Post-hatching development of the porcine and bovine embryo—defining criteria for expected development in vivo and in vitro
Introduction
While there seems to be no scarcity in the literature describing in vitro production (IVP) of bovine embryos up to the stage of hatching from the zona pellucida, relatively little work has been done on characterizing the events leading into the post-hatching process of gastrulation in embryos of the domestic species. This period is of outmost interest for at least two major reasons.
First, it is clear, at least in cattle and pigs, that embryos derived by different technologies such as IVP or cloning by somatic cell nuclear transfer display molecular deviations despite appearing rather normal from the light microscopical point of view. Hence, blastocyst development is a poor indicator of embryo viability. Although blastocyst formation requires initial steps of cell differentiation, this process is far less complex and demanding than the subsequent phase of post-hatching embryonic development. The prospect of being able to support post-hatching embryonic development in vitro would potentially allow for establishment of tools for evaluating accurately the developmental potential of embryos without using recipient animals for transfer. Such an evaluation would require (1) definition of milestones of expected post-hatching embryonic development in vivo; and (2) development of suitable culture systems allowing for such development to take place in vitro. Currently, the in vitro development of bovine embryos beyond hatching allows for establishment of hypoblast and contemporary signs of shedding of the polar trophoblast, i.e. Rauber's layer. Moreover, elongation may be achieved upon physical constraints in agar tunnels [1], [2]. However, further development of the epiblast is compromised. In the pig, IVP of embryos is still hampered by the high incidence of polyspermy and low rates of pronuclear formation as well as suppressed subsequent development of the embryos, at least when compared to cattle [3], [4], [5]. Thus, in this species, development of embryos in vitro after hatching poses even more scientific challenges than in cattle.
Second, under in vivo conditions, the majority of embryonic and fetal mortality in both pigs and cattle occurs within the post-hatching period when the embryo accomplishes a complex development and a critical dialogue between the embryo and the dam decisive for continued pregnancy to be established [6], [7], [8], [9].
The first part of this review includes an up-to-date description of the morphology of post-hatching or pre-implantation development in pigs and cattle with special attention to the period around gastrulation. Molecular insights from the mouse and rabbit on especially the pre-gastrulation period are included. Moreover, a simple stereomicroscopical staging system for post-hatching embryos is presented. More knowledge in domestic species on these stages of development may provide clues to overcome the present obstacles, as discussed in the second part of this review, in contemporary post-hatching in vitro culture systems.
Section snippets
Stages in the post-hatching development of bovine and porcine embryos in vivo
Here, a brief summary of the post-hatching development of porcine and bovine embryos is presented with the aim to define common landmarks on which to stage specimens by simple stereomicroscopy. Such a precise staging of embryos is critical when data on morphological aspects of embryonic development are to be compared with data gained from molecular analyses of, e.g. gene expression where the morphology of the embryo may be lost. Due to the fact, firstly, that the rate of embryo development
Development of post-hatching bovine and porcine embryos in vitro
Extended in vitro culture of bovine post-hatching embryos has been achieved after IVP [2], [57]. To establish a culture system allowing for post-hatching development, a three-dimensional structure inducing elongation and appropriate supporting medium for further in vitro development are crucial points. A miniature agarose gel composed of micro tunnels of 60 mm length and 1.2 mm width has been prepared for physical induction of elongation. The induction of rapid growth after hatching is attributed
Conclusion
The development of porcine and bovine embryos after hatching includes a complex cascade of cell differentiation events where certain compartments of the embryo induce others to differentiate. These processes include (1) the differentiation of ICM to hypoblast and epiblast; (2) the removal of the polar trophoblast covering the epiblast resulting in the establishment of the embryonic disk; (3) the differentiation of the epiblast to the three basic germ layers, i.e. ectoderm, mesoderm and
Acknowledgements
The authors would like to thank Jytte Nielsen for assistance with image processing and the personnel at the abattoir in Skaerbaek for their kindness and interest in relation to the embryo collection. The study was supported by the Danish Agricultural and Veterinary Research Council and the National Committee for Pig production.
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