FIGURE 1-30A-F. Abnormally close proximity of the nasal placodes and subsequent deficiency in medial nasal prominence development results in the development of a small nose and a long upper lip (from nose to mouth) with a deficient philtrum. Variable degrees of severity of effect in ethanol-exposed mouse fetuses results in phenotypes comparable to those in humans with fetal alcohol syndrome (A,B), cebocephaly (C,D), and premaxillary agenesis (E,F). (From Siebert JR, Cohen MM Jr, Sulik KK. Holoprosencephaly: an overview and atlas of cases. New York: Wiley-Liss, 1990, with permission.)

derived from two conjoined nasal placodes, and lateral nasal prominences and hypotelorism is marked (Fig. 1-30C,D). Deficiencies that involve not only the anterior midline region but also the neural crest cells that contribute to the maxillary prominences (i.e., the crest cells derived from the mesencephalic neural folds) appear to be the basis for the premaxillary agenesis malformation complex illustrated in Figure 1-30E,F. In some rodent models, as in humans, mandibular deficiencies can also occur in conjunction with upper midface abnormalities, yielding the malformation complex termed agnathia-holoprosencephaly.

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