text: “In situ” is a latin phrase whose literal translation is “on site.” It is a term that has been used in the English language since the mid-eighteenth century to describe something that is in its original place or position. In the context of muscle tissue engineering, in situ tissue engineering involves the introduction and implantation of an acellular scaffold into the site of injury or degenerated tissue. The goal of in situ muscle tissue engineering is to encourage host cell recruitment, natural scaffold formation, and proliferation and differentiation of host cells. The main idea which in situ muscle tissue engineering is based on is the self-healing, regenerative properties of the mammalian body. The primary method for in situ muscle tissue engineering is described in the following section:
  As described in Biomaterials for In Situ Tissue Regeneration: A Review (Abdulghani & Mitchell, 2019), in situ muscle tissue engineering requires very specific biomaterials which have the capability to recruit stem cells or progenitor cells to the site of the muscle defect, thus allowing regeneration of tissue without implantation of seed cells. The key to a successful scaffold is the appropriate properties (i.e. biocompatibility, mechanical strength, elasticity, biodegradability) and the correct shape and volume for the specific muscle defect in which they are implanted. This scaffold should effectively mimic the cellular response of the host tissue, and Mann et al. have found that Polyethylene glycol-based hydrogels are very successful as in situ biomaterial scaffolds because they are chemically modified to be degraded by biological enzymes, thus encouraging cell migration and proliferation. Beyond Polyethylene glycol-based hydrogels, synthetic biomaterials such as PLA and PCL are successful in situ scaffolds as they can be fully customized to each specific patient. These materials stiffness, degradation, and porosity properties are tailored to the degenerated tissues topology, volume, and cell type so as to provide the optimal environment for host cell migration and proliferation.
  In situ engineering promotes natural regeneration of damaged tissue by effectively mimicking the mammalian body's own wound healing response. The use of both biological and synthetic biomaterials as scaffolds promotes host cell migration and proliferation directly to the defect site, thus decreasing the amount of time required for muscle tissue regeneration. Furthermore, in situ engineering effectively bypasses the risk of implant rejection by the immune system due to the biodegradable qualities in each scaffold.