LAUSR

Molecular left-right patterning of themammalian body plan appears to follow two principally different schedules: In mouse and rabbit asymmetrical, left-sided expression of the gene coding for the signaling molecule nodal starts in the lateral plate mesoderm at early somite stages while in pig and cattle asymmetrical nodal expression is seen in so-called parachordal cells prior to somite formation already. Mechanisms breaking initial symmetry of the bodyplanmay, therefore, differ in these twomammalian groups as is, indeed, suggested by the ciliary flow hypothesis, which is convincingly formulated for mouse and rabbit but precluded to be universal amongst amniotes for the following reason: the ventral, possibly cilia-bearing, surface of the notochordal process in chick, pig and cattle is covered by subchordal mesoderm prior to somite formation and this makes ciliary flow impossible to be required for establishing left-sided nodal expression in these species. The neurenteric canal at the base of the early notochord was recently suggested to be a atavistic characteristic of primate embryos at the time of left-right symmetry breaking and now opens the possibility that motile cilia may exist and protrude into its lumen making primates including man possible candidates for the cilia-bearing, late molecular asymmetry group of mammals. We therefore analyzed nodal expression in a presomite embryo of the New World monkey Callithrix jacchus by in situ hybridization and 3-dimensional reconstruction of serial histological sections and found an early asymmetrical nodal mRNA expression in parachordal cells near the patent neurenteric canal, similar to the situation seen in chick, pig and cattle. Therefore, if notochordal cilia were to exist in this primate their activity would follow, but not cause, leftright patterning as is known for many amniotes to date.