Microtia is a congenital condition in which a child is born with an underdeveloped outer ear on one or both ears. Ears are formed on a fetus during the first trimester of pregnancy. Children born with microtia generally have a fully functioning inner ear but tend to suffer from hearing loss due to the closure of an external ear canal. Many children with microtia can now be grown new ears in laboratories by using the child’s own stem cells. The new ear(s) can then be used to surgically repair the condition.

Which of the following processes allows scientists to grow new ears for children with microtia?