Abnormal facial development in the mouse mutant first arch

The first arch mutation in mice, far, causes specific severe craniofacial defects, different from those caused by other craniofacial mutations. The purpose of the present study was to define the facial defects in far homozygotes, to identify heterozygote expression, if any, and to investigate the embryogenesis of the defect. A genetic study and a developmental study were done. The genetic study showed that only far homozygotes, observed on day 18 of gestation, have cleft palate, pointed snout, and deficiency and disorganized pattern of maxillary vibrissae. Approximately 75% have open eye(s). In addition, only far homozygotes lack infraorbital vibrissa(e) (76%) or have facial skin tags (62%). Thus, the missing infraorbital vibrissa(e) and disturbed maxillary vibrissa pattern could be used to identify far/far embryos for developmental study. The developmental study showed that all of the defects observed in far homozygotes at birth can be accounted for by abnormalities present on day 12 of gestation. Both the palatal shelves and the vibrissal ridges are deficient and irregular. The maxillary branch of the trigeminal nerve is abnormal and associated with a streaming and swirling pattern of mesenchymal cells between vibrissae and palate. No aberrations in cell density are apparent on day 12. The primary defect in the far mutation is not known, but all of the defects derive specifically from the maxillary facial process, one portion of the first branchial arch.