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

## TL;DR

This study describes the first histological analysis of the gastrointestinal tract and immune cells in a harbour porpoise, revealing structural and immune cell distribution patterns.

## Contribution

The study provides the first detailed histological characterization of the GIT and lymphoid structures in a harbour porpoise, including immune cell distribution.

## Key findings

- The GIT layers were thickest in the stomach and anal canal, with no significant differences between intestinal segments.
- B lymphocytes were the most prevalent immune cell type in lymph nodes, while all three cell types were more concentrated in the distal intestine and anal tonsil.
- Peyer’s patches and morphological differences helped distinguish the duodenal ampulla and distal segments from other intestinal regions.

## Abstract

Here, we characterized the histological structure and distribution of immune cells (macrophages and T and B lymphocytes) in the gastrointestinal tract (GIT) and associated lymphoid tissue, including lymph nodes, of a harbour porpoise (Phocoena phocoena) that died due to bycatch in the Bay of Biscay. Despite the lack of a clear distinction between the small and large intestine, the difference in thickness, folds, and the presence of Peyer’s patches allowed differentiation of the duodenal ampulla and the distal segments from the rest of the intestine. Within the lymph nodes, B lymphocytes represented the predominant cell population.

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.