Intrinsic innervation in the intestine of the lizard Podarcis hispanica

The aim of this study was the description of the morphology and distribution of nerve structure elements in the intestine of the lizard Podarcis hispanica using different histochemical methods; namely acetylcholinesterase (AChE), formol-induced fluorescence for catecholamines (FIF), nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), and immunohistochemistry for vasoactive intestinal peptide (VIP), as well as substance P (SP) and electron microscopy. The AChE method showed fibres in the myenteric and submucosal plexus, with a higher fibre density in the large intestine. The highest number of related neurons was located in the myenteric plexus ganglia. Noradrenergic innervation was distributed through the myenteric and submucosal plexus, and also around blood vessels, with the highest fibre density in the large intestine. VIP immunohistochemistry showed a wide distribution of positive fibres throughout the intestine, although the highest density was again detected in the large intestine. Small positive cells for VIP were located at internodal segments in the plexus. SP labeling, although subtle, was present all along the intestine. It showed delicate varicose nets and few fibres innervating blood vessels. Small positive cells for SP were located in the large intestine. The indirect method to detect nitric oxide (NO)-producing system showed neural cells in the myenteric plexus ganglia of the large intestine. Electron microscopy showed ganglion neurons with scattered chromatin condensations, glial cells with higher electron density, and axons with varicosities occupied by different vesicles. We also identified certain cells as interstitial cells of Cajal due to their ultrastructural features. They were mostly located in the region of the myenteric plexus.