EXPERIMENTAL URETERAL OBSTRUCTION IN THE FETAL OPOSSUM - HISTOLOGIC ASSESSMENT

In the North American opossum, Didelphis virginiana unilateral complet e ureteral obstruction (ECO) and partial unilateral (EPO) ureteral obs truction were created during the early metanephric stage of kidney dev elopment in pups attached to the teat (approximately 4.5 cm. long, 20 days old). At 70 days of pouch life (full-term equivalent in the human ) some completely obstructed ureters were unobstructed with reanastomo sis to the bladder (RECO). Other pups underwent unilateral complete ur eteral ligation at this full-term equivalent (70 days of pouch life) a nd constituted the late obstruction (LCO) group. Unoperated animals co nstituted the control (C) group. All animals were harvested when full grown (.7 to 2.2 kg.), and paraffin sections of the kidneys were stain ed with hematoxylin and eosin and Masson's Trichrome. In each a semiqu antitative assessment of 24 histologic features was made and a digital score assigned. All experimental groups except EPO demonstrated signi ficant epithelial and mesenchymal alterations. The changes can be broa dly categorized as those secondary to obstruction of urine flow (dilat ion and cystic changes), those affecting epithelial differentiation in both cortex and medulla and those affecting mesenchymal differentiati on. The EPO group demonstrated significantly fewer glomerular generati ons than did control kidneys. For all other characteristics studied, t he EPO group did not differ from control. The ECO group demonstrated s ignificant changes when compared with control for most parameters stud ied. Medullary dysplasia was more prominent in kidneys obstructed earl y. Reanastomosis (RECO) at a full-term equivalent did ameliorate the c hanges of medullary dysplasia and cortical atrophy. More collecting du ct hyperplasia, cortical and medullary aplasia were present in the LCO group compared with the ECO. The LCO group also had less primitive du ct formation and less medullary dysplasia than the ECO group. Renal bl astema, previously unreported in experimental obstruction, was present in 41% of experimental kidneys.