89BVs (Tan et al. 2016 ). Notably, this cell-to-cell communication is mediated by inte-
grin/laminin interactions, as has been described for NSCs of the adult brain (Kazanis
et al. 2010 ; Shen et al. 2008 ). Disruption of these interactions leads to decreased
proliferation, aberrant formation of interneurons and reduced cortical synaptic inhi-
bition (Tan et al. 2016 ), because interneurons are generated at the ventral telen-
cephalon before migrating and populating the cortex (Alifragis et al. 2004 ).
The other, obvious, contribution of BVs to the microenvironment of NSCs is the
transfer of components of the circulation; primarily oxygen, but also a myriad of
other factors. It should be noted that in Drosophila, in which NSCs are bathed in the
blood precursor called haemolymph (Limmer et al. 2014 ), the stereotypic succes-
sive changes in the expression of different transcription factors, which defines the
cellular output of NSCs and which was thought to be entirely cell-autonomous, is
partly regulated by ecdysone, a systemic steroid hormone (Syed et al. 2017 ). The
haemolymph also provides insulin-like peptides; some of which are produced
locally in the brain and some are of systemic origin (reviewed in Liu et al. ( 2014 )).
The same seems to apply for the coupling of the insulin/IGF metabolic pathway to
NSC proliferation in mammals, with low quantities of both insulin and IGF-1 and 2
being able to cross the blood brain barrier and the occurrence of local synthesis (Liu
et al. 2014 ). Notably, changes in local IGF expression lead to changes in brain
growth and size (D’Ercole et al. 2002 ). Another family of hormones that was
recently revealed to regulate proliferation of NSCs, with an evolutionary twist in
their role, are thyroid hormones. Their activity on intermediate progenitors of the
SVZ, via integrin αvβ 3 expressed on their membrane, regulates proliferation; hence,
the size of this progenitor pool and the size of neocortex (Stenzel et al. 2014 ).
During the last decade another area in which increasing amount of work has been
performed is the assessment on how the biomechanics of the tissue can influence the
behaviour of stem cells (reviewed in detail in Lin et al. ( 2016 )). Results from in vitro
assays have shown that specification of both embryonic stem cells and human
induced pluripotent stem cells towards the neuronal fate is affected by the stiffness
of the microenvironment (Keung et al. 2012 ; Kothapalli and Kamm 2013 ) and that
oligodendrogenesis depends on the mechanical properties of the microenvironment
(Jagielska et al. 2012 ). Importantly, in vivo analysis of the developing rodent brain
using atomic force microscopy showed significant changes in the stiffness of the
tissue over time (gradual increase in the VZ/SVZ and decrease in the cortical plate)
and differences between areas (e.g. higher stiffness in the SVZ compared to the VZ)
(Iwashita et al. 2014 ). These data suggest that the triple microenvironment to which
NEP cells and RGCs are exposed during cortical development (as described in
section 6.2.1) also vary in mechanical properties, although it is very difficult to dif-
ferentiate if NSC behaviour is affected primarily by these differences, or just by the
ECM composition variations that also control mechanical properties (Kothapalli
and Kamm 2013 ). In practical terms, in order to achieve maximal results in direct-
ing cell cultures towards the optimal direction, the mechanical properties of the
microenvironment (that include stiffness, elasticity, surface topography and compo-
sition) have to be combined with micro-patterning with specific growth factors,
morphogens and ECM molecules (Kothapalli and Kamm 2013 ), altogether mimick-
ing the complex in vivo situation.
6 Being a Neural Stem Cell: A Matter of Character But Defined...