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Microencapsulating stem cells in injectable microbeads can enhance delivery and localization, but their ability to act as growth factor production sources is still unknown. To address this concern, growth factor mRNA levels and production from alginate microbeads with encapsulated human adipose stem cells (ASC microbeads) cultured in both growth and chondrogenic media (GM and CM) were measured over a two week period. Human ASCs in microbeads were either commercially purchased (Lonza) or isolated from six human donors and compared to human ASCs on tissue culture polystyrene (TCPS). The effects of crosslinking and alginate compositions on growth factor mRNA levels and production were also determined. Secretion profiles of IGF-I, TGF-ß3 and VEGF-A from commercial human ASC microbeads were linear and at a significantly higher rate than TCPS cultures over two weeks. For human ASCs derived from different donors, microencapsulation increased pthlh and both IGF-I and TGF-ß3 secretion. CM decreased fgf2 and VEGF-A secretion from ASC microbeads derived from the same donor population. Crosslinking microbeads in BaCl2 instead of CaCl2 did not eliminate microencapsulation's beneficial effects, but did decrease IGF-I production. Increasing the guluronate content of the alginate microbead increased IGF-I retention. Decreasing alginate molecular weight eliminated the effects microencapsulation had on increasing IGF-I secretion. This study demonstrated that microencapsulation can enhance chondrogenic growth factor production and that chondrogenic medium treatment can decrease angiogenic growth factor production from ASCs, making these cells a potential source for paracrine factors that can stimulate cartilage regeneration.

Osteoblasts respond to microarchitectural features of their substrate. On smooth surfaces (tissue culture plastic, tissue culture glass, and titanium), the cells attach and proliferate but they exhibit relatively low expression of differentiation markers in monolayer cultures, even when confluent. When grown on microrough Ti surfaces with an average roughness (Ra) of 4-7 µm, proliferation is reduced but differentiation is enhanced and in some cases, is synergistic with the effects of surface microtopography. In addition, cells on microrough Ti substrates form hydroxyapatite in a manner that is more typical of bone than do cells cultured on smooth surfaces. Osteoblasts also respond to growth factors and cytokines in a surface-dependent manner. On rougher surfaces, the effects of regulatory factors like 1alpha,25(OH)2D3 or 17beta-estradiol are enhanced. The response to the surface is mediated by integrins, which signal to the cell through many of the same mechanisms used by growth factors and hormones. Studies using PEG-modified surfaces indicate that increased differentiation may be related to altered attachment to the surface. When osteoblasts are grown on surfaces with chemistries or microarchitectures that reduce cell attachment and proliferation, and enhance differentiation, the cells tend to increase production of factors like TGF-beta1 that promote osteogenesis while decreasing osteoclastic activity. Thus, on microrough Ti surface, osteoblasts create a microenvironment conducive to new bone formation.

Implant osseointegration is reduced in patients with systemic conditions that compromise bone quality, such as osteoporosis, disuse syndrome, and type 2 diabetes. Studies using rodent models designed to mimic these compromised conditions demonstrated reduced bone-to-implant contact (BIC) or a decline in bone mineral density. These adverse effects are a consequence of disrupted intercellular communication. A variety of approaches have been developed to compensate for the altered microenvironment inherent in compromised conditions, including the use of biologics and implant surface modification. Chemical and physical modification of surface properties at the microscale, mesoscale, and nanoscale levels to closely resemble the surface topography of osteoclast resorption pits found in bone has proven to be a highly effective strategy for improving implant osseointegration. The addition of hydrophilicity to the surface further enhances osteoblast response at the bone-implant interface. These surface modifications, applied either alone or in combination, improve osseointegration by increasing proliferation and osteoblastic differentiation of osteoprogenitor cells and enhancing angiogenesis while modulating osteoclast activity to achieve net new bone formation, although the specific effects vary with surface treatment. In addition to direct effects on surface-attached cells, the communication between bone marrow stromal cells and immunomodulatory cells is sensitive to these surface properties. This article reports on the advances in titanium surface modifications, alone and in combination with novel therapeutics in animal models of human disease affecting bone quality. It offers clinically translatable perspectives for clinicians to consider when using different surface modification strategies to improve long-term implant performance in compromised patients. This review supports the use of surface modifications, bioactive coatings, and localized therapeutics as pragmatic approaches to improve BIC and enhance osteogenic activity from both structural and molecular standpoints. [ABSTRACT FROM AUTHOR]

Efforts to improve bone response to biomaterials have focused on ligands that bind [alpha]5[beta]1 integrins. However, antibodies to [alpha]5[beta]1 reduce osteoblast proliferation but do not affect differentiation when cells are grown on titanium (Ti). [beta]1-silencing blocks the differentiation stimulus of Ti microtopography, suggesting that other [beta]1 partners are important. Stably [alpha]2-silenced MG63 human osteoblast-like cells were used to test whether [alpha]2[beta]1 specifically mediates osteoblast response to Ti surface micron-scale structure and energy. WT and [alpha]2-silenced MG63 cells were cultured on tissue culture polystyrene (TCPS) and Ti disks with different surface microtopographies: machined pretreatment (PT) surfaces [mean peak to valley roughness ([R.sub.a]) < 0.02 [micro]m], PT surfaces that were grit-blasted and acid-etched (SLA; [R.sub.a] = 4 [micro]m), and SLA with high surface energy (modSLA). Alkaline phosphatase (ALP), [alpha]2 and [beta]1 mRNA, but not [alpha]5, [alpha]v, [beta]3, type-I collagen, or osteocalcin, increased on SIP, and modSLA at 6 days. [alpha]2 increased at 8 days on TCPS and PT, but remained unchanged on SLA and modSLA. [alpha]2-protein was reduced 70% in [alpha]2-siRNA cells, whereas [alpha]5-mRNA and protein were unaffected, [alpha]2-knockdown blocked surface-dependent increases [beta]1 and osteocalcin and decreases in cell number and increases in ALP and local factors typical of MG63 cells grown on SLA and modSLA [e.g., prostaglandin [E.sub.2], osteoprotegerin, latent and active TGF-[beta]1, and stimulatory effects of 1[alpha],25[(OH).sub.2][D.sub.3] on these parameters]. This finding indicates that [alpha]2[beta]1 signaling is required for osteoblastic differentiation caused by Ti microstructure and surface energy, suggesting that conclusions based on cell behavior on TCPS are not predictive of behavior on other substrates or the mechanisms involved. [alpha]-2 integrin siRNA | MG63 human osteoblasts | titanium surface roughness