At what point during plant development the epidermal cell layer is specified remains an open question. In many angiosperms, including the model species Arabidopsis thaliana, embryonic cytokinesis commences with an asymmetric division partitioning the fertilized zygote into a small, highly cytoplasmic, apical cell and a larger basal cell. In Arabidopsis, the progeny of the apical cell are destined to give rise to most of the cells in the mature plant, including all epidermal cells, whilst one descendent from the basal cell forms the organising centre (quiescent centre) of the root meristem (Jurgens 2001). In Arabidopsis an outer cell layer (or protoderm/dermatogen) is demarcated after just four rounds of cell division in the apical cell. After this so-called dermatogen stage, protodermal cells undergo principally anticlinal cell divisions and thus only give rise to more protodermal cells (Goldberg 1994; Willemsen and Scheres 2004). The point at which protodermal demarcation occurs during embryogenesis in different plant species depends largely upon patterns of cell division early in embryogenesis. The division of the embryos in other dicotyledonous species, such as cotton (Pollock 1964), Citrus (Bruck 1985a) and apple (Meyer 1958), and of monocotyledonous species, such as maize (Randolph 1936) and barley (Merry 1941), lack the pleasing predictability of those observed in Arabidopsis. Despite this they lead to the production of an organised protodermal cell layer, albeit in an embryo comprising many more than 16 cells. Moreover, disruption of the cell division patterns of the early Arabidopsis embryos in some developmental mutants, so that dermatogen demarcation occurs later or asynchronously in different embryonic regions, does not appear to affect the propensity of embryos to produce a discrete protoderm and epidermis later in development (Torres-Ruiz and Jurgens 1994). Thus, acquisition of protodermal identity appears solely a function of "outside" cell position. The question of the positional information required for the specification of protodermal identity is complex, and will be addressed later.
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