Breaking of symmetry in the early embryo is a crucial developmental event. The Nodal signalling pathway is very important in establishment of the left-right (L-R) body axis and is highly conserved among vertebrates. In the mouse, motile cilia in the node of the embryo are responsible for moving secreted proteins from the pathway towards the left side of the embryo. These cilia are located at the posterior end of each cell, which favours a cilium-mediated flow of extracellular proteins towards the left side. The internal structure of motile cilia consists of 9 pairs of microtubules in an outer circle plus a central pair, giving them a (9+2) constellation. Immotile/sensory cilia lack the central pair of microtubules (9+0). In the chicken, motile cilia are not present in the node, in contrast to in most other vertebrates. The tammar wallaby, a marsupial, would be expected to resemble eutherian mammals. However, the presence of certain pathway genes and their sequences in the tammar is more similar to that of the chicken.
Using Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and immunohistochemistry (IHC) we investigated whether the tammar has motile cilia in the node, as in the mouse and most other vertebrates, or whether it resembles the chicken in the establishment of the L-R axis. We found that cilia were located at the posterior end of the cells in the node at tammar embryonic stages ranging from the mid-primitive streak to the early head fold stage. These cilia had a (9+2) confirmation, demonstrating that they are most likely motile cilia. Thus our results show similarities with both the eutherian and avian species, showing that the tammar wallaby has its own unique mechanism for establishing the L-R body axis.