Osteoprogenitor Cells and Bone Regeneration Are Factually Accurate

They can be present within the endosteum, the cellular layer of the periosteum, and the lining of the osteogenic cells. Osteoprogenitor cells exist as flattened spindle-shaped structures in matured bones that no longer display active bone remodeling or formation.

Periosteum and endosteum contain cells (osteoblasts, osteoclasts, and osteoprogenitor cells) required for bone development and remodeling of the bone.

They also have potential to differentiate into fibroblasts, adipocytes, chondrocytes and muscle cells under appropriate circumstances. Osteoprogenitor cells are located on the endosteal and periosteal surface of the bone and inner surface of the Haversian canals (see Fig.

In addition, this tissue was described as a richer and more effective source of osteoprogenitor cells than bone marrow, one that could easily be modified to express BMP-2 (Dragoo, 2003). While all these in vitro considerations were very intriguing, only a bone regeneration model in vivo would address whether or not these cells were suitable for bone regeneration. Cowan et al. (2004) used these cells to investigate the in vivo osteogenic capability of adipose-derived stromal (ADAS) cells to heal critical-sized mouse calvarial defects.

Comparison of adolescent versus aged subjects revealed higher numbers (5-fold) of circulating osteoprogenitor cells in adolescent individuals likely reflecting the bone growth at that age. Moreover, circulating osteoprogenitors were also higher in subjects who had experienced a recent fracture [30]. Expression of the chemokine receptor CXCR4 on the circulating osteoprogenitor may assist in homing to bone [31**]. These studies raise the possibility that osteoprogenitors may not only be recruited locally but recruited from distant sites during times of heightened bone formation or fracture.

Cells derived from fat do not exhibit an inherent skeletal potential that is observed in the mSSC lineage. However, extraskeletal mSSC formation is inducible with application exogenous BMPs and additional soluble factors which regulate the differentiation toward bone, cartilage, or stromal cells ...

Osteoprogenitor cells, also known as osteogenic cells, are stem cells in the bone that play a prodigal role in bone repair and growth (see Image. Cells in the Bone). These cells are the precursors to the more specialized bone cells (osteocytes ...

Moreover, there is a conceptual problem in terms of postulating that these cells are osteoblast precursors at sites of bone remodeling on trabecular surfaces adjacent to red marrow and yet having to posit potentially entirely different mechanisms for the origins of osteoblasts at sites of cortical bone remodeling distant from red marrow. Thus, the identification and characterization in recent years of non-adherent stem and osteoprogenitor cells in the bone marrow, of similar cells in the peripheral circulation, and of stem/osteoprogenitor cells arising either from the perivascular compartment (pericytes) or within the developing vascular wall itself, has suggested alternative candidate cell populations that may help to resolve the problem of postulating different mechanisms of remodeling in trabecular versus cortical bone.

This has been supported by parabiotic mouse models involving transplantation of green fluorescence protein positive (GFP+) bone marrow into one paired animal and stimulation of bone formation in the other [19,20]. Once osteogenesis was initiated in the paired mouse, GFP+ cells were found at the site of bone formation, indicating a circulating osteogenic cell, though one study of similar methodology did not identify the circulating osteoprogenitors [13]. Despite this evidence that the bone marrow is the tissue of origin, the precise cellular lineage of COP cells remains unclear. It has been suggested that hematopoietic stem cells (HSCs) are possible progenitors for osteoblasts [21,22]. This, combined with newer information on hematopoietic marker expression by COP cells, suggests that COP cells may be an intermediary between HSCs and osteoblasts.