The Small eye (Sey) gene in the mouse is lethal in the homozygous stat
e. It is located on chromosome 2, is a mutation in the Pax-6 gene, and
is genetically homologous with the human aniridia 2 (AN2) gene mutati
on. Numerous studies over the last few years, using genetic and molecu
lar biological approaches, have investigated both the location of the
gene as well as its possible mode of action. In the homozygous state,
the primary defect appears to be limited to the failure of differentia
tion of the presumptive lens and nasal placodes. Such mice therefore d
isplay a characteristic phenotype; they possess neither eyes nor any n
asal derivatives. Their heterozygous (Sey/+) and normal(+/+) littermat
es may be distinguished before birth only by a detailed examination of
their eyes. Few detailed morphological/histological studies have been
undertaken to date in the Sey/Sey embryos and newborn, and in the pre
sent study we describe a variety of craniofacial abnormalities that ha
ve not previously been reported. We observed, with one exception, dela
yed closure of the palate, and the presence in 80 % of mice of an abno
rmal complement of upper incisor teeth, so that 35 % possessed 1 super
numerary tooth while 45 % possessed 2 supernumerary teeth. In these mi
ce, a total of either 3 or 4, rather than the normal complement of 2,
upper incisor teeth were present. Possibly the most unexpected finding
, however, was the presence of a median cartilaginous rod-like structu
re which protruded between the 2 maxillae to give the Alizarin red S a
nd Alcian blue-stained 'cleared' skulls of the newborn mice a characte
ristic 'unicorn-like' appearance. While this structure appeared to be
a rostral. extension of the chondrocranium, its exact derivation is un
clear.