tightly organized and controlled. It suggests that these character- istics may have developmental origins. SE elongation by both cell rearrangement and cell proliferation Spacing and mediolateral cell intercalation. The spacing of the clonally related P-DVCUs is characteristic of dispersive growth during the first phase of SE formation. Since, in addition, P-DVCUs are organized into a small number of sectors (manifest as the few discrete DV positions possible for the lines along which P-DVCUs are spaced), this implies a special cell rearrangement process. In the absence of cell rearrangement, clonally related cells would remain close, in columns or stripes, as they do later in the DV stripes and as other cells do in other systems (the clonal columns in the cortical structures of the CNS [40,41] and the longitudinal columns of the tubular structure of the kidney [42,43]. Visibly, in the SE, this is not the case: each sector has a polyclonal origin and the cells of the polyclone rearrange. For the rearrangement to produce lines that are parallel to the axis of the embryo the underlying process must be ordered. It must separate daughter cells without changing their relative DV position. As this ordered process involves several adjacent sectors, it must act throughout the DV dimension of the structure. In the trunk and the tail the spacing is stronger dorsally than ventrally, suggesting that the process involved is oriented towards the midline. An obvious consequence of this spacing is elongation of the head, trunk and tail SE. In lower vertebrates AP extension is due to convergent extension [2,44,45,46] by medio-lateral intercala- tion during gastrulation. Examples include the zebrafish [1,47] and Xenopus [48], in which convergent extension has many similarities with the process described above: most importantly, it respects the medio-lateral order of cells [47]. In the mouse convergent extension has been proposed to play a role in the developing neural plate [49,50]. So the most obvious explanation for the spacing of the clonally related P-DVCUs of the SE, their organization in lines parallel to the AP axis and the DV sectorisation, is convergent extension, a hypothesis that can now be tested. Several observations suggest that the process at the origin of the spacing of P-DVCUS follows a spatio-temporal progression from anterior to posterior. The smallest clones induced at E6.5–E7.5 in the head have only one or two P-DVCUs although those in the trunk or the tail have numerous P-DVCUs indicating that the spacing stopped in the head at a stage when it is still operating in the trunk and the tail. This suggests a caudally oriented progression of the initiation of the spacing. We conclude that the spacing of the PUCDVs is very likely the consequence of medio-lateral convergent extension of the SE. As the SE of the posterior territories (the tail) is organized into sectors and shows similar spacing, convergent extension continue even after the regression of the primitive streak and the closure of the neuropore. Combination of distinct modes of cell proliferation with spacing. The analysis of the mode of growth of the cells producing the P-DVCUs suggests a sequential mode in the trunk: production from a pool of caudally positioned cells that self renews during the process and produces the P-DVCUs anteriorly. The caudally polarized clones that start in the middle of the trunk, in the E6.5 lox-LacZ library, indicate that this system produces the P- DVCUs for the anterior trunk by E7.5. However, as no clones corresponding to only one P-DVCU were found in the anterior trunk, spacing is not yet started. Consequently production and spacing of the P-DVCUs are likely two separate and sequential operations. Both progress from anterior to posterior. Surface Ectoderm Morphogenesis In the head, the production of the P-DVCUs is regional. If the spacing of the cells in the head uses the same cellular operations as in the trunk these operations could be combined with different modes of division, suggesting a modular organisation [51,52] of these operations. Hair follicle morphogenesis is another system for which a separation of growth and cell rearrangement has been observed [20]. A reorganisation of the pool of producers of P-DVCUs at the rhombencephalon level The organization of clonally related DVCUs in certain DV positions suggests DV organization in the pool of P-DVCU producer cells. This DV organization is established very early: clones from the ancestral and founder cells of SE already exhibit a DV restriction. This organization is maintained during the elongation of the head and of the trunk, as demonstrated by most clones having only one line parallel to the AP axis of the embryo in each of these structures. This striking observation shows that the process of elongation is not accompanied by random cell mixing along the DV axis, but respects some sort of coherence. This coherence could be part of the process of oriented AP elongation and/or preliminary DV organization. Many lines continue unchanged in the head and the trunk and in particular in the more dorsal regions, revealing a continuity of properties. This is accompanied by a clonal continuity between the pools of cells that produce the head and the trunk: indeed, the pool that produces the trunk P-DVCUs is probably entirely derived from cells that have produced head P-DVCUs. Nevertheless neither the clonal nor the cellular organization is totally conserved. First, not all cells of the head contribute to the trunk pool, but mainly the dorsal and dorso-lateral ones. Second, many of these cells move ventrally (the so called shifts). These indicate that there is a transition involving cell rearrangement and the acquisition of novel properties (such as a change in the fluidity of the tissue). This transition requiring a change in the developmental programmes of the cells occurs at the level of the rhombencephalon, that is, before the production of the anterior trunk. It may be as early as the initiation of gastrulation, raising the possibility that the trunk pool is organized by signals from the anterior primitive streak. The transition may accompany the individualization of the trunk organizer from a structure that contains the anterior organizer [8] and controls early SE development in the head. This would require novel programming of the precursor cells of the SE (acquisition of a novel mode of growth and for a short period of time, novel dispersal behaviour). It should be noted that the transition occurs in a context of a closed SE pool: all cells that contribute to the trunk SE are cells that have previously produced SE. Clearly, clonal continuity in this case does not mean continuity of cell properties. The precursor cells of a structure can change. Whether these changes occur in concert with the evolution of other long-term axial progenitors [14,16,53,54,55,56,57] in particular neural progenitors with which SE progenitors share many properties, can now be experimentally tested. Dorso-ventrally oriented stripes The DV oriented stripes of cells are a general characteristic of the growth of the head, the trunk and the tail SE. This type of oriented growth implies a particular behaviour: either oriented cell division [13,58] and/or realignment of daughter cells after mitosis [59]. In both cases, this requires planar polarization of the epithelium, suggesting that the planar cell polarity pathway [60,61] may be involved. This planar polarity of the whole PLoS ONE | www.plosone.org 12 February 2009 | Volume 4 | Issue 2 | e4353