Gene expression in elongating and gastrulating embryos from ruminants

In ruminants, more than 30% of the embryonic losses observed after artificial insemination (AI) have an early origin, coincident with a marked elongation of the trophoblast which occurs before implantation. Several observations provide clear evidence that early elongation of the conceptus relies on cell multiplication, cell growth and cell shape remodeling. Recent results indicating an intense multiplication of a non-fully differentiated trophoblast, which still expresses some epiblast genes, has to be considered at the onset of elongation. It has also been shown in the last two years that general metabolism and protein trafficking are characteristic of the onset of elongation whereas cellular interactions, cell to cell signaling and cell adhesion become predominant at the end of elongation. Accordingly, expression of most of the single genes identified so far increases during elongation and is related to the establishment of embryo-maternal exchanges before implantation. However, not much is known of what controls the induction of the elongation process or the coordinated development of the embryonic and extra-embryonic tissues. This review highlights new information on this developmental phase and summarizes the views on the complex cross-talk among molecules which might govern conceptus development and lead to successful implantation.