Cell-based engineered tissue models have been increasingly useful in the field of tissue engineering, in in vitro drug screening systems, and in complex cell biology studies. While techniques for engineering tissue models have advanced, there have been few imaging technique capable of assessing the complex 3-D cell behaviors in real-time and at the depths that comprise thick tissues. Understanding cell behavior requires advanced imaging tools to progress from characterizing 2-D cell cultures to complex, highly-scattering, thick 3-D tissue constructs. In this study, we demonstrate that it is possible to use OCT to non-destructively evaluate dynamic cell behavior and function in a quantitative fashion in four dimensions (3-D space plus time). Dynamic processes including cell migration, proliferation, apoptosis, necrosis, and mechanical restructuring are observed during engineering tissue development. With high penetration depth and increased spatial and temporal resolution in 3-D space, OCT will be a useful tool for improving our understanding of cell dynamics in situ and in real-time, for elucidating the complex biological interactions, and for directing our designs toward functional and biomimetic engineered tissues.
Biomechanical elastic properties are among the many variables used to characterize in vivo and in vitro tissues. Since these properties depend highly on the micro- and macro- scopic structural organization of tissue, it is useful to understand the mechanical properties and the alterations that occur when tissues are given biomechanical stimuli by applying external forces under different circumstances. Recent advances in tissue engineering have explored and utilized the significant role that externally-applied forces play during the development of engineered tissues. However, current methods for investigating the microscopic biomechanical changes in complex three-dimensional engineered tissues have been limited. Using Optical Coherence Elastography (OCE), we map the spatially-distributed mechanical displacements and strains in a representative model of a developing engineered tissue as cells begin to proliferate and attach within a three-dimensional collagen matrix. OCE is also preformed in the complex developing tissue of the Xenopus laevis (African frog) tadpole. Displacements were quantified by a cross-correlation algorithm on pre- and post- compression images, which were acquired using Optical Coherence Tomography (OCT). The differences in strain were observed over a certain period of time in various regions. OCE was able to differentiate changes in strain over time, which correspond with cell proliferation and matrix deposition as confirmed with histological observations. By anatomically mapping the regional variation of stiffness with micron resolution, it may be possible to provide new insight into the complex process by which engineered and natural tissues develop complex structures.