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Experimental analysis of embryogenesis of cerebellum in rat. II. Morphogenetic malformations following X‐ray irradiation on day 18 of gestation

Rat embryso of 18 days gestation were exposed in utero to 170 R of X‐ray irradiation. Embrysos were collected six hours, 1, 2 and 3 days after irradiation, and animals of 2‐, 6‐, 15‐ and 30–day‐old postnatal age were sacrificed. Six hours after irradiation pyknosis of cells was noticed in the external granular layer along the posterior aspect of the cerebellum. Neuroblasts, destined to differentiate into Purkinje cells, were found arrested in their migratory path. During subsequent peoiods of embryogenesis the external granular layer was found recovered, and clustering of the neuroblasts was disorganized and fragmented. This abnormal clustering of neuroblasts was permanent, and the external granular layer followed the same abnormal pattern in its growth. During postnatal development the internal granular layer also was found to follow the abnormal pattern of Purkinje cell layer. These abnormal developmental events were seen to lead to malformed folia in the anterior regions of the cerebellum. In addition to it the cerebellum of X‐ray irradiated animals appeared smaller than the normal. Issues having abearing on the differential radiosensitivity of different cells, factors determining the small size of the cerebellum, and cellular events determining the morphogenetic malformations are discussed.