Timing of Blastocyst Expansion Affects Spatial Messenger RNA Expression Patterns of Genes in Bovine Blastocysts Produced In Vitro1

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Abstract
Blastocyst formation and expansion are dependent on the differentiation and function of a proper transport of nutrients through the trophectoderm (TE) enclosing the inner cell mass (ICM). Coincident with compaction and cavitation, glucose becomes the preferred energy substrate of the early embryo. These hallmarks in early development require well-orchestrated gene expression patterns specifically with regard to timing and localization. The present study investigated the relative abundance (RA) of gene transcripts in the two lineages of in vitro-produced expanded bovine blastocysts in relation to timing of development, i.e., blastocyst expansion and localization of specific mRNAs. Expanded blastocysts from either Day 7 or Day 8 or isolated ICMs derived thereof were analyzed with the aid of a semiquantitative reverse transcriptase-polymerase chain reaction assay for gene transcripts, which are thought to play a pivotal role in blastocyst expansion, i.e., Na/K-ATPase α1 subunit (Na/K), E-cadherin (E-cad), zonula occludens protein-1 (ZO-1), desmocollin II (Dc II), plakophilin (Plako), trophoblastic function (interferon τ [IFτ]), and glucose transport (glucose transporter-1, -3, -4 [Glut-1, -3, -4]). Total cell number, ICM cell number, or ICM/total cells ratio were similar in Day 7 and Day 8 expanded blastocysts. Significant differences were determined in the RA for Na/K, E-cad, Dc II, Plako, and ZO-1 transcripts between TE cells of expanded blastocysts derived from either Day 7 or Day 8. The RA of Dc II, Glut-1, and Glut-4 was significantly decreased in the ICM compared with the TE at Day 7. Similarly, the RA of Na/K, Dc II, Glut-1, and Glut-4 at Day 8 of development was significantly decreased in the ICM compared with the TE. Interestingly, no differences were observed when comparing ICMs originating from blastocysts expanded at either Day 7 or Day 8. Plako and IFτ transcripts were not detected in isolated ICMs, indicating that expression of these mRNAs is restricted to the TE. In contrast, similar expression patterns within the ICM and TE were determined for Na/K, E-cad, ZO-1, and Glut-3 mRNA. Dc II, Glut-1, and Glut-4 were more abundant in the TE than in ICM. Results show that expression of developmentally important genes is related to the two cell lineages in the early embryo and emphasize the critical role of a well controlled spatial gene expression pattern for regular preimplantation development.