Abstract
The mouse blastocyst consists of the trophectoderm, the inner cell mass, and a fluid‐filled cavity, the blastocoel. Formation and subsequent expansion of this cavity is important for further differentiation of the inner cell mass and successful implantation. Previous work provided evidence that vectorial transport of Na^+^ and Cl^−^ ions through the trophectoderm into the blastocoel generates an osmotic gradient that drives fluid across this epithelium. As the activity of the Na^+^/H^+^ exchanger (NHE) has been implicated as the exchanger responsible for facilitating the transtrophectodermal Na^+^ flux, the functional role of NHE in mouse blastocoel development was determined. Embryos were cultured in the presence of subtype‐specific NHE inhibitors to examine the role of NHEs in blastocoel development. When 2‐cell stage embryos were treated continuously with a specific inhibitor of NHE‐1, cariporide, the embryos passed beyond the 8‐cell stage and became blastocysts. However, in the presence of a specific inhibitor of NHE‐3, S3226, the 2‐cell stage embryos developed to the morula stage but formation of the blastocyst were inhibited in a dose‐dependent manner. Cariporide did not inhibit the formation of the blastocoel cavity from the morula stage whereas S3226 did inhibit that process. S3226 also reduced the rate of re‐expansion of blastocysts collapsed by cytochalasin D upon transfer to the control medium. An immunofluorescence study showed that NHE‐3 was detected in the vicinity of the cell membrane of the trophectoderm, especially in the apical cell margins of the trophectoderm. These results suggest that NHE‐3 is likely involved in blastocyst formation. J. Exp. Zool. 301A:767–775, 2004. © 2004 Wiley‐Liss, Inc.