell proliferation.
• Donor stem cells lodge near endothelial lining and contact myofibroblasts post-transplantation.
• Stromal cells from old mice exhibit reduced replicative rates in vivo and in vitro.
• Stromal populations include alkaline phosphatase-positive myofibroblasts and cells producing bone matrix proteins.

Conclusions:

• A balance between growth stimulation (myofibroblasts) and suppression (endothelial-like cells) likely regulates the marrow stroma.
• Age-related decline in stromal cell function may increase susceptibility to microenvironment damage in older individuals.
• The stromal cell population includes cells with characteristics of both myofibroblasts and osteogenic precursors, with lineage relationships under investigation.