Morphological and Gene Expression Changes in Cattle Embryos from Hatched Blastocyst to Early Gastrulation Stages after Transfer of In Vitro Produced Embryos

Abstract

A detailed morphological staging system for cattle embryos at stages following blastocyst hatching and preceding gastrulation is presented here together with spatiotemporal mapping of gene expression for BMP4, BRACHYURY, CERBERUS1 (CER1), CRIPTO, EOMESODERMIN, FURIN and NODAL. Five stages are defined based on distinct developmental events. The first of these is the differentiation of the visceral hypoblast underlying the epiblast, from the parietal hypoblast underlying the mural trophoblast. The second concerns the formation of an asymmetrically positioned, morphologically recognisable region within the visceral hypoblast that is marked by the presence of CER1 and absence of BMP4 expression. We have termed this the anterior visceral hypoblast or AVH. Intra-epiblast cavity formation and the disappearance of the polar trophoblast overlying the epiblast (Rauber’s layer) have been mapped in relation to AVH formation. The third chronological event involves the transition of the epiblast into the embryonic ectoderm with concomitant onset of posterior NODAL, EOMES and BRACHYURY expression. Lastly, gastrulation commences as the posterior medial embryonic ectoderm layer thickens to form the primitive streak and cells ingress between the embryonic ectoderm and hypoblast. At this stage a novel domain of CER1 expression is seen whereas the AVH disappears. Comparison with the mouse reveals that while gene expression patterns at the onset of gastrulation are well conserved, asymmetry establishment, which relies on extraembryonic tissues such as the hypoblast and trophoblast, has diverged in terms of both gene expression and morphology.

Beyond the mouse: non-rodent animal models for study of early mammalian development and biomedical research

“…Unlike in mouse and human embryos, gastrulation in cattle (and other ungulates) begins prior to implantation and takes place around 16 dpi. Mural TE proliferation causes further embryo extension, which finally reaches up to 20cm in length before the implantation (Degrelle et al, 2005, van Leeuwen et al, 2015. Due to the differences mentioned above, for a long time, it was believed that bovine/ ungulate blastocyst may not be similar to the known mouse or human model of cell differentiation, lineage-specific signalling pathways activity and/or ICM/TE marker gene expression.…”

The evolution of amniote gastrulation: the blastopore‐primitive streak transition

“…However, information from a wider sampling of mammalian species is essential to understand what features are mammo‐typic. More recently, work in rabbit has started to incorporate culture and live imaging experiments while a handful of studies have begun to provide staging systems, EM data, and molecular insight into gastrulation through the expression pattern of key genes in both cow and pig . These species all have substantial differences in early development, including the size and composition of extraembryonic tissues, timing and manner of implantation, but they all have bilaminar blastodiscs prior to gastrulation and can be considered, topographically, to be broadly more representative of mammalian gastrulation than that of the egg‐cylinder morphology of the rodents.…”

“…These species all have substantial differences in early development, including the size and composition of extraembryonic tissues, timing and manner of implantation, but they all have bilaminar blastodiscs prior to gastrulation and can be considered, topographically, to be broadly more representative of mammalian gastrulation than that of the egg‐cylinder morphology of the rodents. Although the embryos of the species are of approximately similar size prior to gastrulation there are also morphological differences, for example the rabbit blastodisc is a flat oval‐like shape which converts to a pyriform (pear‐like) shape during gastrulation, while in the cow there is an oval blastodisc during the onset of primitive streak formation that proliferates to form a lens‐shaped thickening, and the pig embryo epiblast converts from circular to oval during primitive streak formation . Morphological changes are also seen in the mouse embryo as the cylinder elongates: transverse sections show that it becomes elongated along one axis forming an ellipse‐like shape—the primitive streak then forms at one end of the long axis of the ellipse .…”

“…In addition to Brachyury expression building from a crescent‐like region at the posterior margin of the epiblast in these three species, there is also evidence that key signaling pathways and tissues are also conserved with that of chick and mouse. Each has an equivalent tissue to the hypoblast/AVE—the AMC in the rabbit, the anterior hyboblast (AHB) of the pig, and the anterior visceral hypoblast (AVH) of the cow, all of which have been shown to have topographical and molecular homology although there are differences in shape between these tissues. Furthermore, gene expression patterns, in addition to Brachyury , are also well conserved including Bmp4 , Nodal , Cer1 in the cow, FoxA2 in the pig, and Nodal , Dkk1 , Wnt3 , Chordin in rabbit .…”

“…Each has an equivalent tissue to the hypoblast/AVE—the AMC in the rabbit, the anterior hyboblast (AHB) of the pig, and the anterior visceral hypoblast (AVH) of the cow, all of which have been shown to have topographical and molecular homology although there are differences in shape between these tissues. Furthermore, gene expression patterns, in addition to Brachyury , are also well conserved including Bmp4 , Nodal , Cer1 in the cow, FoxA2 in the pig, and Nodal , Dkk1 , Wnt3 , Chordin in rabbit . Together these studies argue that the signaling pathways, gene expression patterns and extraembryonic tissues involved in gastrulation in these mammalian species has been highly conserved and has adapted to a range of morphologies upon the flat blastodisc.…”