A complex network of nerve fibers of the enteric nervous system and enteroendocrine cells is known to regulate the gastrointestinal tract. The distribution and frequency of the argyrophil, argentaffin and serotonin immunoreactive endocrine cells and of the submucosal and myenteric nervous ganglia were studied in the small intestine of the capybara Hydrochoerus hydrochaeris, aiming to verify the existence of possible numerical correlations between endocrine cells and nervous ganglia. Fragments of the duodenum, jejunum and ileum of adult animals were collected and processed according to routine histological techniques. To study the nervous ganglia, hematoxylin and eosin staining was used, while specific staining techniques were used to study the argyrophil, argentaffin and serotonin immunoreactive endocrine cells: Grimelius, modified Masson-Fontana and peroxidase anti-peroxidase, respectively. Endocrine cells were more abundant in the area of the crypts and, in relation to their morphology, ‘open type’ endocrine cells prevailed. The population of argyrophil cells was larger than that of argentaffin cells, and these cells were larger than serotonin immunoreactive cells. The frequency of endocrine cells was apparently greater in the duodenum, indicating the importance of this intestinal segment in digestive and absorptive functions. Prominent nervous ganglia were observed in the submucosal and myenteric plexi, and were larger and more frequent in the myenteric plexus. A numerical correlation was found among the endocrine cells (argentaffin and serotonin immunoreactive cells) and the myenteric nervous ganglia, suggesting the presence of physiological interactions among the endocrine and nervous systems for the control of intestinal activities. The findings in this study contribute to the understanding of the digestive processes of this species, which may also help in its conservation and future survival.
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