The brown area is composed mainly of fibrovascular tissue containing many small vessels and a few large-caliber arteries and a medial muscular layer (arrowheads). The translucent area consists of sclera (arrows). (A) From the section corresponding to the line marked X in Figure 4, no apparent iris or ciliary body is formed in the lower half. Sagittal sections of an eye with an ectopic pupil from a 5-wk-old male RCS rat. The RCS strain is a suitable model for human ocular coloboma, and inferior ectopic pupil appears to be a strong indicator of ocular coloboma. Neither F(1) rats nor N(2) (progeny of RCS × BN matings) displayed any ocular anomalies, including ectopic pupils. Histopathologic examination of primordia showed incomplete closure of the optic fissure in 4 eyeballs of RCS fetuses. The morphologic characteristics closely resembled those of chorioretinal coloboma in humans. Fundic examination revealed severe chorioretinal coloboma in all cases of inferior ectopic pupil. Ectopic pupils in RCS rats were found exclusively in an inferior position, where the iris was shortened. In a prenatal study, coronal serial sections of eyeballs from fetuses at gestational day 16.5 were examined by using light microscopy. Both eyes of RCS rats were examined clinically, and those with inferior ectopic pupils underwent morphologic and morphometric examinations. The objective of the current study was to evaluate whether inferior ectopic pupil is associated with iridal coloboma and other types of ocular coloboma in RCS rats. We found that inferior ectopic pupil is associated exclusively with small-sized eyes in this strain. The RCS strain of rats has a low incidence of microphthalmia. Ocular coloboma is sometimes accompanied by corectopia in humans and therefore ectopic pupil may indicate ocular coloboma in experimental animals.
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