Le Guyader, D., Redd, M.J., Colucci-Guyon, E., Murayama, E., Kissa, K., Briolat, V., Mordelet, E., Zapata, A., Shinomiya, H., and Herbomel, P. (2008) Origins and unconventional behavior of neutrophils in developing zebrafish. Blood. 111(1):132-141.
The first leukocytes that arise in the development of vertebrate embryos are the primitive macrophages, which differentiate in the yolk sac, and then quickly invade embryonic tissues. These macrophages have been considered to constitute a separate lineage giving rise to no other cell type. Using an in vivo photoactivatable cell tracer in the transparent zebrafish embryo, we demonstrate that this lineage also gives rise to an equal or higher number of neutrophilic granulocytes. Surprisingly, the differentiation of these primitive neutrophils occurs only after primitive myeloid progenitors have dispersed in the tissues. By 2 days post-fertilization, these neutrophils have become the major leukocyte type found wandering in the epidermis and mesenchyme. Like the primitive macrophages, all primitive and larval neutrophils express PU.1 and L-plastin, they are highly attracted to local infections, yet only a small fraction of them phagocytose microbes, and to a much lesser extent per cell than the macrophages. They are also attracted to variously stressed or malformed tissues, suggesting a wider role than anti-microbial defence.