TERATOLOGICAL EFFECTS OF N-ETHYL-N-NITROSOUREA ON THE DEVELOPMENT OF THE RAT SPINAL CORD
Abstract
The term terato generally refers to a grossly observable malformation in development which in the case of the central nervous system (CNS) often results in the death of the individual. However, recent studies have demonstrated that more subtle structural alterations of the CNS may be present in animals exposed to teratogenic agents during their parenatal development. These alterations, though not necessarily fatal, are permanent in nature and may manifest themselves in some form of sensorimotor or behavioral deficiency. In order to understand the relationship between teratologic agents and observed functional deficits it is important to analyze the underlying abnormal morphological changes wrought by such treatments. The present studies were initiated to examine the immediate cytotoxic effects as well as the permanent alterations occurring in the rat spinal cord following transplacental exposure to N-ethyl-N-nitrosourea (ENU). For the first study pregnant albino rats in the 12th through 16th day of gestation received a single injection of ENU intravenously (60 mg/kg body weight). At 6 hrs, post injection one rat from each gestational age was anesthetized with Nembutal (35 mg/kg), and the embryos were removed and histologically processed for Paraplast embedding. The same procedure was carried out at 12, 24, and 48 hrs. post injection. The spinal cords of these embryos were examined microscopically for distribution of necrotic cells, presence of mitotic figures, removal of necrotic debris, and regeneration of the neuroepithelium over the 48 hr. period. Semiquantitative measurements of the number of pyknotic cells were made. In the second study pregnant rats in the 14th through 22nd day of gestation received in injection of ENU intravenously (60 mg/kg body weight). These females were allowed to sustain their pregnancies and give birth. The pups were raised to 60 days of age, when they were sacrificed and their spinal cords removed and histologically processed for Paraplast embedding or Golgi-Cox impregnation. This material was analyzed quantitatively by the measurement of spinal cord length and volume, estimation of the number of neurons of various classes, and measurement of the length and number of dendrites formed by these cells. Exposure to ENU during spinal cord neurogenesis resulted in extensive cell death throughout the proliferating neuroepithelium with a concomitant delay in mitosis lasting 6 to 24 hrs. Postmitotic neuroblasts appeared to be spared. Regeneration of neuroepithelium as defined by resumption of mitosis, increase in cell number, restoration of tissue continuity, and removal of necrotic debris proceeded very quickly in older embryos (E14, 15, 16) as all spinal cords appeared normal cytoarchitecturally by 48 hrs. In younger embryos (E12, 13) regeneration following cell death due to ENU was not as efficient as considerable amount of debris remained even 48 hrs. after injection. Quantitative analysis of adult spinal cords revealed some permanent deficits. Animals exposed during neurogenesis displayed significantly smaller volumes of gray matter and reduced numbers of all cell types. Animals exposed during cellular differentiation exhibited stunted dendritic length although the dendritic number was significantly higher than normal. This data indicated that regenerative events during embryonic development did not achieve complete restoration of the lost cells. Furthermore, all experimental animals sacrificed as adults showed the presence of multiple demyelinating foci throughout both gray and white matter regions of the spinal cord.
Degree
Ph.D.
Subject Area
Anatomy & physiology
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