# Whole Organ Engineering: Approaches, Challenges, and Future Directions
## 3 History of Whole Organ Engineering
### Extracellular Matrix  
In [PERSON] and Vacanti's 1993 review publication, they identified research that fell under the umbrella of the new field of tissue engineering [10]. Several of these early tissue engineering works incorporated ECM components by coating synthetic scaffolds with ECM proteins [25]. In 1992, [PERSON] et al. performed experiments to assess what ECM proteins and what configurations of those proteins could dictate hepatocyte behavior, specifically whether the cells would proliferate or differentiate [25]. They found that rather than a specific protein or scaffold geometry, it was the ECM density that determined hepatocyte fate by modulating cell shape. Other work has shown ECM-derived scaffolds to be capable of directing cell adhesion and tissue growth according to the scaffold orientation [26; 27]. [PERSON] and colleagues demonstrated the repair capacity of engineered ECM scaffolds in their 1999 publication outlining their studies on the effects of using porcine small intestinal submuc (SIS) ECM scaffolds as grafts for goat anterior cruciate ligament injuries [27]. The implanted SIS graft underwent significant remodeling by the host and thus notably increased in strength to have similar failure force as a patellar tendon autograft implanted in the same goat, with histology revealing no difference in inflammatory cell behavior in the two grafts [27]. Similar remodeling and regenerative mechanisms driven by ECM scaffolds have been observed in animal heart models as well [28]. [PERSON] et al. implanted type I collagen scaffolds into injured left ventricles of mice, which were degraded by matrix metalloproteinases and replaced by vascularized ECM [28]. The ECM's capacity for cell adhesion and ability to direct cell behavior and integrate with host tissue after implantation, as it did in these studies, have caused it to be one of the foremost scaffold biomaterials used in tissue engineering. The ECM is a complex and dynamic composite network of biopolymers laden with bioactive compounds like growth factors, and is responsible for providing tissue with its structural properties as well as directing cellular activity [29; 30]. The main components of the ECM are collagens, elastin, proteoglycans, hyaluronic acid, laminin, and fibronectin. There are at least 16 types of collagen, the most abundant being collagen type I, II, and III, which are interstitial collagens that bundle together to form fibrils with high tensile strength, type IV which forms X-shaped units that connect  
Figure 2: Timeline of significant discoveries that have contributed to and advanced the field of whole organ engineering. to form a fibrous network and is abundant in the basement membrane, and type VI which plays a role in connecting interstitial collagen to cells and aids in cell adhesion to the ECM [30; 31]. Elastin is a protein that is highly elastic, with a higher amount of elastin in tissue resulting in greater mechanical compliance [30]. Proteoglycans are proteins with several glycosaminoglycan (GAG) side chains and are highly hydrophilic, allowing tissue to retain water [30]. HJauronic acid (HA) is a GAG that is not covalently attached to a protein, distinguishing it from proteoglycans, though HA does share proteoglycans' hydrophilicity and tissue hydrating role [30]. Laminin, which has 15 isoforms at the time this article was written, is an adhesion protein that is plentiful in the basal lamina, binding to other ECM molecules such as collagen and other laminins [30]. Fibronectin is another adhesion protein that contains binding domains for collagens, integrins, and heparin sulfate [30]. These biomolecules form the composite network of the ECM to provide a substrate, which directs how cells grow and interact with one another, with changes in the ECM structure changing how cells within it interact. One way in which changes in the ECM affect cell behavior is in the formation of new tissue.