Ontogeny of American paddlefish lymphoid tissues

The temporal and spatial distribution of American paddlefish Polyodon spathula immune cell populations was determined using enzyme cytochemistry and immunohistochemistry. Monocytes and macrophages were present in the renal haematopoietic tissue, spleen, meningeal myeloid tissue, cardiac myeloid tissue and lamina propria of the spiral valve at 7 days post-hatch (dph). Immature lymphocytes were present in the renal haematopoietic tissue, spleen, meningeal myeloid tissue, cardiac myeloid tissue, thymus and lamina propria of the spiral valve at 7 dph. Type A lymphocytes (T-cell like) were demonstrated in the thymus by 21 dph. Type B immunoglobulin positive lymphocytes (B-cell like) were present in the renal haematopoietic tissue, cardiac myeloid tissue and lamina propria of the spiral valve by 7 dph, the thymus at 21 dph, the spleen by 56 dph, and were not observed in the meningeal myeloid tissue of paddlefish aged 7–28 dph. Granulocytes were present in the renal haematopoietic tissue, thymus, spleen, meningeal myeloid tissue, cardiac myeloid tissue and lamina propria of the spiral valve by 7 dph. The spleens in 7–28 dph fish were predominately red pulp. Differentiation of leukocytic and erythrocytic compartments (white and red pulp, respectively) was not apparent in the spleen until 56 dph. Remarkable thymic cortical and medullary differentiation was not yet present at 28 dph, and the thymus was not sampled at 56, 84 or 112 dph. The cardiac myeloid tissue was not developed until 56 dph. Peyer's patches were present in the lamina propria by 56 dph. Paddlefish lympho-myeloid structures are therefore slow to develop, and vaccination procedures should be performed at 2–4 months post-hatch.