In vivo acoustic patterning of endothelial cells for tissue vascularization

Strategies to fabricate microvascular networks that structurally and functionally mimic native microvessels are needed to address a host of clinical conditions associated with tissue ischemia. The objective of this work was to advance a novel ultrasound technology to fabricate complex, functional microvascular networks directly in vivo. Acoustic patterning utilizes forces within an ultrasound standing wave field (USWF) to organize cells or microparticles volumetrically into defined geometric assemblies. A dual-transducer system was developed to generate USWFs site-specifically in vivo through interference of two ultrasound fields. The system rapidly patterned injected cells or microparticles into parallel sheets within collagen hydrogels in vivo. Acoustic patterning of injected endothelial cells within flanks of immunodeficient mice gave rise to perfused microvessels within 7 days of patterning, whereas non-patterned cells did not survive. Thus, externally-applied ultrasound fields guided injected endothelial cells to self-assemble into perfused microvascular networks in vivo. These studies advance acoustic patterning towards in vivo tissue engineering by providing the first proof-of-concept demonstration that non-invasive, ultrasound-mediated cell patterning can be used to fabricate functional microvascular networks directly in vivo.

Ultrasound patterning technologies for studying vascular morphogenesis in 3D

Investigations in this report demonstrate the versatility of ultrasound-based patterning and imaging technologies for studying determinants of vascular morphogenesis in 3D environments. Forces associated with ultrasound standing wave fields (USWFs) were employed to non-invasively and volumetrically pattern endothelial cells within 3D collagen hydrogels. Patterned hydrogels were composed of parallel bands of endothelial cells located at nodal regions of the USWF and spaced at intervals equal to one half wavelength of the incident sound field. Acoustic parameters were adjusted to vary the spatial dimensions of the endothelial bands, and effects on microvessel morphogenesis were analyzed. High-frequency ultrasound imaging techniques were used to image and quantify the spacing, width and density of initial planar cell bands. Analysis of resultant microvessel networks showed that vessel width, orientation, density and branching activity were strongly influenced by the initial 3D organization of planar bands and, hence, could be controlled by acoustic parameters used for patterning. In summary, integration of USWF-patterning and high-frequency ultrasound imaging tools enabled fabrication of vascular constructs with defined microvessel size and orientation, providing insight into how spatial cues in 3D influence vascular morphogenesis.

Mesenchymal stem cells produce functional cartilage matrix in three-dimensional culture in regions of optimal nutrient supply

Mesenchymal stem cells (MSCs) are a promising cell source for the treatment of musculoskeletal disease. However, MSC chondrogenesis in 3D culture generates constructs whose macroscopic (bulk) mechanical properties are inferior to constructs formed with chondrocytes. To investigate where and why these deficits in functionality arise, we assessed the local (microscopic) properties of cell-laden hydrogel constructs. Both chondrocyte- and MSC-laden constructs showed pronounced depth dependency, with ~3.5 and ~11.5 fold decreases in modulus from the surface to central regions, respectively. Importantly, in the surface region, properties were similar, suggesting that MSCs can produce matrix of mechanical equivalence to chondrocytes, but only in conditions of maximal nutrient support. Dynamic culture on an orbital shaker (which enhances diffusion) attenuated depth-dependent disparities in mechanics and improved the bulk properties compared to free swelling conditions (225 to 438 kPa for chondrocytes, 122 to 362 kPa for MSCs). However, properties in MSC-based constructs remained significantly lower due to persistent mechanical deficits in central regions. MSC viability in these central regions decreased markedly, with these changes apparent as early as day 21, while chondrocyte viability remained high. These findings suggest that, under optimal nutrient conditions, MSCs can undergo chondrogenesis and form functional tissue on par with that of the native tissue cell type. However, the lack of viability and matrix production in central regions suggests that chondrogenic MSCs do not yet fully recapitulate the advanced phenotype of the chondrocyte, a cell that is optimized to survive (and thrive) in a mechanically challenging and nutrient-poor environment.