Oxygen gradients correlate with cell density and cell viability in engineered cardiac tissue

Abstract

For clinical utility, cardiac grafts should be thick and compact, and contain physiologic density of metabolically active, differentiated cells. This involves the need to control the levels of nutrients, and most critically oxygen, throughout the construct volume. Most culture systems involve diffusional transport within the constructs, a situation associated with gradients of oxygen concentration, cell density, cell viability, and function. The goal of our study was to measure diffusional gradients of oxygen in statically cultured cardiac constructs, and to correlate oxygen gradients to the spatial distributions of cell number and cell viability. Using microelectrodes, we measured oxygen distribution in a disc-shaped constructs (3.6 mm diameter, 1.8 mm thickness) based on neonatal rat cardiomyocytes cultured on collagen scaffolds for 16 days in static dishes. To rationalize experimental data, a mathematical model of oxygen distribution was derived as a function of cell density, viability, and spatial position within the construct. Oxygen concentration and cell viability decreased linearly and the live cell density decreased exponentially with the distance from the construct surface. Physiological density of live cells was present only within the first 128 microm of the construct thickness. Medium flow significantly increased oxygen concentration within the construct, correlating with the improved tissue properties observed for constructs cultured in convectively mixed bioreactors.

Modeling of encapsulated cell systems

“…3(A) and (B) shows the sensitivity of the steady-state cell and oxygen profiles for different values of μ g,max /μ d,max around the baseline value of 0.25. The effect of the two oxygen consumption parameters, ν max and K m , on the steady-state solutions was comparable to previous reported findings (Bassom et al, 1997;Radisic et al, 2006) and was as follows. Increasing ν max resulted in steeper cell density gradients and a decrease in the number of viable cells that could be supported towards the center, as oxygen concentrations quickly reached hypoxic levels away from the bead periphery.…”

Endothelial cells guided by immobilized gradients of vascular endothelial growth factor on porous collagen scaffolds

“…Engineered Heart Tissues (EHTs) suffer from poor cell survival in the center regions of the constructs due to poor oxygen and nutrient supply [67]. Although strategies such as growth factor immobilization have partially succeeded in enhancing cell survival and proliferation, there still remains the need for a more long-term and natural process of enhancing the cell survival in the center of the scaffolds.…”

Cardiac tissue engineering using perfusion bioreactor systems

“…Whereas human cardiac muscle is ~1 cm thick, diffusion alone can support only four to seven cell layers, that is, a 100-μm thick layer of viable and compact tissue 3,4,7,12 . We recently measured oxygen gradients in statically grown cardiac constructs and correlated them with the decrease in cell viability and density 13 (Fig. 1, panels a-c).…”