| In this thesis, I describe studies identifying and characterizing two putative stem cell populations of the neural and pancreatic lineages. The mechanisms governing the emergence of the earliest mammalian neural cells during development and the ontogeny of neural stem cells remain incompletely characterized. A default mechanism has been suggested to underlie neural fate acquisition, however an instructive process has also been proposed. I utilized mouse ES cells to explore the fundamental issue of how an uncommitted, pluripotent mammalian cell will self-organize in the absence of extrinsic signals, and what cellular fate will result. Individual ES cells were found to rapidly transition directly into neural cells by a default mechanism, a process shown to be independent of suggested instructive factors. Further, I provide evidence that the default neural identity is that of a primitive neural stem cell, the earliest identified stem cell of the neural lineage. The exiguous conditions used to reveal the default state were found to present primitive neural stem cells with a survival challenge, which could be mitigated by survival factors or genetic interference with apoptosis. I also report the clonal identification of multipotent precursor cells, PMPs, from the adult mouse and human pancreas. These cells proliferate in vitro to form clonal colonies and display both pancreatic and neural cell multipotentiality. Importantly, the newly generated beta cells demonstrate glucose-dependent Ca2+ responsiveness and regulated insulin release. PMP colonies do not express markers of embryonic stem cells, nor genes suggestive of mesodermal or neural crest origins. Moreover, genetic lineage-labeling experiments excluded the neural crest, and established the embryonic pancreatic lineage, as the developmental source of PMPs. The PMP cell was further found to express insulin in vivo, and insulin+ stem cells were shown to contribute to multiple pancreatic and neural cell populations in vivo. These findings demonstrate that the adult mammalian pancreas contains a population of insulin+ multipotent stem cells, capable of contributing to the pancreatic and neural lineages. In the final section of this thesis, I consider the relationships between neural and pancreatic tissues, as well as discussing the relevance of these two novel stem cell populations. |
