Ex situ bioengineering of bioartificial endocrine glands : a new frontier in regenerative medicine of soft tissue organs

Ex situ bioengineering is one of the most promising perspectives in the field of regenerative medicine allowing for organ reconstruction outside the living body; i.e. on the laboratory bench. A number of hollow viscera of the cardiovascular, respiratory, genitourinary, and digestive systems have been successfully bioengineered ex situ, exploiting biocompatible scaffolds with a 3D morphology that recapitulates that of the native organ (organomorphic scaffold). In contrast, bioengineering of entire soft tissue organs and, in particular endocrine glands still remains a substantial challenge. Primary reasons are that no organomorphic scaffolding for endocrine viscera have as yet been entirely assembled using biocompatible materials, nor is there a bioreactor performance capable of supporting growth within the thickness range of the regenerating cell mass which has proven to be reliable enough to ensure formation of a complete macroscopic gland ex situ. Current technical options for reconstruction of endocrine viscera include either biocompatible 3D reticular scaffolds lacking any organomorphic geometry, or allogenic/xenogenic acellular 3D matrices derived from a gland similar to that to be bioengineered, eventually recellularized by autologous/heterologous cells. In 2007, our group designed, using biocompatible material, an organomorphic scaffold-bioreactor unit for bioengineering ex situ the human thyroid gland, chosen as a model for its simple anatomical organization (repetitive follicular cavities). This unit reproduces both the 3D native geometry of the human thyroid stromal/vascular scaffold, and the natural thyrocyte/vascular interface. It is now under intense investigation as an experimental tool to test cellular 3D auto-assembly of thyroid tissue and its related vascular system up to the ex situ generation of a 3D macroscopic thyroid gland. We believe that these studies will lay the groundwork for a new concept in regenerative medicine of soft tissue and endocrine organs; i.e. that the organomorphism of a biocompatible scaffold–bioreactor complex is essential to both the 3D organization of seeded stem cells/precursor cells and their phenotypic fate as glandular/parenchymal/vascular elements, eventually leading to a physiologically competent and immuno-tolerant bioconstruct, macroscopically suitable for transplantation and clinical applications.