Characterisation of the progenitors for the mouse anteroposterior axis

• Main Author: Cambray, Noemí
• Published: University of Edinburgh 2006
• Subjects: 571.861
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.642336

Retrospective single cell marking experiments indicate a stem cell progenitor for the myotome and the spinal cord in the primitive streak and its descendant, the tail bud. However, the identity and exact location of the progenitors is unclear from these analyses. First, I performed a detailed gene expression analysis using genes known to play a role in axis elongation <i>(T (Brachyury), Fgf8 </i>and <i>Wnt3a) </i>and the homologues of genes expressed in the <i>Xenopus </i>tail <i>(Evx1 (Xhox3), Foxa2(Pintallavis) </i>and <i>Cdx2 (Xcad3). </i>Comparison of the gene expression patterns showed that either these genes were expressed in domains of the primitive streak that corresponded to specific tail bud regions or showed a consistent set of expression domains that was continuous throughout axis elongation. This gives further support to the idea that tail bud formation in the mouse, as shown in other vertebrates, is a continuation of gastrulation. However, in the mouse, we have not detected a new wave of gene expression coinciding with tail elongation, as seen in <i>Xenopus.</i> Secondly, from experiments started during my MRes and finished during my PhD, we have shown that ingression of ectodermal cells to the mesoderm layer continues even after posterior neuropore closure around the 35­somite stage, as previously observed in chick. Furthermore, grafting of one of the regions of the tail bud, named the chordoneural hinge (CNH) region by analogy to an equivalent structure in Xenopus and chick, showed it had properties expected of a stem cell population. When transplanted to earlier (8.5dpc) embryos, which were then cultured in vitro during a period of extensive axis elongation, the CNH could incorporate to all dorsal host axial tissues and still retain progenitor cells in the tail bud. Indeed, these cells could be serially passaged through 3 successive generations of embryos without apparent loss of their ability to differentiate and retain progenitors in the tail bud. Therefore, the CNH has the characteristics of a population of axial stem cells. A third set of experiments was then performed to localise the putative axial stem cells at earlier stages. Using isotopic grafts to 8.5 dpc embryos, I tested the fate of the node region itself, the region just posterior to it that represents the border between the node and streak (named border) and the anterior primitive streak (APS) region, and found that only cells from the border region contribute to all the dorsal axial tissues and to the CNH, while its neighbours showed a much restricted set of fates. Taken together, these results point to the existence of axial stem cells with equivalent gene expression that reside in the border region at primitive streak stages and later in the CNH in the tail bud.