text: "In vivo" is a latin phrase whose literal translation is "in a living thing." This term is used in the English language to describe a process which occurs inside of a living organism. In the realm of muscle tissue engineering, this term applies to the seeding of cells into a biomaterial scaffold immediately prior to implantation. The goal of in vivo muscle tissue engineering is to create a cell-seeded scaffold that once implanted into the wound site will preserve cell efficacy. In vivo methods provide a greater amount of control over cell phenotype, mechanical properties, and functionality of the tissue construct.
  As described in Skeletal Muscle Tissue Engineering: Biomaterials-Based Strategies for the Treatment of Volumetric Muscle Loss (Carnes & Pins, 2020), in vivo muscle tissue engineering builds on the concept of in situ engineering by not only implanting a biomaterial scaffold with specific mechanical and chemical properties, but also seeding the scaffold with the specific cell type needed for regeneration of the tissue. Reid et al. describe common scaffolds utilized in the in vivo muscle tissue engineering process. These scaffolds include hydrogels infused with hyaluronic acid (HA), gelatin silk fibroin, and chitosan as these materials promote muscle cell migration and proliferation. For example, a biodegradable and renewable material derived from chitin known as chitosan, has unique mechanical properties which support smooth muscle cell differentiation and retention in the tissue regeneration site. When this scaffold is further functionalized with Arginine-Glycine-Aspartic Acid (RGD), it provides a better growth environment for smooth muscle cells. Another scaffold commonly used is decellularized extracellular matrix (ECM) tissue as it is fully biocompatible, biodegradable, and contains all of the necessary protein binding sites for full functional recovery and integration of muscle tissue. Once seeded with cells, this material becomes an optimal environment for cell proliferation and integration with existing tissue as it effectively mimics the environment in which tissue naturally regenerates in the mammalian body.
  The in vivo muscle tissue engineering technique provides the wound healing process with a "head start" in development, as the body no longer needs to recruit host cells to begin regeneration. This approach also bypasses the need for cell manipulation prior to implantation, thus ensuring that they maintain all of their mechanical and functional properties.