What step of the cloning process requires an egg cell? Have alternatives been attempted?
egg cells are not required for cloning, plants for instance can be cloned just fine without them. Oocytes are commonly used in animals becasue they already contain some of the chemical markers and enzymes to begin development in the mother's body and keep her immune system from attacking it. In more derived animals development is not solely controlled by the the embryo but also by the mother, so using an egg cell helps trigger those processes of the mother. If you used a standard stem cell likely the mother immune system would just destroy it as a foreign entity.