First Histological Study of the Gastrointestinal Tract and Associated Lymphoid Structures of a Harbour Porpoise (Phocoena phocoena).

The current knowledge on the histological structure of the gastrointestinal tract (GIT) in cetaceans is based on general descriptions. The aim of this study was to characterize the histology and expression of immune cell markers in samples from the GIT and lymph nodes (LNs) in a harbour porpoise (Phocoena phocoena) bycaught in the Cantabrian Sea. The thickness of the histological layers of the GIT was measured, being greater in the stomach and anal canal, although no significant differences were found among any intestinal segment (p = 0.448). Variation in thickness, morphology of the folds, and the presence of Peyer's patches allowed the duodenal ampulla and the distal segments to be distinguished from the rest of the intestine. An immunohistochemical technique was performed to identify the following markers: IBA1 for macrophages, CD3 for T lymphocytes, and CD20 for B lymphocytes. The distribution of immune cells varied significantly along the GIT, with higher percentages of all three cell types in the distal intestine and the anal tonsil. Within the LNs, B lymphocytes represented the predominant cell population. This study provides the first description of the histological structure of the GIT and associated lymphoid tissue in a harbour porpoise, which will be useful for future research studies.