Hemocytes and neural injury in freshwater crayfish : Does the melanization reaction matter?

Abstract: Neurogenesis primarily occurs during embryonic development in decapod crustaceans, ending when the embryonic precursor cells die. However, areas in the central olfactory pathway are exceptions to this. Here, neurons are produced throughout the animal’s life from precursor cells in the neurogenic niche. Cells within the niche divide and migrate to clusters in the olfactory pathway where they eventually differentiate into neurons. The number of cells in the niche correlates with the total number of hemocytes, which have been suggested to be a source of adult-born neurons. Hemocytes are further an important part of the innate immunity since containing the compounds of the proPO-system needed for the melanization reaction. The purpose of this study was to find out whether the melanization reaction matters when it comes to neural injuries. Neural injury was induced by cutting of the first pair of antennae, where neurons are connected to the olfactory pathway. Hemocytes in the hemolymph were counted and characterized and phenoloxidase activity in the brain was measured before and after neural injury. mRNA expression was measured for prophenoloxidase, the neurogenic niche marker glutamine synthetase 2 as well as for astakine 1. Astakine 1 protein had been found in increased levels after neural injury in previous studies. Significant differences were detected for number of hemocytes in injured crayfish and for glutamine synthetase 2 and prophenoloxidase in control crayfish. However, these findings did not provide strong enough evidence to suggest that the melanization reaction plays a role after neural injury. More research is still needed, perhaps by studying the distribution of hemocytes in the brain at different times post-injury by histological sectioning.