Preheat oven to 325 degrees F (165 degrees C). Place sausage, mushrooms, celery, and onions in a large, deep skillet. Cook over medium-high heat until evenly brown. Drain, crumble, and set aside. In a large bowl combine sausage mixture with cornbread, chicken broth, poultry seasoning, and sage. Mix well and transfer to a 9×12 inch baking dish. Bake, covered, for 45 minutes or until well set and cooked through.