Allometric growth of the spinal cord in relation to the vertebral column during prenatal and postnatal life in the sheep (Ovis aries).

Allometric growth of the spinal cord in relation to the vertebral column was studied for the whole cord and for regional and segmental levels in 30 Iranian sheep of the Mehraban breed, divided into 5 groups comprising 3, 4 and 5 month fetuses, new born lambs, and adults, each group consisting of 6 animals. Observations in all 5 age groups revealed that although the growth of the vertebral column was greater than that of the spinal cord, the difference in growth was limited to the lumbar region and other more caudal regions. The most marked change was observed in the coccygeal region. The growth rate of the coccygeal vertebral column when compared with that of the spinal cord in this region was 2.27, 4.16, 4.76, 4.54 and 7.69 times in the 3, 4, 5 month fetuses, new born lambs and adult sheep, respectively. The allometric growth rate for individual regions indicated that in the cervical region in all age groups, the highest growth rate was at the level of C1 and the lowest at C6 and C7 vertebrae. In the thoracic region, it was higher in the caudal than the cranial part. In the lumbar and sacral regions, growth rate decreased craniocaudally. As a consequence of the negative allometric growth of the spinal cord in relation to the vertebral column in the caudal half of spine, the conus medullaris was displaced from S4 or 3 vertebrae in the 3 month fetus to S2 in adult sheep.