Stem cells niches during development--lessons from the cerebral cortex.

Adult stem cells are typically dependent on their specific niche environment, here we discuss to which extent this is applicable in the embryonic cortex. During development, signals regulating both proliferation and differentiation are derived from local sources within the stem and progenitor cell zone of the cerebral cortex as well as from extrinsic sources, such as the meninges, blood vessels and the cerebrospinal fluid. However, neural stem cells isolated as single cells in vitro self-renew and progress through a normal sequence of lineage decisions in the absence of the above signals. Taking these data together we propose a concept of intrinsic specification of the major lineage decisions with environmental niche signals influencing quantitative aspects, such as numbers of stem cells and timing of lineage progression, which are the parameters affected during brain evolution.