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This retrospective study analyses 23 children treated with vertical expandable prosthetic titanium rib (VEPTR) for correction of non-congenital early onset spine deformities. After the index procedure (IP), the device was lengthened at 6-month intervals. The average (av) age at the time of IP was 6.5 years (1.11-10.5). The av follow-up time was 3.6 years (2-5.8). Diagnosis included 1 early onset idiopathic scoliosis, 11 neuromuscular, 2 post-thoracotomy scoliosis, 1 Sprengel deformity, 2 hyperkyphosis, 1 myopathy and 5 syndromic. Surgeries (187) included 23 IPs, av 6.5 (4-10) device expansions per patient (149) and 15 unplanned surgeries. 23 complications (0.13 per surgery) included 10 skin sloughs, 5 implant dislocations, 2 rod breakages and 6 infections. Coronal Cobb angle was av 68°(11°-111°), at follow-up av 54°(0°-105°). Pelvic obliquity was av 33°( 13°-60°), at follow-up av 16°(0°-42°). T1 tilt was av 29°( 5°-84°), two remained unchanged, the remainder improved 10°-68°. Sagittal plane: All but two had stable profiles, two hyperkyphosis of 110°/124°improved to 56°/86°. Space available for lung ratio was less than 90% in ten before the IP, improved in nine and deteriorated in one. Originally designed for thoracic insufficiency syndromes related to rib and vertebral anomalies, VEPTR proved to be a valuable alternative to dual growing rods for non-congenital early onset spine deformities. The complication rate was lower, the control of the sagittal plane and the pelvic obliquity was as good, but the correction of the coronal plane deformity was less than growing rods. However, VEPTR's spine-sparing approach might provoke less spontaneous spinal fusion and ease the final correction at maturity.

Engineered osteogenic constructs for bone repair typically involve complex and costly processes for cell expansion. Adipose tissue includes mesenchymal precursors in large amounts, in principle allowing for an intraoperative production of osteogenic grafts and their immediate implantation. However, stromal vascular fraction (SVF) cells from adipose tissue were reported to require a molecular trigger to differentiate into functional osteoblasts. The present study tested whether physiological doses of recombinant human BMP-2 (rhBMP-2) could induce freshly harvested human SVF cells to generate ectopic bone tissue. Enzymatically dissociated SVF cells from 7 healthy donors (1 x 10(6) or 4 x 10(6)) were immediately embedded in a fibrin gel with or without 250 ng rhBMP-2, mixed with porous silicated calcium-phosphate granules (Actifuse(®), Apatech) (final construct size: 0.1 cm(3)) and implanted ectopically for eight weeks in nude mice. In the presence of rhBMP-2, SVF cells not only supported but directly contributed to the formation of bone ossicles, which were not observed in control cell-free, rhBMP-2 loaded implants. In vitro analysis indicated that rhBMP-2 did not involve an increase in the percentage of SVF cells recruited to the osteogenic lineage, but rather induced a stimulation of the osteoblastic differentiation of the committed progenitors. These findings confirm the feasibility of generating fully osteogenic grafts using an easily accessible autologous cell source and low amounts of rhBMP-2, in a timing compatible with an intraoperative schedule. The study warrants further investigation at an orthotopic site of implantation, where the delivery of rhBMP-2 could be bypassed thanks to the properties of the local milieu.

It is generally accepted that adult human bone marrow-derived mesenchymal stromal cells (hMSCs) are default committed toward osteogenesis. Even when induced to chondrogenesis, hMSCs typically form hypertrophic cartilage that undergoes endochondral ossification. Because embryonic mesenchyme is obviously competent to generate phenotypically stable cartilage, it is questioned whether there is a correspondence between mesenchymal progenitor compartments during development and in adulthood. Here we tested whether forcing specific early events of articular cartilage development can program hMSC fate toward stable chondrogenesis. Inspired by recent findings that spatial restriction of bone morphogenetic protein (BMP) signaling guides embryonic progenitors toward articular cartilage formation, we hypothesized that selective inhibition of BMP drives the phenotypic stability of hMSC-derived chondrocytes. Two BMP type I receptor-biased kinase inhibitors were screened in a microfluidic platform for their time- and dose-dependent effect on hMSC chondrogenesis. The different receptor selectivity profile of tested compounds allowed demonstration that transient blockade of both ALK2 and ALK3 receptors, while permissive to hMSC cartilage formation, is necessary and sufficient to maintain a stable chondrocyte phenotype. Remarkably, even upon compound removal, hMSCs were no longer competent to undergo hypertrophy in vitro and endochondral ossification in vivo, indicating the onset of a constitutive change. Our findings demonstrate that adult hMSCs effectively share properties of embryonic mesenchyme in the formation of transient but also of stable cartilage. This opens potential pharmacological strategies to articular cartilage regeneration and more broadly indicates the relevance of developmentally inspired protocols to control the fate of adult progenitor cell systems.

Despite the various reported approaches to generate osteochondral composites by combination of different cell types and materials, engineering of templates with the capacity to autonomously and orderly develop into cartilage-bone bi-layered structures remains an open challenge. Here, we hypothesized that the embedding of cells inducible to endochondral ossification (i.e. bone marrow derived mesenchymal stromal cells, BMSCs) and of cells capable of robust and stable chondrogenesis (i.e. nasal chondrocytes, NCs) adjacent to each other in bi-layered hydrogels would develop directly in vivo into osteochondral tissues. Poly(ethylene glycol) (PEG) hydrogels were functionalized with TGFβ3 or BMP-2, enzymatically polymerized encapsulating human BMSCs, combined with a hydrogel layer containing human NCs and ectopically implanted in nude mice without pre-culture. The BMSC-loaded layers reproducibly underwent endochondral ossification and generated ossicles containing bone and marrow. The NC-loaded layers formed cartilage tissues, which (under the influence of BMP-2 but not of TGFβ3 from the neighbouring layer) remained phenotypically stable. The proposed strategy, resulting in orderly connected osteochondral composites, should be further assessed for the repair of osteoarticular defects and will be useful to model developmental processes leading to cartilage-bone interfaces.

Bone marrow (BM) mesenchymal stem/stromal cells (MSC) are a heterogeneous population of multipotent progenitors currently under investigation for a variety of applications in regenerative medicine. While self-renewal of stem cells in different tissues has been demonstrated to be regulated by specialized microenvironments called niches, it is still unclear whether a self-renewing niche also exists for MSC. Here, we show that primary human BM cultures contain a population of intrinsically non-adherent mesenchymal progenitors (NAMP) with features of more primitive progenitors than the initially adhering colonyforming units-fibroblast (CFU-f). In fact, NAMP could generate an adherent progeny: (a) enriched with early mesenchymal populations (CD1461, SSEA-11, and SSEA-41); (b) with significantly greater proliferation and multilineage differentiation potential in vitro; and (c) capable of threefold greater bone formation in vivo than the corresponding CFU-f. Upon serial replating, NAMP were able to regenerate and expand in suspension as non-adherent clonogenic progenitors, while also giving rise to an adherent progeny. This took place at the cost of a gradual loss of proliferative potential, shown by a reduction in colony size, which could be completely prevented when NAMP were expanded on the initially adhering BM fraction. Mechanistically, we found that NAMP crucially depend on fibroblast growth factor (FGF)-2 signaling through FGFR2c for their survival and expansion. Furthermore, NAMP maintenance depends at least in part on humoral signals distinct from FGF-2. In conclusion, our data show a niche/progenitor organization in vitro, in which the BM adherent fraction provides a self-renewing microenvironment for primitive NAMP.

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