The surface of the human brain has many folds (is gyrified), quite unlike the smooth surface of the mouse brain. This gyrification correlates with the increase in size of the neocortex, the part of the brain responsible for most higher order functions such as vision and speech. During development, the outer subventricular zone in higher primates expands dramatically due to increased cell proliferation. The progenitor cells in this region, described so far only by their morphology and position, have been thought to be responsible for this dramatic expansion. The progenitors themselves are a very small percentage of all the cell types that are found in this region and hence have been difficult to define molecularly. While they express transcription factors such as Sox2 and Pax6, we did not know much else about them.
Purifying these progenitor populations has been difficult due to a lack of surface markers that identify them. Antibody staining of transcription factors leads to the degradation of RNA and thus intracellular antibody staining has not been usable to target single cells for RNA profiling. We have developed a method for extracting very high quality RNA from single cells even after staining them with antibodies for intracellular proteins. This technique allows us to purify cells expressing Sox2 and Pax6 from human fetal tissue, and identify different classes of progenitor cells. In particular we identify molecular markers for the outer radial glial cells thought to be responsible for driving proliferation in the subventricular zone. The marker HopX is a transcriptional cofactor, whose mis-regulation has been previously associated with colon cancer.
We believe that our findings are the first steps toward identifying and understanding novel progenitor cell types that are specific to higher primates. We also believe our method will be useful in working with patient tissue, allowing for much better transcriptional profiling and storage than is currently possible.
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