Fig. 1

Critical period for orofacial cleft formation in humans. During critical period I (approximately embryonic days 30–40), cleft lip and jaw (CL) can be induced by a harmful environmental factor. During critical period II, cleft palate (CP) develops as a result of either hypoplasia of the palatal shelves (period II.a – approximately embryonic day 40–55) and/or by retarded growth of the mandible (period II.b – approximately embryonic day 55–60). As a result of its retarded forward growth, the short mandible does not withdraw the tongue from the space between the palatal shelves and so prevents their horizontalization. The total cleft lip and palate (CLP) develops during the critical period of CL and CP. (Adapted according to [63])