FOETAL BRAIN DEVELOPMENT IN THE SHEEP

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Abstract
The pattern of normal fetal brain development was described for the Merino sheep. Controlled flock matings were used and fetuses removed by hysterotomy at 40, 54, 67, 81, 90, 95, 109, 121, and 150 days of gestation. Lambs at 7 days after birth and adults were also studied. The CNS was dissected, separated and weighed as 4 segments-cerebral hemispheres, brain stem, cerebellum and spinal cord. Determinations of DNA, RNA, protein, cholesterol and water content were subsequently carried out on each segment. Brain weight increased from 0.264 g (40 days) to 52.74 g at 150 days. The ratio to body weight decreased from 6.7% (40 days) to 1.5% (150 days). DNA increased more rapidly from 40-90 days in the cerebral hemispheres and brain stem than in the cerebellum and spinal cord and this increase preceded the main increase in weight. Increases in cholesterol and decreases in water content were more marked in the cerebral hemispheres and brain stem. Increase in brain weight occurred in 2 phases, 1 up to 90 days followed by a more rapid and larger increase after 90 days which continued to birth. These 2 phases appear to reflect an increase in neuroblast multiplication followed by neuroglial multiplication and myelination, respectively. At birth, brain weight had reached 50% of adult size, the cerebral hemispheres 52% of adult size, the cerebellum 40% and spinal cord 40%. The relative maturity of the brain at birth justifies the classification of the sheep as a prenatal brain developer.