Morphogenesis of the hereditary microphthalmia in a new strain of rat

Morphogenesis of the eye was studied in a new strain of microphthalmic rat. Abnormalities were noted immediately after the formation of the optic cup. The inner layer in the central part of the optic cup was relatively thick and contained many mitotic figures, whereas that of the marginal part was thin and contained only a few. The transitional point in the inner layer between the central and the marginal parts was well marked. This is evidently due to the extreme growth inhibition of the inner layer at the marginal part. At the early developmental stage, an area of the inner layer corresponding to the transitional point protruded toward the lens because the central part of the inner layer continued to differentiate. The differentiation and the protrusion of the inner layer proceeded variably at the later stages depending on the degree of the growth inhibition. The eyes were classified into three groups: Group A-the retina was recognized as a cyst consisting of the pigment layer and the pigment-layerlike structure which originated from the inner layer; group B-the neural retina and its layered structure were inverted; group C-abnormalities, such as the destruction of the lens, were observed. Although previous authors who studied eye mutants suggested the vascular abnormality as the primary cause of the production of abnormal eyes, we feel that this is not the case in our animals.