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Optimization of regenerative medicine strategies includes the design of biomaterials, development of cell-seeding methods, and control of cell-biomaterial interactions within the engineered tissues. Among these steps, one paramount challenge is to non-destructively image the engineered tissues in their entirety to assess structure, function, and molecular expression. It is especially important to be able to enable cell phenotyping and monitor the distribution and migration of cells throughout the bulk scaffold. Advanced fluorescence microscopic techniques are commonly employed to perform such tasks; however, they are limited to superficial examination of tissue constructs. Therefore, the field of tissue engineering and regenerative medicine would greatly benefit from the development of molecular imaging techniques which are capable of non-destructive imaging of three-dimensional cellular distribution and maturation within a tissue-engineered scaffold beyond the limited depth of current microscopic techniques. In this review, we focus on an emerging depth-resolved optical mesoscopic imaging technique, termed laminar optical tomography (LOT) or mesoscopic fluorescence molecular tomography (MFMT), which enables longitudinal imaging of cellular distribution in thick tissue engineering constructs at depths of a few millimeters and with relatively high resolution. The physical principle, image formation, and instrumentation of LOT/MFMT systems are introduced. Representative applications in tissue engineering include imaging the distribution of human mesenchymal stem cells embedded in hydrogels, imaging of bio-printed tissues, and in vivo applications.

Three-dimensional imaging of thick tissue constructs is one of the main challenges in the field of tissue engineering and regenerative medicine. Optical methods are the most promising as they offer noninvasive, fast, and inexpensive solutions. Herein, we report the use of mesoscopic fluorescence molecular tomography (MFMT) to image function and structure of thick bioprinted tissue hosted in a 3-mm-thick bioreactor. Collagen-based tissue assembled in this study contains two vascular channels formed by green fluorescent protein- and mCherry-expressing cells. Transfected live cell imaging enables us to image function, whereas Flash Red fluorescent bead perfusion into the vascular channel allows us to image structure. The MFMT optical reconstructions are benchmarked with classical microscopy techniques. MFMT and wide-field fluorescence microscopy data match within 92% in area and 84% in location, validating the accuracy of MFMT reconstructions. Our results demonstrate that MFMT is a well-suited imaging modality for fast, longitudinal, functional imaging of thick, and turbid tissue engineering constructs.