FIG. 5.

The OC probe discriminates chondrogenesis from osteogenesis in vitro and in vivo. (A) Transduced hMSCs exhibit a basal OC expression, which decreases with time until the beginning of the fourth week when terminal differentiation is characterized by its presence. Control human chondrocytes are negative throughout the same time frame, while same hMSCs subject to osteogenic conditions (B) show the expected activation of the probe. n = 3, with each experiment run in triplicate; one representative experiment shown. (C) Transduced hMSCs implanted in vivo (immunocompromised mice) in a porous hydroxyapatite ceramic matrix (n = 8, made from three donors; four shown; all eight with signal quantified) exhibit a characteristic increasing OC expression (inside the back of the mice and after harvesting) with a signal peak at day 28 and associated full osteoblastic differentiation evidenced histologically at day 50 (black arrows). (D) Separate experiment in which hMSCs (n = 3, from independent donors) exhibited a similar ultimate osteoblastic differentiation, while human chondrocytes (n = 3) show a progressive increase in the signal, suggesting hypertrophic differentiation within an endochondral bone formation environment. Human fibroblasts (n = 3) and NT hMSCs (n = 3, from independent donors) were negative throughout the time frame as expected. NT, nontransduced; OC, osteocalcin. Color images available online at www.liebertpub.com/tea