Abstract
New neurons continue to be born and integrated into the brains of adult decapod crustaceans. Evidence in crayfish indicates that the 1st-generation neural precursors that generate these adult-born neurons originate in the immune system and travel to the neurogenic niche via the circulatory system. These precursors are attracted to the niche, become integrated amongst niche cells, and undergo mitosis within a few days; both daughters of this division migrate away from the niche toward the brain clusters where they will divide again and differentiate into neurons. In the crustacean brain, the rate of neuronal production is highly sensitive to serotonin (5-hydroxytryptamine, 5-HT) levels. These effects are lineage-dependent, as serotonin's influence is limited to late 2nd-generation neural precursors and their progeny. Experiments indicate that serotonin regulates adult neurogenesis in the crustacean brain by multiple mechanisms: via direct effects of serotonin released from brain neurons into the hemolymph or by local release onto target cells, or by indirect influences via a serotonin-mediated release of agents from other regions, such as hormones from the sinus gland and cytokines from hematopoietic tissues. Evidence in crayfish also indicates that serotonin mediates the attraction of neural precursors generated by the immune system to the neurogenic niche. Thus, studies in the crustacean brain have revealed multiple roles for this monoamine in adult neurogenesis, and identified several pathways by which serotonin influences the generation of new neurons.
Highlights
When the embryonic precursor cells die during late embryonic life, neural proliferation stops in most areas of the decapod crustacean brain (Harzsch, 2003; Sintoni et al, 2012)
This paper reviews the intense hunt for the source of these cells, which led to the immune system and the identification of a specific type of circulating blood cell that is attracted to the neurogenic niche, where these go through their first division as neural precursors; their daughters migrate along streams arising from the niche, arriving at brain cell clusters containing interneurons in the olfactory pathway
R-astakine 1 (AST1) increases the number of BrdU-labeled cells in the niche and streams, suggesting an effect on cell cycle time. These results demonstrate that the immune system regulates the neurogenic niche dynamically and implicates semi-granular hemocytes in this process, because AST1 regulates the release of this blood cell type and is capable of rescuing the reduction in niche cell counts after hematopoietic tissue ablations
Summary
When the embryonic precursor cells die during late embryonic life, neural proliferation stops in most areas of the decapod crustacean brain (Harzsch, 2003; Sintoni et al, 2012) Exceptions to this are in the central olfactory and higher order processing pathways, where mitotic activity and the integration of new interneurons continue throughout life (Schmidt, 1997; Schmidt and Demuth, 1998; Harzsch et al, 1999; Schmidt and Harzch, 1999). This paper reviews the intense hunt for the source of these cells, which led to the immune system and the identification of a specific type of circulating blood cell (hemocyte) that is attracted to the neurogenic niche, where these go through their first division as neural precursors; their daughters migrate along streams arising from the niche, arriving at brain cell clusters containing interneurons in the olfactory pathway These cells undergo at least two more divisions over 1–2 weeks, before the progeny differentiate into neurons
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