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 unde rtaken. Phenoloxidase and peroxidase assays showed that LOS cells have char acteristics similar to semi-granular and, in particular, large granular hae mocytes. The mean percentage of LOS cells positive for phenoloxidase and pe roxidase was 85 +/- 23 and 82 +/- 23 %, respectively. There was no signific ant difference between the sites of phenoloxidase and peroxidase activity i n LOS cells (t = 1.617, df = 29, p > 0.05). The relative sectional area occ upied by LOS cells relative to that of the stromal matrix cells from both l aboratory-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 m ean 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, gr anular haemocytes that have phagocytosed foreign material, particularly vir uses, and probably constitute a major mechanism for penaeid antiviral defen se.