Histological studies on ontogenic development of lymphoid organs in Indian major carp, Catla catla

Aim: Immunostimulation, before the fish is able to mount an effective immune response, can induce tolerance. Therefore, present study was carried out to investigate the structural development of lymphoid organs, namely thymus, kidney and spleen of Indian major carp, Catla (Catla catla), which will help in deciding the appropriate age, when immunocompetence is established. Methodology: In this study, ontogeny of the lymphoid organs of catla was studied from hatching up to 54 days post hatch (dph). For this, whole fish were fixed, processed and embedded in paraffin wax. Further, serial sections were cut, stained and observed under microscope. Results: The thymus anlage appeared in the dorso-anterior part of the gill cavity at 1 dph. At 5 dph, an undifferentiated mass of stem cells was observed and by 8 dph, it appeared lymphoid. At 11 dph, clear dark and light zones of cells were observed. At 26 dph, Hassall's corpuscles were observed and no further changes were observed except increase in number of lymphoid cells till end of the sampling period. Kidney was observed as an undifferentiated mass of stem cells at 1 dph with few renal tubules. At 5 dph, undifferentiated stem cells increased in number, and by 8 dph, few erythropoietic cells were observed. At 11 dph, lymphoid cells were present and by 17 dph, renal tubules started degenerating. At 26 dph, anterior kidney completely lacked excretory tissue and appeared lymphoid. Spleen was observed at 3 dph as an undifferentiated mass of stem cells, and at 8 dph, erythrocytes were seen. At 11 dph, spleen was heavily packed with erythrocytes, whereas at 20 dph, the number of erythrocytes decreased and lymphocytes were observed. Fully developed spleen was observed at 33 dph with presence of red and white pulp. Interpretation: Thymus, head kidney and spleen were fully developed on 33 dph. The information generated in the present study would be helpful in deciding the appropriate age for vaccination