Hematopoietic derived cells do not contribute to osteogenesis as osteoblasts

Area of Science:

• Cell Biology
• Developmental Biology
• Skeletal Biology

Background:

• The cellular origins of heterotopic ossification (HO) remain unclear, with conflicting reports on the contribution of hematopoietic-derived cells versus bone marrow-derived osteoblast progenitors.
• Previous studies suggested both CD45- and CD45+ cell populations contribute to HO, necessitating a clearer model to distinguish their roles.

Purpose of the Study:

• To investigate whether CD45+ hematopoietic-derived cells can differentiate into mature osteoblasts in the context of HO, fracture healing, and normal skeletogenesis.
• To develop and utilize a novel triple transgenic mouse model for simultaneous visualization of hematopoietic-derived cells and mature osteoblasts.

Main Methods:

• Generation of a triple transgenic mouse line by crossing CD45-Cre, Z/RED (DsRed expression post-Cre recombination), and Col2.3GFP (GFP expression in mature osteoblasts) mice.
• Analysis of DsRed (hematopoietic lineage) and GFP (mature osteoblasts) co-localization in normal skeletogenesis, fracture healing, and BMP-2 induced HO models.
• In vitro osteogenic assays on DsRed-positive bone lining cells (BLCs) derived from hematopoietic cells.

Main Results:

• DsRed and GFP fluorescence did not co-localize in any condition, indicating hematopoietic-derived cells do not differentiate into mature osteoblasts during skeletogenesis, fracture healing, or HO.
• Unidentified DsRed-positive BLCs, derived from hematopoietic cells but lacking CD45 expression, were observed.
• These red BLCs did not express GFP even under in vitro osteogenic conditions, further supporting their non-osteogenic potential.

Conclusions:

• Hematopoietic-derived cells do not contribute to osteogenesis in fracture healing or HO, despite both cell types originating from mesoderm.
• The study clarifies the cellular origin of HO and fracture healing, excluding a direct osteogenic role for CD45+ hematopoietic cells.
• Novel transgenic mouse model provides a valuable tool for future research into skeletal development and disease.