Osterix Marks Distinct Waves of Primitive and Definitive Stromal Progenitors during Bone Marrow Development

Abstract

Mesenchymal stem and progenitor cells (MSPCs) contribute to bone marrow (BM) homeostasis by generating multiple types of stromal cells. MSPCs can be labeled in the adult BM by Nestin-GFP, whereas committed osteoblast progenitors are marked by Osterix expression. However, the developmental origin and hierarchical relationship of stromal cells remain largely unknown. Here, using a lineage-tracing system, we describe three distinct waves of contributions of Osterix+ cells in the BM. First, Osterix+ progenitors in the fetal BM contribute to nascent bone tissues and transient stromal cells that are replaced in the adult marrow. Second, Osterix-expressing cells perinatally contribute to osteolineages and long-lived BM stroma, which have characteristics of Nestin-GFP+ MSPCs. Third, Osterix labeling in the adult marrow is osteolineage-restricted, devoid of stromal contribution. These results uncover a broad expression profile of Osterix and raise the intriguing possibility that distinct waves of stromal cells, primitive and definitive, may organize the developing BM.

Wnts produced by Osterix-expressing osteolineage cells regulate their proliferation and differentiation

“…Lineage tracing with Axin2, a Wnt target gene, initiated at the neonatal stage, showed that the Wnt-responding population contained stromal cells that were able to robustly contribute to bone development, maintenance, and injury repair over the long term. The in vivo fate and longevity of the Axin2-labeled population is similar to those observed from the lineage traces using Osx-CreERT (20,21), suggesting significant overlap in the Wntresponding and Osx-expressing stromal populations during the neonatal stage, both of which contain progenitors that contribute to the osteolineage over the long term.…”

“…As the Osx population has been shown to contain long-lived stromal progenitors that support osteogenesis (20,21), our observation of the overlap in Axin2 and Osx expression (Fig. 1 B-D) prompted us to ask if the Axin2-expressing population contains osteolineage progenitors.…”

“…Descendants of Wnt-Responding Osteolineage Cells Contribute to Bone Injury Repair. Recent studies have shown that progenitors of the osteolineage can participate in bone tissue repair (18)(19)(20)22). Thus, in addition to lineage tracing during homeostasis, we probed for progenitor activity in the AXCT2-traced population by asking if the progeny of neonatal AXCT2-labeled cells could contribute to bone injury repair in adulthood.…”

“…Osteolineage cells arise from multipotent mesenchymal progenitors, which subsequently give rise to osteolineage-restricted progenitors (18)(19)(20)(21)(22)(23). In perinatal mice, Osterix (Osx) appears to be expressed by both populations (20,21,24) and continues to be expressed as the cells divide and differentiate into osteoblasts.…”

“…In perinatal mice, Osterix (Osx) appears to be expressed by both populations (20,21,24) and continues to be expressed as the cells divide and differentiate into osteoblasts. Osteoblasts begin expressing Col1a1 at an immature stage, followed by Osteocalcin expression as they fully mature.…”