The haemocytic origin of lymphoid organ spheroid cells in the penaeid prawn Penaeus monodon

Studies on lymphoid organ spheroid (LOS) cells of Penaeus monodon were undertaken. Phenoloxidase and peroxidase assays showed that LOS cells have characteristics similar to semi-granular and, in particular, large granular haemocytes. The mean percentage of LOS cells positive for phenoloxidase and peroxidase was 85 +/- 23 and 82 +/- 23%, respectively. There was no significant difference between the sites of phenoloxidase and peroxidase activity in LOS cells (t = 1.617, df = 29, p > 0.05). The relative sectional area occupied by LOS cells relative to that of the stromal matrix cells from both laboratory-held and farmed prawns was not correlated to increasing weight or total length of the prawns (p > 0.05). An apoptosis detection assay showed that LOS cells were often apoptotic whilst stromal matrix cells were not. There was a significant difference (t = -5.533, df = 58, p < 0.05) in the mean percentage of apoptotic spheroid cells between laboratory-held prawns (52 +/- 24%) and farmed prawns with midcrop mortality syndrome (MCMS) (80 +/- 12%). In conclusion, LOS cells have the characteristics of exocytosed, granular haemocytes that have phagocytosed foreign material, particularly viruses, and probably constitute a major mechanism for penaeid antiviral defense.