Paraxial <b><i>Nodal</i></b> Expression Reveals a Novel Conserved Structure of the Left-Right Organizer in Four Mammalian Species

Abstract

Nodal activity in the left lateral plate mesoderm is a conserved sign of irreversible left-right asymmetry at early somite stages of the vertebrate embryo. An earlier, paraxial nodal domain accompanies the emergence and initial extension of the notochord and is either left-sided, as in the chick and pig, or symmetrical, as in the mouse and rabbit; intriguingly, this interspecific dichotomy is mirrored by divergent morphological features of the posterior notochord (also known as the left-right organizer), which is ventrally exposed to the yolk sac cavity and carries motile cilia in the latter 2 species only. By introducing the cattle embryo as a new model organism for early left-right patterning, we present data to establish 2 groups of mammals characterized by both the morphology of the left-right organizer and the dynamics of paraxial nodal expression: presence and absence of a ventrally open surface of the early (plate-like) posterior notochord correlates with a symmetrical (in mice and rabbits) versus an asymmetrical (in pigs and cattle) paraxial nodal expression domain next to the notochordal plate. High-resolution histological analysis reveals that the latter domain defines in all 4 mammals a novel ‘parachordal' axial mesoderm compartment, the topography of which changes according to the specific regression of the similarly novel subchordal mesoderm during the initial phases of notochord development. In conclusion, the mammalian axial mesoderm compartment (1) shares critical conserved features despite the marked differences in early notochord morphology and early left-right patterning and (2) provides a dynamic topographical framework for nodal activity as part of the mammalian left-right organizer.

On the Enigma of the Human Neurenteric Canal

“…g 50 µm. primitive streak, this mesenchymal axial mesoderm will be addressed here as SCM in accordance with observations made in the chick [Tsikolia et al, 2012] and several non-primate mammals [Schröder et al, 2016]. The SCM itself is covered -similar to the primitive streak mesoderm further posteriorly -by a single layer of squamous hypoblast cells (Fig.…”

“…42502-04-12/0708) [Aeckerle et al, 2015]. Parts of the embryos not required for this purpose were used for this study and subjected to different embedding protocols after dissection: embryo GM79.01 (recovered at 46 dpc and classified as stage 8b) was dehydrated in a graded series of ethanol and stored in ethanol at -20 ° C until its later use for whole-mount in situ hybridization (WISH, see below), after which Technovit ® 8100 (Heraeus Kulzer GmbH, Wehrheim, Germany) served as embedding medium for high-resolution histology and cellular localization of nodal mRNA expression as described previously [Schröder et al, 2016]. Sections (5 µm) were transferred in pairs onto glass slides and a few glass slides were used for test staining (their WISH staining reaction remaining accidentally unphotographed) to identify the staining technique which would enable easy identification of cellular borders and thus detailed 3-dimensional reconstruction (3DR) afterwards (s.b.).…”

“…Schau insland, 1903] -may thus be suitable to serve as a new model organism for notochord development and neurulation (see below) in primates, in particular. Finally, the primarily asymmetrical nodal expression in the stage 8 Callithrix embryo together with morphological asymmetries in the node suggests that primates make use of an early non-cilium-dependent left-right symmetry breaking mode similar to the situation in the chick, pig, and cattle [Schröder et al, 2016].…”

“…The notochordal canal is a longitudinal canal between the (epithelial cells of the) developing notochordal plate dorsally and mesenchymal cells of the axial mesoderm ventrally [van Beneden, 1888;Sternberg, 1927;Heuser, 1932]. This ventral stretch of the axial mesoderm was recently found also in the chick (where a typical notochordal canal has not been described so far) and was coined subchordal mesoderm (SCM) in contradistinction to the notochordal plate lying dorsal to it [Tsikolia et al, 2012]; its presence was soon confirmed in several mammalian species such as rabbit, pig, and cattle, and furthermore shown to be topographically related to an early non-cilium-dependent molecular left-right asymmetry [Schröder et al, 2016]. During further development in mammals, the notochordal canal loses its ventral wall as the SCM disintegrates through an as yet unknown mechanism with the effect that the notochordal plate has direct contact to the yolk sac cavity [Heuser, 1932].…”

Cilia in vertebrate left–right patterning

“…Indeed, cell-tracking experiments revealed that asymmetric cell movements around Hensen's node set up asymmetric signalling in the chicken embryo [81]. Intriguingly, the node in the pig embryo also lacks cilia [81] and, unlike the mouse ventral node that is exposed to extraembryonic fluid, pig node cells are covered by a layer of cells referred to as subchordal mesoderm [82]. The node of the cow embryo is also enclosed in subchordal mesoderm [82], suggesting multiple vertebrates use ciliaindependent mechanisms to establish LR asymmetry.…”