Identification and structural composition of the blood-spleen barrier in chickens

- The blood-spleen barrier of chickens was identified in the ellipsoid and PELS region.
- The morphological architecture of the chicken blood-spleen barrier was demonstrated.
- The sheathed capillary in chicken spleens is similar to a high endothelial venule.
- The blood-spleen barrier provides a structural basis for lymphocyte homing.

To identify the existence and composition of the blood-spleen barrier (BSB) in chickens, the microanatomical features of the spleen were investigated by light and transmission electron microscopy, intravenous injection of ink, acid phosphatase reaction, and silver impregnation. The results showed that the white pulp in chicken spleen consists of lymphoid nodules, periarteriolar lymphatic sheaths (PALS) and periellipsoidal lymphatic sheaths (PELS). There was no evidence for the presence of a marginal zone.The splenic ellipsoid was a unique structure, which functioned as a barrier for filtering and phagocytosis. Uptake of carbon particles was limited to the ellipsoid and PELS, 60 min after injection of carbon particles. Reticular fibres were densely distributed in the ellipsoid and extended into the PELS. Ellipsoid-associated cells (EACs), reticular cells and macrophages were acid phosphatase positive. The sheathed capillaries, surrounded by the ellipsoid, were similar to high endothelial venules (HEVs). These findings suggest that the BSB of chickens is present in the ellipsoid and PELS, protecting the spleen from invasion from circulating pathogens. The BSB was a reticular framework, between the arterial and venous vessels, which included cuboidal-shaped endothelial cells, supporting cells, EACs, macrophages, reticular cells and fibres. Lymphocyte migration into the spleen parenchyma is most likely via the HEV-like vessels. These research findings contribute to better understanding of avian immunology and provide an insight into evolutionary differences in the immune system.