Abstract
Microscopic organized lymphoid tissues in the small intestine, such as cryptopatches (CP) and isolated lymphoid follicles (ILF), have been well characterized in the laboratory mouse. Descriptions of these structures have also been reported in rat and human, but the presence or absence of CP and ILF in other animals has not been explored. Given the evolutionary development of the adaptive immune system beginning in jawed fish, we hypothesized that CP and ILF would be present in the intestines of fish. We examined the intestines of 10 wild-caught Catostomus commersonii, a bottom-feeding freshwater fish found in local streams in Western Maryland and throughout the United States. The intestines of these fish were opened longitudinally and arranged in flattened layers for horizontal sectioning. Samples of mouse small intestine were prepared simultaneously as a positive control. After quick-freezing and sectioning, the tissue was stained with hematoxylin and eosin. Interestingly, a complete absence of organized lymphoid tissue was noted, suggesting that CP and ILF, in addition to the well-known Peyer’s patches, are limited to higher-order animals and potentially only mammals. Plans to study other fish species, especially carnivorous varieties (since the composition of the intestinal immune system can change in response to diet) are currently underway to further support this finding.
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