In ruminants, early embryonic development involves a unique step, elongation: before implanting, the conceptus leaves its ovoid form to become first tubular, then filamentous, while the extra-embryonic tissues develop and differentiate in preparation for implantation. This process, orchestrated by numerous genes and signaling pathways, can be disrupted when the conceptus is shorter than expected or when the embryo develops after splitting. To dissect the molecular mechanisms of elongation in cattle, the researchers relied on trophoblastic vesicles: short tubular fragments of extra-embryonic tissue lacking an embryo but retaining the ability to elongate in vivo. Until now, however, these vesicles had not been subjected to molecular analyses at the ovoid or filamentous stages capable of shedding light on the expression changes involved.
To address this gap, the team collected bovine conceptuses developed in vivo, from the ovoid stage (day 12) to the filamentous stage (day 18), then cut them into small fragments with or without their embryonic disc before transferring them into a receptive bovine uterus in order to assess their elongation capacities. Spherical blastocysts cultured in vitro until day 8 were subjected to the same treatment. Gene expression profiles were then compared between the different samples and normally elongating controls, at several stages, using a bovine microarray (10,000 genes) and an expanded qPCR panel covering 224 genes distributed across 24 pathways.
In vivo, the extent of vesicle elongation depended on the stage at which they had been created and the time spent in utero. Their daily elongation rates differed from those of control extra-embryonic tissues, sometimes resembling those of tissues at earlier stages. Overall, the molecular signatures of the vesicles followed a developmental trajectory comparable to that of intact tissues between days 12 and 18. Within each stage, however, vesicles and intact tissues displayed distinct expression dynamics, some of which were shared with other short epithelial models.
According to the authors, these discrepancies likely result from several factors: a reduction in the length and signaling capacities of the vesicles, elongation delayed by inadequate uterine signals, and an alteration of the dialogue between the conceptus and the uterus. This work confirms that close coordination between uterine, embryonic, and extra-embryonic tissues is required to orchestrate proper elongation. The only partial differentiation observed also raises questions about the presence or absence of certain developmental signals, or even their asynchronies.