Morphological characteristics of neutrophils in amphioxus (Cephalochordata) based on the literature review

Based on the findings of past literature, we speculated the existence of two types of stratified [two-layer; inner layer (L0) and outer layer (L1)] granules in the neutrophils (granulocytes; blood cells) of amphioxus (Cephalocordata), especially Branchiostoma japonicum. Type 1 neutrophil granules (NG1a) consist of chromophobic L0 and chromatophilic L1. Other type (type 2; NG2) have chromatophilic L0 and chromophobic L1. The L1 of NG1a and L0 of NG2 show metaazurophilic (purple to blackish purple) in Giemsa staining preparation. Ultrastructurally, these two granule types will not be separately identified. These granules contain intragranular particles (granulons, g) in the L0 of granules. At least, three types of granulons (g-1, g-2, and g-3) are recognized in a granule and localized in L0. Two of them form an agglomerate with short rod shape and two layers (inner layer consisted by g-2, and outer layer by g-3). The agglomerate have been considered as tubular structure or microtubule (misinterpretation). Neutrophils of cyclostome (hagfish and lamprey) also have NG1 [NG1a or NG1b (chromophobic L0 and L1)] and NG2. Further, neutrophil granules of cyclostome also contain granulons (without forming of agglomerate).