The diverse origin of bone-forming osteoblasts

Abstract

Osteoblasts are the only cells that can give rise to bones in vertebrates. Thus, one of the most important functions of these metabolically active cells is mineralized matrix production. Because osteoblasts have a limited lifespan, they must be constantly replenished by preosteoblasts, their immediate precursors. Because disruption of the regulation of bone-forming osteoblasts results in a variety of bone diseases, a better understanding of the origin of these cells by defining the mechanisms of bone development, remodeling, and regeneration is central to the development of novel therapeutic approaches. In recent years, substantial new insights into the origin of osteoblasts-largely owing to rapid technological advances in murine lineage-tracing approaches and other single-cell technologies-have been obtained. Collectively, these findings indicate that osteoblasts involved in bone formation under various physiological, pathological, and therapeutic conditions can be obtained from numerous sources. The origins of osteoblasts include, but are not limited to, chondrocytes in the growth plate, stromal cells in the bone marrow, quiescent bone-lining cells on the bone surface, and specialized fibroblasts in the craniofacial structures, such as sutures and periodontal ligaments. Because osteoblasts can be generated from local cellular sources, bones can flexibly respond to regenerative and anabolic cues. However, whether osteoblasts derived from different cellular sources have distinct functions remains to be investigated. Currently, we are at the initial stage to aptly unravel the incredible diversity of the origins of bone-forming osteoblasts. © 2021 American Society for Bone and Mineral Research (ASBMR).

Keywords: BONE DEVELOPMENT; BONE MARROW STROMAL CELLS; BONE REGENERATION; CHONDROCYTES; OSTEOBLASTS; SKELETAL STEM CELLS.

Figure 1.. Active cuboidal osteoblasts on the bone surface.

(Left) Fluorescent pseudo-confocal microscopy images of the endosteal surface of the femur, osteocalcin (Bglap)-GFP mice at postnatal day 21 (3 weeks of age). A layer of large cuboidal osteocalcin-GFP⁺ osteoblasts (green) overlay the mineralized bone matrix (visualized by differential interference contrast, DIC, gray). Note that osteocalcin-GFP is also expressed in osteocytes embedded in the bone matrix. In the lower panel, osteoblasts are often covered by spindle-shaped pre-osteoblasts (yellow dotted line). Green: GFP; gray: DIC. Scale bar: 20 μm. The images were captured by Dr. Yuki Matsushita, University of Michigan. (Right) Diagram showing the cellular component of the endosteal space. The illustration was created by Dr. Naoko Sakagami, University of Michigan.

Figure 2.. Chondrocyte-to-osteoblast transition at the end of the growth plate.

(Upper left) Fluorescent confocal microscopy images of the proximal growth plate of the femur, parathyroid hormone-related protein (Pthrp)-creER; R26R^tdTomato mice at postnatal day 36, after a tamoxifen pulse at postnatal day 6 (one month of chase). Lineage-marked PTHrP⁺ chondrocytes (red) form the entire layer of columns of chondrocytes in a clonal fashion, originating from the resting zone. In right panel, a large hypertrophic chondrocyte encased in “cocoons” is observed to escape from its sheath at the bottom of the growth plate, and then dramatically change its shape and produces its progeny in the underlying metaphyseal marrow space. Red: tdTomato; gray: DIC. Scale bars: 50 μm (left) and 10 μm (right). The images were captured by the author N.O. (Lower left) Fluorescent pseudo-confocal microscopic images of the proximal metaphyseal marrow space of the femur, Cxcl12^GFP/+; Pthrp-creER; R26R^tdTomato (left) and Col1a1(2.3kb)-GFP; Pthrp-creER; R26R^tdTomato (right) mice at postnatal day 67, after a tamoxifen pulse at postnatal day 6 (two months of chase). Green: Cxcl12-GFP (left) or Col1a1(2.3kb)-GFP (right), red: tdTomato, gray: DIC. Scale bars: 20 μm. The images were captured by Dr. Koji Mizuhashi, University of Michigan. (Right) Diagram demonstrating chondrocyte-to-osteoblast and stromal cell transformation at the sub-hypertrophic zone of the growth plate. The illustration was created by Dr. Naoko Sakagami, University of Michigan.

Figure 3.. The diverse origin of bone-forming osteoblasts.

Diverse origins of bone-forming osteoblasts at the fetal and postnatal stages and regeneration. Osteoblasts are matrix-secreting cells with well-developed cytoplasm, organelles, and euchromatin. Osteoblasts have a limited lifespan; these cells soon become embedded in the matrix as osteocytes or undergo apoptosis. Osteoblasts are continuously provided by their immediate pre-osteoblast precursors. These pre-osteoblasts are formed from a diverse array of cells, including those termed as “skeletal stem cells,” under a variety of conditions such as bone development, remodeling and regeneration, bone anabolism, and heterotopic ossification. Osteoblasts originate from growth plate chondrocytes, bone marrow stromal cells, quiescent bone-lining cells, and other fibroblasts, such as suture fibroblasts or dental follicle cells. The illustrations were created by Dr. Naoko Sakagami, University of Michigan.