The primitive immune system of amphioxus provides insights into the ancestral structure of the vertebrate immune system

Amphioxus is considered to be the basal chordate. However, the structural and anatomical features of the amphioxus immune system are still elusive. Here we report a profile of structural studies of the amphioxus gill and gut, the first line of defending against microbes, through optical and electron microscopy. The amphioxus gut and gill are characterized by the following morphological criteria compared with vertebrates: primary and secondary lymphoid-like tissue clustered in the gill, a thicker basement membrane with a large villus channel and lack of muscular layer in the gut, along with blood vessels that fill with phagocytes following microbial challenge. The phenomena of tissue repair after microbial invasion was observed, though no phagocytes were observed in the region of tissue necrosis. The epithelium cells of amphioxus gut showed active phagocytosis after the microbial challenge. A small number of free and fixed macrophage-like cells were also found in the amphioxus gut. The current results described the structure of the immune system and cellular defense against infection in a protochordate, which may help us in understanding the structural origin of the vertebrate immune system.