Who was recruited for the Janissaries, Ottoman Sultan's household troops and bodyguards?

When a Christian boy was recruited, first he would be sent to selected Turkish families to learn to speak Turkish and culture of Ottoman society. Later they were trained in tough Enderun school, eventually converting to Islam. The more talented ones received a higher standard of education to become engineers, architects, physicians and scientists. Janissaries belonged to the Sultan, carrying the title kapıkulu "slaves of the Porte".