Vascular cells Vascular cells are known to play a prominent role in regulating proliferation of adult neural precursors. In the adult
subgranular zone (SGZ), dense clusters of dividing cells were found to be anatomically close to the vasculature, especially capillaries. Contacts between adult SVZ neuronal precursors and blood vessels are unusually permeable and frequently devoid of
astrocyte and
pericyte interferences, suggesting that blood-derived cues are gaining direct access to adult neural precursors and their progeny. The vasculature also provides the substrate for new neuron migration after injury in the adult
striatum.
Astrocytes Astrocytes form
gap junctions and are closely associated with the vasculature and its
basal lamina in the adult SVZ and subsequently in the RMS. They may serve as an interface to modulate influences of
endothelial and circulation-derived factors as well as the availability of
cytokines and
growth factors in this system. In addition, astrocytes derived from the neurogenic hippocampus and SVZ, but not from the non-neurogenic
spinal cord, promote proliferation and neuronal fate commitment of multipotent adult neural stem cells in culture, suggesting a role in the RMS. Astrocytes express a number of secreted and membrane-attached factors both
in vitro and
in vivo that are known to regulate proliferation and fate specification of adult neural precursors as well as neuronal migration, maturation, and
synapse formation. In the adult SVZ, astrocytes express
Robo receptors and regulate the rapid migration of
SLIT1-expressing neuroblasts through the RMS. Additionally, it has been proposed that the neuroblasts themselves play a role in modulating the astrocytes through Slit-Robo interactions. In the absence of Slit, astrocytic processes do not align correctly, or create the "tubes", instead running across the migrating neurons. Adult SVZ astrocytes also appear to release
glutamate to regulate the survival of
neuroblasts. Unique to the adult SVZ,
ependymal cells lining the ventricular wall are in close association with neural precursors and their progeny, acting like a shield to protect the "neurogenic niche", a zone in which stem cells are retained after embryonic development for the production of new cells of the nervous system.
Other glial cells Ependymal cells actively regulate neuronal fate specification of adult neural precursors through release of
Noggin. Beating of the
cilia of ependymal cells appears to set up concentration gradients of guidance molecules, such as
cytokines TNF-α (tumor necrosis factor) and
IGF-1 (insulin-like growth factor), to direct migration of neuroblasts, such as in the RMS.
Microglia also actively regulate adult neurogenesis. Under basal conditions,
apoptotic corpses of newly generated neurons are rapidly
phagocytosed from the niche by unactivated microglia in the adult SGZ. Under
inflammatory conditions, reactivated
microglia can have both beneficial and detrimental effects on different aspects of adult neurogenesis, depending on the balance between secreted molecules with pro- and anti-inflammatory action. In one study, the activation of microglia and recruitment of
T cells were suggested to be required for enriched environment-induced SGZ neurogenesis, suggesting a possible role in the RMS. ==Migration mechanics==