EARLY GESTATIONAL EXPRESSION OF TRANSFORMING GROWTH-FACTOR-BETA ISOFORMS BY THE OVINE PLACENTA

The ruminant blastocyst metamorphoses from a 1-mm sphere to a 1 X 190-mm thread between Days 12 and 15 of gestation. The transforming growth factor beta (TGF beta) family of polypeptide growth factors are potential regulators of embryonic elongation because of their ability to regulate cellular replication, differentiation, and extracellular matrix formation. In mammalian development three isoforms of TGF beta-TGF beta 1, beta 2, and beta 3- have been identified. Through the use of isoform-specific probes, ovine embryonic TGF beta 1 and beta 2 transcripts were identified and characterized in the present study. TGF beta 1 was expressed as a single transcript 2.6 kb in length. Levels increased by 14.8-fold from Day 13, when levels were barely detectable, to Day 27, when they were highest (p = 0.0001). Enriched samples of Day 20 chorionic and allantoic membranes demonstrated that the allantoic membrane contained 3.9-fold greater levels (p = 0.001) of TGF beta 1 message than the chorion. TGF beta 2 was expressed as five transcripts 6.2, 5.2, 4.8, 4.0, and 2.7 kb in length. From Day 13 to Day 23, there was a 6.2-fold increase (p = 0.0005) in TGF beta 2 expression; thereafter, expression declined 20% by Day 30 (p = 0.03). Extra-embryonic membrane expression of TGF beta 2 was slightly greater (1.5-fold; p = 0.01) in allantois than in chorion. Expression of TGF beta 3 was not detectable by Northern blotting, and only trace quantities of TGF beta 3 transcript were detected by slot-blot analysis. Antisera specific to TGF beta 1, beta 2, and beta 3 were used to immunolocalize the proteins within tissues. TGF beta 1 and beta 2 were identified in Day 16 trophectoderm and yolk sac. Both were also localized in chorion, allantois, and placental endothelium through Day 30. TGF beta 3 protein could not be detected in any conceptus tissue at any of the stages examined. The increase in expression of TGF beta 1 and beta 2 mRNA coincidental with conceptus elongation and placental development, as well as tissue localization of the protein, suggests their involvement in early gestational development in sheep.