# Whole Organ Engineering: Approaches, Challenges, and Future Directions
## 5 Recent Developments and Accomplishments
### Bladder  
Much of the research on bladder whole organ engineering in the past couple decades have been focused on developing decellularized, biomaterial, or composite scaffolds for bladder regeneration in animal models [133; 134; 135]. These scaffolds can pose challenges due to the inability to effectively control cellular remodeling of the graft, resulting in incomplete cellular infiltration, fibrotic scarring, and infection of the graft, leading to a loss in function and even death in animal models [136]. This is why more recent approaches to bladder tissue engineering have focused on reconstructing the cellular components of the bladder, primarily focusing on the urothel and smooth muscle layers.  
A phase II clinical trial conducted by [PERSON] et al., used bladder tissue biopsies from children and adolescents with Spina Bifida to create an autologous bladder graft on a biodegradable PLGA scaffold as an alternative to augmentation cystoplasty [137]. Patient-specific grafts were taken from the bladder dome, and then urothelial cells and smooth muscle cells were cultured separately to form layers that were then grafted onto the spherical PLGA scaffold [137]. This construct was then sutured to the inside of the patient's bladder to improve bladder capacity and compliance [137]. This approach failed however due to a lack of bladder cycling after the operation, the limited proliferation capacity of the cells taken from the biopsies in a diseased bladder, and possible expression of the disease phenotype in the grafted cells taken from the diseased bladder [137]. Due to the limitations in proliferative capacity and ease of access of adult bladder cells, pluripotent and adult stem cells have been explored extensively as an alternative for autologous regenerative bladder grafts.  
A study conducted by [PERSON] et al., used bladder acellular matrix grafts seeded with adipose-derived stem cells in a rat model of bladder augmentation [138]. Compared to an unseeded control group, the stem cell-seeded group had greater bladder capacity as well as greater regeneration of smooth muscle and nerve cells [138]. This complete regeneration of tissue in adipose stem cell-seeded BAM's has significant advantages over using autologous cells from bladder biopsies. This is because harvesting adipose tissue from a patient is less invasive than bladder tissue and adipose-derived mesenchymal stem cells (ADMSC) are cheaper and easier to proliferate in culture than terminally differentiated bladder cells. Additionally, ADMSCs are less likely to express a disease phenotype than cells derived from a diseased bladder [138]. Overall, the advantages of using adipose-derived stem cells significantly improves the potential for future clinical translation of [PERSON] et al.'s technique tested in rats, even compared to other stem cell methods using human embryonic stem cells or bone marrow-derived stem cells due to ethical concerns and the invasiveness of the procedure, respectively [138]. This study has inspired several similar follow-up studies, including one by [PERSON] et al., where human ADMSCs were seeded on a decellularized rat bladder scaffold to regenerate a rat bladder after partial cystectomy [139]. The seeded bladder scaffolds resulted in regeneration of urothel, smooth muscles, nerves, and even vasculature in the neo bladder to a much greater degree than non-seeded controls, leading the authors to conclude that with further advancements, this technique could be effectively applied to bladder transplantations in humans [139].