Fate-map analysis of the epiblast of the dasyurid marsupial Sminthopsis macroura (Gould).

Allocation of cells in the marsupial epiblast to embryonic and extra-embryonic domains has to date been studied only histologically. An unresolved issue in marsupial embryology has been the existence of a medullary plate. We re-examined the hypotheses that the medullary plate, or neurectoderm, arises before notochord formation and that the integumentary ectoderm is segregated from the ectoderm after the formation of the medullary plate. By marking epiblast cells in 65 Day-8 embryos of the dasyurid marsupial Sminthopsis macroura, with the lipophilic cell-surface marker, DiI, we demonstrated that the so-called medullary plate contains progenitors of the integumentary ectoderm of the neck, chest, limbs and flank of the embryo. Thus, cell-allocation processes in the epiblast must have segregated the entire complement of embryonic ectoderm in one event, not separate events. It is concluded that the embryonic structure called 'medullary plate' in marsupial embryos is misnamed since, in fact, it consists of both integumentary ectoderm and neurectoderm.