FOETAL THYROIDECTOMY AND BRAIN DEVELOPMENT IN THE SHEEP

Abstract

Sheep fetuses were surgically thyroidectomized at 50-60 days gestation, when thyroid function begins, and the pregnancy was then allowed to continue until 90, 120 or 150 days (term). At these times the fetuses were removed by cesarean delivery, exsanguinated, weighed and dissected. The CNS was divided into cerebral hemispheres, brain stem, cerebellum and spinal cord for subsequent analysis. Comparison was made with sham operations on 7 fetuses. Thyroidectomy led to a fall in body wt (24.7%) and brain wt (11.7%) at 120 days and 150 days, 31.1 and 24.5%, respectively. The hemispheres showed the greatest change and the cerebellum the least. DNA and protein contents were reduced in the hemispheres and cerebellum at 150 days, while in the brain stem and spinal cord the DNA content was not significantly affected. Protein content was reduced indicating a reduced cell size, but not a reduced cell number as in the other 2 regions. Somatic changes included reduced wool growth, delayed osseous development in the limbs (X-ray assessment), a reduced heart wt (39.1%) and an increased pituitary wt (48.1%). The thyroidectomized lambs failed to survive for more than a few hours after birth or cesarean delivery at 150 days. The findings indicate significant effects of fetal thyroidectomy on brain development in the sheep late in pregnancy along with other evidence of fetal hypothyroidism.