Primary Neuronal Precursors in Adult Crayfish Brain: Replenishment from a Non-neuronal Source

Abstract

BackgroundAdult neurogenesis, the production and integration of new neurons into circuits in the brains of adult animals, is a common feature of a variety of organisms, ranging from insects and crustaceans to birds and mammals. In the mammalian brain the 1st-generation neuronal precursors, the astrocytic stem cells, reside in neurogenic niches and are reported to undergo self-renewing divisions, thereby providing a source of new neurons throughout an animal's life. In contrast, our work shows that the 1st-generation neuronal precursors in the crayfish (Procambarus clarkii) brain, which also have glial properties and lie in a neurogenic niche resembling that of vertebrates, undergo geometrically symmetrical divisions and both daughters appear to migrate away from the niche. However, in spite of this continuous efflux of cells, the number of neuronal precursors in the crayfish niche continues to expand as the animals grow and age. Based on these observations we have hypothesized that (1) the neuronal stem cells in the crayfish brain are not self-renewing, and (2) a source external to the neurogenic niche must provide cells that replenish the stem cell pool.ResultsIn the present study, we tested the first hypothesis using sequential double nucleoside labeling to track the fate of 1st- and 2nd-generation neuronal precursors, as well as testing the size of the labeled stem cell pool following increasing incubation times in 5-bromo-2'-deoxyuridine (BrdU). Our results indicate that the 1st-generation precursor cells in the crayfish brain, which are functionally analogous to neural stem cells in vertebrates, are not a self-renewing population. In addition, these studies establish the cycle time of these cells. In vitro studies examining the second hypothesis show that Cell Tracker™ Green-labeled cells extracted from the hemolymph, but not other tissues, are attracted to and incorporated into the neurogenic niche, a phenomenon that appears to involve serotonergic mechanisms.ConclusionsThese results challenge our current understanding of self-renewal capacity as a defining characteristic of all adult neuronal stem cells. In addition, we suggest that in crayfish, the hematopoietic system may be a source of cells that replenish the niche stem cell pool.

Neurogenesis in the central olfactory pathway of adult decapod crustaceans: development of the neurogenic niche in the brains of procambarid crayfish

“…The first-generation precursors in the crayfish P. clarkii undergo geometrically symmetrical divisions and both daughter cells migrate away [32], suggesting that these divisions are not self-renewing. This has been confirmed in experiments that directly tested the self-renewal capacity of the niche cell population [30]. Hence, the first-generation neuronal precursors are not typical neuronal stem cells.…”

Invertebrate Hematopoiesis: An Anterior Proliferation Center As a Link Between the Hematopoietic Tissue and the Brain

“…Earlier, this tissue was thought to be restricted to the dorsal part of the stomach. However, since previous reports have indicated that neuronal stem cells in the crayfish brain are derived from an external source [13,14] and because several genes that we have isolated from crayfish are limited in expression to the HPT and cells in nerve and brain tissues [9,12], we asked whether there is a physical link between the HPT and the brain. By a careful dissection of this thin and fragile tissue, we can now demonstrate that the HPT extends forward and connects to the brain ( Fig.…”

“…For example, it has been reported that hemocyte number and the plasma level of Color images available online at www.liebertpub .com/scd the hematopoietic cytokine, Ast1, oscillates in a typical circadian rhythmic manner in crayfish [18]. Moreover, a study conducted on adult Procambarus clarkia crayfish suggested that HPT cells possibly serve as a primary source of neuronal precursor cells [13]. In mammals, a brain-bone-blood link has been proposed to require signals delivered from the central nervous system directly to the hematopoietic stem and progenitor cells or indirectly to the niche that controls HSC proliferation, mobilization, and differentiation [22].…”

Adult neurogenesis: Examples from the decapod crustaceans and comparisons with mammals

“…Song et al (2009) were also unable to demonstrate a connection between the vascular system and the niche in P. clarkii . However, injection of a low molecular weight fluorescent dye into the perfused brain via the dorsal artery (Figure 4D) or directly into the pericardium of P. clarkii resulted in the clear presence of this dye within the cavity (Sullivan et al, 2007a; Benton et al, 2010), as did injections into the dorsal artery of Cherax destructor (Sandeman et al, 2009). Dye injection into the cavity itself also suggests some form of physical connection, as dye fills not only the cavity, but also nearby vessels (Sandeman and Beltz, unpublished results).…”

“…These precursors undergo symmetrical divisions, unlike traditional neuroblasts, and their divisions are not self-renewing (Zhang et al, 2009; Benton et al, 2010). The cells are bipolar, a structural feature that is clearly distinct from neuroblasts found in the embryonic brain; it has been proposed that the processes of the bipolar cells guide the migration of the 2nd generation cells in the neural precursor lineage to sites of further proliferation and differentiation, proliferation zones in CL9 and CL10.…”