# Whole Organ Engineering: Approaches, Challenges, and Future Directions
## 4 Approaches to Engineering Whole Organs  
### High-Level Strategies  
There are two major approaches to whole organ engineering: (1) top-down construction and (2) bottom-up construction (Figure 3). The concept of top-down engineering is to develop scaffolds that direct cells to form functional tissues. Macroscopic, or even whole organ scaffolds, are seeded with one or many cell types, where a combination of scaffold remodeling and self-assembly leads to the formation of a complex 3D tissue or whole organ [47; 48; 49]. In contrast, bottom-up engineering utilizes the smallest component elements of tissue, such as collagen molecules, as building blocks and combines them to assemble a larger construct [50; 51; 52; 53; 54; 55]. Similar to erecting a building by adding one brick at a time, bottom-up engineering methods are designed to control and organize the interaction of cells with each other and their surroundings. Using stem cell engineering, it is possible to utilize a single building block, human pluripotent stem cells (hPSCs), to engineer constructs such as organoids, and organ-specific functional tissue through a bottom-up approach [56].  
The most complete top-down methods use whole organs from donor or cadaveric sources that are decellularized and then re-seeded with autologous somatic cells, induc  
Figure 3: Comparison of top-down and bottom-up whole organ engineering strategies.  
(iPSCs), or adult stem cells [43; 48; 57]. Using the native matrix composition and complex architecture to regenerate functional whole organs has distinct advantages over other whole organ regeneration methods and is the closest to becoming a clinically viable substitution for organ transplantation. Despite these advantages, several barriers to clinical translation of this approach, such as efficient methods of maturing cells seeded on decellularized organs as well as proliferating cells to physiologically viable numbers for recapitulating whole organ structure and function, still need to be overcome [56]. Bottom-up approaches require fabrication methods that can replicate the bulk structure of organs along with their complex internal geometries. A bottom-up method that has been widely used due to its ability to generate complex constructs is additive manufacturing, or 3D printing [51]. Three-dimensional (3D) bioprinting is useful in bottom-up whole organ engineering because organ structure can be replicated using highly accurate models generated from imaging technology like magnetic resonance imaging (MRIs) and can print constructs with internal tubular networks resembling vasculature ranging from arterial to capillary diameters [58]. Additionally, the bioinks used in 3D bioprinting can be derived from native ECM molecules, such that printed organ constructs have an internal microenvironment conducive to cell adhesion and proliferation [51; 58; 59]. Bioprinted organ structures have demonstrated the ability to replicate specific organ functions. However, increasing the structural and physiological complexity of organ constructs to be able to produce multiple organ functions in a single engineered organ remains a challenge.  
A significant barrier impeding the creation of whole organs is developing an organ engineering approach that incorporates functional angiogenesis, lymphangiogenesis, and neurogenesis within a single organ or organ system [60]. All of these components must be present in an engineered whole organ, but no one has successfully accomplished this yet.