In the freshwater crayfish species Procambarus clarkia and Pacifastacus leniusculus, the production of neurons continues throughout the organisms' lives. New neurons are continually added to visual areas located in the eyestalks and to two groups of neurons in the sensory area of the brain called cell clusters 9 and 10, which contain neurons in the olfactory pathway. These new neurons appear to arise from neuronal precursors located in a niche that is loosely attached to the underside of the brain. The precursors are not self-renewing, however, raising the question of how the pool of neuronal precursors is replenished.

The importance of communication between the nervous system and the immune system is becoming more and more apparent in mammals as well as in invertebrates. Hemolymph, the arthropod equivalent of blood, contains cells called hemocytes, which underlie innate immunity in invertebrate organisms instead of blood cells. The neurogenic niche of crayfish is continually bathed in hemolymph, which is also funneled into the niche via the vascular cavity. Neuroscientist Jeanne Benton hypothesized that these hemocytes might be the source of new neuronal stem cells.

Benton and her colleagues at Wellesley College (MA) tested this idea by altering the total number of hemocytes in crayfish using an immune system factor called astakine 1, which controls hemocyte production. As the scientists increased the number of hemocytes produced in the crayfish, they saw a corresponding increase in the number of neuronal stem cells in the niche (Dev. Cell 30, 322–333; 2014). They determined that hemocytes are attracted to the neurogenic niche in the crayfish brain via a serotonergic signaling mechanism, extending processes into the niche and integrating among niche cells within 6 h.

Credit: MikeLane45/iStock/Thinkstock

In a second experiment, Benton's team extracted hemocytes from donor crayfish, labeled them with a cell marker and transfused them into receptor crayfish. Three days later, the label was observed in cells in the niche, and later, labeled cells were found migrating away from the niche. After niche cells divided, they migrated along designated streams toward proliferation zones in cell clusters 9 and 10, where they divide at least one more time before differentiating into neurons. The labeled cells also expressed the neurotransmitters characteristic of neurons found in this region.

The study demonstrates a natural occurrence of transdifferentiation, or the process by which one cell type turns into another cell type, which may be useful for applications in regenerative medicine.