Expression patterns of signalling molecules and transcription factors in the early rabbit embryo and their significance for modelling amniote axis formation

Ruben Plöger,Christoph Viebahn

doi:10.1007/s00427-021-00677-w

Abstract

The anterior-posterior axis is a central element of the body plan and, during amniote gastrulation, forms through several transient domains with specific morphogenetic activities. In the chick, experimentally proven activity of signalling molecules and transcription factors lead to the concept of a ‘global positioning system’ for initial axis formation whereas in the (mammotypical) rabbit embryo, a series of morphological or molecular domains are part of a putative ‘three-anchor-point model’. Because circular expression patterns of genes involved in axis formation exist in both amniote groups prior to, and during, gastrulation and may thus be suited to reconcile these models, the expression patterns of selected genes known in the chick, namely the ones coding for the transcription factors eomes and tbx6, the signalling molecule wnt3 and the wnt inhibitor pkdcc, were analysed in the rabbit embryonic disc using in situ hybridisation and placing emphasis on their germ layer location. Peripheral wnt3 and eomes expression in all layers is found initially to be complementary to central pkdcc expression in the hypoblast during early axis formation. Pkdcc then appears — together with a posterior-anterior gradient in wnt3 and eomes domains — in the epiblast posteriorly before the emerging primitive streak is marked by pkdcc and tbx6 at its anterior and posterior extremities, respectively. Conserved circular expression patterns deduced from some of this data may point to shared mechanisms in amniote axis formation while the reshaping of localised gene expression patterns is discussed as part of the ‘three-anchor-point model’ for establishing the mammalian body plan.

Highlights

As forerunner of the cranio-caudal body axis, the anteriorposterior (a.-p.) axis of the vertebrate embryo emerges through a series of transient morphological hallmarks and molecularly defined zones during early gastrulation
The four genes selected for this study reveal specific axis and cell layer-related expression patterns and may be relevant for the models considered here in the following way: Wnt3 and eomes are expressed in both epi- and hypoblast in a marginal zone (MZ, Fig. 4) at the beginning of axis formation, and at mid-axis formation, they show a basket-like pattern complementary to pkdcc expression in the hypoblast of a central zone (CZ, Fig. 4)
Wnt3 expression described to start at stage 2 and not a stage 0 in the rabbit by Yoshida et al (2016) is in contrast to the present study and may be explained by (1) the dedicated low-background protocol of the present study, which allows exposure to the staining reaction for more than a week, and (2) by the direct comparison of sagittal sections at early stages which reveal subtle but stage-specific histological characteristics of the anterior margin

Summary

Introduction

As forerunner of the cranio-caudal body axis, the anteriorposterior (a.-p.) axis of the vertebrate embryo emerges through a series of transient morphological hallmarks and molecularly defined zones during early gastrulation (see Wolpert et al 2015). The interaction between activating genes of the ‘dual’ GPS and genes of the concentric system normally initiates PS formation in the posterior marginal zone (pMZ) (cf Skromne and Stern 2001) or, formation of multiple PSs (and twinning) in the case of misexpression of activating genes in ectopic parts of the marginal zone (Shah et al 1997; Skromne and Stern 2001) In support of this sequence of events, the ring-like patterns of wnt8c and tbx (Torlopp et al 2014; Lee et al 2020), on the one hand, develop a marked posterior-to-anterior gradient originating from the posterior pole, while eomes, a conserved T-boxrelated transcription factor heavily involved in early axis related events (Arnold et al 2008; van Leeuwen et al 2015), on the other hand, shows a peripheral, partially extraembryonic, ring-like pattern in addition to its posterior expression domain in the pMZ (Pernaute et al 2010)

Methods

Results

Conclusion