The equine enteric nervous system--neuron characterization and distribution in adults and juveniles

A study of myenteric and submucosal plexuses was undertaken in the jejunum and ileum of horses and ponies in which no clinical or pathological evidence of intestinal abnormality was apparent. Complete transverse sections of the intestine, stained by a modified haematoxylin and eosin method, were examined using up to 20 sequential sections per animal. Information was gathered from adult, juvenile and fetal equidae. In adults, the longitudinal muscle layers were thinner than the circular muscle layers and the ileum had thicker layers compared to the jejunum. In adults, the submucosal plexus had more neurons per section than the myenteric plexus by mean ratios of 1:3 in the jejunum and 1:1.9 in the ileum. In juveniles, the ratios were respectively 1:1.8 and 1:1.5 and in the fetus 1:2.5 and 1:1.3. The three-dimensional distribution of neurons in both plexuses varied from animal to animal and no consistent pattern was observed. Groups of neurons contained between one and 42 cells per section examined and their length in a cranio-caudal direction varied from 10 to over 100 microns. There were few statistical differences observed between the cranial, middle and caudal portions of either the jejunum or the ileum when neuron groups or neuron numbers per section were examined in 10 adult animals.