Submitted to: Animal Reproduction Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/9/2012
Publication Date: 5/1/2012
Citation: Vallet, J.L., Miles, J.R. 2012. Comparison of myelination between large and small pig fetuses during late gestation. Animal Reproduction Science. 132(1-2):50-57.
Interpretive Summary: As swine producers select for increased number of piglets born, there has been a simultaneous reduction in piglet birth weight and an increase in piglet mortality. However, the reasons why piglets with reduced birth weights have reduced survival are not entirely clear. Previous studies indicate that piglet brain weights are reduced in low birth weight piglets, even though brain weight is proportionately preserved in small piglets relative to the weights of other newborn piglet organs. This illustrates the importance of the piglet brain in avoiding the hazards of the newborn piglet’s environment, which includes being crushed by the sow, a primary cause of piglet mortality. Another aspect of brain development, besides size, that can affect survival is myelination. Myelin forms the white matter of the brain and consists of multiple layers of a specialized cell membrane that wraps around specific nerves in the brain. This encasement of nerves by myelin increases the speed of transmission of information by nerves and has broad effects on the coordination of movement and the speed of reflexes, both of which are important to the piglet’s ability to avoid the sow when necessary. In this experiment, myelin content of specific brain regions involved in coordination of movement (cerebellum) and reflexes (brain stem and spinal cord) were compared between the largest and smallest piglet fetuses during late pregnancy. To measure myelin, myelin basic protein (the major protein component of myelin) and myelin phospholipids (the phosphate containing fats that make up the majority of myelin) were measured. Results indicated that myelination of the spinal cord was not different between the largest and smallest fetuses during late pregnancy; but reductions of myelin basic protein and all measured phosphate containing fats in myelin from the cerebellum and brain stem were observed in the smallest piglets compared to the largest piglets, which indicates that myelination of these brain regions is impaired in the smallest piglets. Results further indicated a specific deficit in phosphatidylethanolamine (the primary phosphate containing fat component of myelin) in the myelin of the brain stem in the smallest piglets. Previous research indicates that myelin basic proteins interact with zinc as part of their role in myelin formation, and the ethanolamine portion of phosphatidylethanolamine must come from the diet. Collectively, these results suggest that reduced brain myelination may contribute to the poor survival of low birth weight piglets, and zinc supplementation, supplementation of essential fats, or supplementation of ethanolamine may all be potential dietary manipulations that could improve myelination and survival of these piglets.
Technical Abstract: We compared myelination of the cerebellum, brain stem, and spinal cord in the largest and smallest pig fetuses within a litter during late gestation. Gilts were killed on days 92, 100, and 110 of gestation and these neural tissues were obtained from the largest and smallest fetuses in each litter. Myelin basic protein (MBP) mRNA was quantified in each tissue using real time reverse transcriptase polymerase chain reaction (rtPCR). Myelin was recovered from each tissue and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and thin layer chromatography (TLC) was used to measure MBP and lipids, respectively. MBP mRNA increased with advancing gestation in all three tissues examined (P = 0.05) and was decreased in brain stem of small piglets compared to large piglets (P < 0.01). Two coomassie stained protein bands (HMBP and LMBP) were observed by SDS-PAGE. Six prominent lipid bands were obtained by TLC (cholesterol, hydroxyl(h)-cerebroside, nonhydroxy(nh)-cerebroside, phosphatidylethanolamine, phosphatidylcholine, and sphingomyelin). Significant day by fetal size interactions for cerebellar MBP and lipids indicated that cerebellar myelination in the smallest fetuses was decreased compared to the largest fetuses on day 100 and 110 of gestation. Myelin MBP and lipid obtained from brain stem increased with advancing gestation and LMBP and lipids were decreased in small piglets compared to large piglets. In contrast, myelination in spinal cord increased with day of gestation but was not different between smallest and largest fetuses. These results confirm that myelination of the cerebellum and brain stem, but not spinal cord, is reduced in small fetuses during late gestation.