Regeneration of bone lost to injury or disease is a major health challenge. Most efforts toward bone regeneration center around the manipulation of exogenous cells from bone stem cell niches, which cannot provide sufficient cell numbers plus poor function, survival and integration of the injected cells with host tissues (cited from “RFA- DE-20-006”). In search for endogenous sources, we have studied potential roles of tendon as an alternative lineage for craniofacial bone growth and bone repairs (beyond its classic role: connecting muscles and bones for joint movement) as most bones of the skull below the calvaria are sheathed in the fibrous. Our unexpected and striking findings are a. The bone cell in bone ridges attached by tendon are Scx+ (a gene highly expressed in tendon); b. Tendon-formed bone (TFB) (similar to alveolar bone) is rich with a mixture of matrix components (such as tendon, cartilage and bone markers) that is distinct from conventional bone; c. TFB is built on the existing bone, and continuously expands, leading to more prominent bone ridges with aging; d. Tendon is not simply connected to bone but rather grows into and merges with the existing bone, making bone-tendon interface a mineralized continuum rather than a simple attachment; e. TFB varies depending on tensile forces with some transdifferentiating into chondrocytes while others directly entering into fibrogenesis and then osteogenesis, as well as on the location (such as zygomatic-temporal suture); and f. Tendon cells rapidly and robustly reprogram into bone-generating cells upon trauma or inflammation, leading to new long lasting large TFB. In response to this RFA (that requests an alternative lineage to promote endogenous healing and regeneration of craniofacial bones, and avoid injection of exogenous cells) we propose that the tendon cell, a natural but overlooked precursor for normal craniofacial bone growth, is a robust alternative lineage for bone regeneration in trauma or inflammation. We will achieve our objective by pursuing the following two highly related but independent specific aims: Aim 1: To define the plasticity of tendon cells in formation of craniofacial bone during development using mandible and temporal bones as testing models; Aim 2: To comprehensively study how an adult tendon cell, as an alternative lineage, switches its natural tendon program to an osteogenic program during trauma or inflammation. By virtue of our exciting discoveries, we are uniquely well positioned for this study. We have developed powerful and proven cell lineage tracing techniques with state of the art imaging and single cell RNA- seq methods to analyze cell plasticity in health and reprogramming in trauma. Upon completion of the proposed work, we expect to fill a glaring gap in our knowledge of craniofacial osteogenesis derived from tendon, and lay the foundation for establish tendon as an alternative lineage cell source for bone augmentation and bone regeneration in trauma ...