Experimentally Induced Coloboma 1n the Golden Hamster: A Preliminary Report

Introduction. The appearance of a coloboma of the iris must have been known long before Walther (15) introduced the term in 1821 since the condition is easiiy visible. The word coloboma is derived from the Greek and means the part which is taken away in mutilation or injury or the part that is lacking, that is, a defect. In coloboma iridis, therefore, a section of the iris is lacking, often bilaterally. After the invention of the ophthalmoscope by von Helmholtz in 1851, it could be seen that the defect is not restricted to the iris but that lens, retina, choroid, and optic nerve can also be involved. However, little was known about the etiology. A fault in the closure of the "choroid fissure" was first mentioned as the cause of the congenital defect by von Ammon ( 1) in 1831 . At that time the pigment epithelium, the most obviously involved layer, was believed to be part of the choroid, and the cleft was, therefore, called choroid fissure. This is a misnomer since the defect lies primarily in the neural retina and the pigment epithelium, both derived from neuroectoderm. The choroid, a derivative of mesoderm, has not formed at the time of the fissure closure and becomes involved only secondarily. The most commonly employed term now is embryonic fissure . Various investigators have proposed different theories regarding the cause of the retarded closure or non-closure of the embryonic fissure: lack of degeneration of mesoderm in the fissure ( 8), inflammation of the optic vesicle ( 3) , pressure from an unusual amount of cerebrospinal fluid ( 4), abnormally large lens (2), and maternal toxins or malnutrition ( 10) . The actual formation of a colo-