In this study, a morphometric analysis of morphological changes in the layers of the small intestine (duodenum and jejunum) and liver occurring during the hatching period (week 0) and postnatal development (weeks 1, 3, 6, and 8) was performed in geese. For this purpose, the staining of samples obtained from tissues collected from geese after culling was carried out. Staining was performed using the Goldner method to visualize all layers of the intestine for morphometric measurements. Our analysis focused mainly on traits such as the thickness of the mucosal, submucosal, and muscular layers, as well as traits related to intestinal absorption, such as the height and width of intestinal villi and crypts. Additionally, we also took into account the number of mononuclear and binucleate hepatocytes and other cells present in the liver. After analyzing the results, an increase in most traits was found during the development of the animals, with slight differences between the sections of the duodenum and jejunum. An interesting phenomenon was also noticed—the greatest increase in most traits was observed between the 3rd and 6th week of life, which coincides with the time of feed change. We hope that our work will highlight how important the digestive system is for birds because research on this topic is limited.

Ducks exhibit substantial ecological and dietary diversity, which drives morphological and functional adaptations in their digestive systems. This study analyzed the small intestine and cecum of three wild duck species: Mallard (Anas platyrhynchos), Tufted Duck (Aythya fuligula), and Green-Winged Teal (Anas crecca) collected post-mortem. Histomorphometric analysis and immunohistochemistry (IHC) with the pan-neuronal marker HuC/D were performed. The Tufted Duck showed the thickest intestinal muscle layers, particularly in the duodenum and ileum, and the largest enteric ganglia, indicating adaptation to a fibrous and protein-rich diet. The Mallard displayed the longest villi and deepest crypts, consistent with its omnivorous diet rich in plant material. The Green-Winged Teal, which consumes highly digestible insect-rich food, had the shortest villi and thinnest muscle layers. Differences in enteric ganglion size and organization among species suggest varying neuroregulatory demands in different gut segments. These findings confirm that intestinal morphology and enteric nervous system (ENS) structure are tightly linked to dietary specialization and ecological strategies. The results highlight the high adaptive plasticity of the avian digestive system in response to feeding behavior.

Within the complex regulatory network of the gastrointestinal tract, enteroendocrine cells play a fundamental role in coordinating digestive processes and maintaining metabolic balance. These cells produce a wide range of hormones that influence secretion, motility, and nutrient utilization, constituting an essential component of gastrointestinal physiology in ruminants. Despite the growing interest in the role of endocrine regulation in digestive system function, comprehensive studies evaluating enteroendocrine cell populations across different segments of the gastrointestinal tract in cattle remain limited. In particular, the organization of these cell populations in key digestive segments such as the abomasum and small intestine requires further investigation to better understand their functional significance. The aim of the present study was to provide a comparative characterization of selected enteroendocrine cell populations in the abomasum and small intestine of young and adult Holstein–Friesian bulls. Tissue samples collected from the abomasum, duodenum, jejunum, and ileum were subjected to histological and immunohistochemical analyses. Chromogranin A (CgA) was used as a general marker of enteroendocrine cells, and the number of cells immunoreactive to CgA as well as to gastrin, secretin, somatostatin (SOM), glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide-1 (GLP-1) was quantified in the examined segments of the gastrointestinal tract. All types of immunoreactive cells were counted using a standardized quantitative method based on the assessment of positively stained cells in randomly selected microscopic fields of histological sections. The results revealed clear differences in the number of enteroendocrine cells between the analyzed segments of the gastrointestinal tract as well as between the studied age groups. These findings indicate regional and age-related variation in enteroendocrine cell populations within the gastrointestinal tract and highlight their association with the functional organization of individual segments of the digestive system.

Background/Objectives: Irisin is an exercise-associated myokine with reported effects on β-cell survival, insulin secretion, and pancreatic tissue protection in experimental models of metabolic stress. However, less is known about its influence on the developing endocrine pancreas under physiological conditions. Accordingly, the aim of the present study was to determine whether exogenous recombinant FNDC5/irisin is associated with changes in pancreatic endocrine-cell immunophenotype and islet-level markers of cell turnover in male and female Wistar rats. Methods: Thirty-two 3-week-old Wistar rats were assigned to control and irisin-treated groups, with males and females analyzed searately. Irisin was administered intraperitoneally at 100 µg/kg body weight once weekly for four weeks. Control animals received 0.9% NaCl. At the end of the experiment, serum cholesterol, fructosamine, and lipase were measured as supportive biochemical endpoints. Pancreatic sections were analyzed immunohistochemically for insulin, glucagon, somatostatin, PCNA, and cleaved caspase-3. Results: Irisin treatment was not associated with statistically significant changes in serum cholesterol, fructosamine, or lipase. In males, irisin increased insulin immunoreactive area, insulin-positive cell percentage, and insulin-positive cell density. In females, irisin decreased glucagon immunoreactive area and glucagon-positive cell density, whereas glucagon-positive cell percentage remained unchanged. Somatostatin immunoreactive area decreased after irisin administration in both sexes, while somatostatin-positive cell percentage and density increased in males and females. PCNA-positive cell percentage decreased in both sexes after irisin treatment. Islet cleaved caspase-3 immunoreactivity, assessed as optical density, was not significantly altered. Conclusions: Exogenous irisin modified pancreatic endocrine-cell immunophenotype in a sex- and cell-type-dependent manner in post-weaning rats. These changes occurred without significant alterations in serum lipase, fructosamine, or cholesterol and without a detectable increase in islet cleaved caspase-3 immunoreactivity under the applied analytical conditions.