Preheat oven to 325. Prepare a 9 "x 13" pan with cooking spray; set aside. Prepare and bake cornbread mixes according to package directions, adding corn to batter. Cool completely. Crumble into a mixing bowl; set aside. In a skillet, cook celery, onions, and bell peppers. Cook ten minutes, or until vegetables are tender. Remove from heat. In a mixing bowl, combine celery mixture, cornbread crumbs, salt, and black pepper. In small bowl, combine egg and milk. Mix well. Add to cornbread mixture. Then, spread into prepared pan. Baked, covered, for 50 minutes.