Embryonic origins of the two main classes of hemocytes - Granular cells and plasmatocytes - In Manduca sexta

Cell-mediated responses of the moth immune system involve the interaction of two main classes of hemocytes - granular cells and plasmatocytes. During embryogenesis, granular cells arise much earlier than plasmatocytes, and the presence of granular cells is closely coupled with the formation of basal laminae that line the hemocoel occupied by hemocytes. Although epithelial cells contribute the large extracellular matrix protein lacunin to embryonic matrices before granular cells begin contributing this protein to basal laminae, the spatial pattern of lacunin expression in early embryos parallels the later distribution of granular cells over surfaces of basal laminae. Plasmatocytes arise late in embryogenesis, after the cessation of the major morphogenetic movements and the establishment of intact basal laminae. Granular cells are intimately involved with remodeling of basal laminae, and disruptions in the structure of basal laminae can trigger an autoimmune response of granular cells and plasmatocytes. By arising after basal laminae have been molded and remodeled by granular cells, plasmatocytes presumably do not encounter the cues that trigger their aggregation and an autoimmune response.