Detection of transcripts for retinoic acid receptors, retinol-binding protein, and transforming growth factors during rapid trophoblastic elongation in the porcine conceptus

Early conceptus development (Days 10-12) in the pig is characterized by rapid remodeling and elongation of the trophoblastic membrane. Endometrial synthesis and secretion of retinol-binding protein (RBP; transporter of retinol) into the uterine lumen parallels the rapid morphological transformation of the conceptus. Hence, retinoic acid, a powerful morphogen, may serve a critical function in trophoblastic remodeling and conceptus elongation through the various isoforms of retinoic acid receptor (RAR). We hypothesize that various transcripts for RAR will be expressed, possibly differentially, in the early developing porcine conceptus on Days 10-12 of gestation. Conceptuses were collected from pregnant gilts and individually frozen in liquid nitrogen after classification for morphological size. A total of 36 conceptuses (4 groups of 9) representing spherical (2-9 mm), early tubular (10-12 mm), late tubular (13-60 mm), and filamentous (> 100 mm) morphologies were assembled for qualitative assessment of mRNA for RAR alpha, RAR beta, RAR gamma, RBP, and transforming growth factor beta (TCF beta)-2 and -3 with reverse transcription polymerase chain reaction (RT-PCR). RBP gene expression was affected by stage of conceptus development (p < 0.05) as RBP expression increased in a linear fashion from 2-mm to 8-mm spherical conceptuses, decreased slightly in the tubular stage, and increased dramatically in filamentous conceptuses. RAR alpha, RAR beta, and RAR gamma gene expression was evident at all stages of conceptus development. RAR alpha gene expression increased (p < 0.05) in a linear fashion from 2- to 8-mm spherical conceptuses, and expression plateaued in the 9- to 12-mm conceptuses. Expression of RAR alpha declined slightly at the tubular stage but dramatically increased in filamentous conceptuses. RAR beta gene expression was present across all stages of conceptus development although in diminutive amounts. RAR gamma gene expression was evident in all stages of conceptus development but was not affected (p > 0.05) by developmental stage. Gene expression for TCF beta-2 was not detected by RT-PCR at any stage of conceptus development, whereas TCF beta-3 expression increased (p < 0.05) from the 2- to 8-mm conceptuses and remained high throughout trophoblastic elongation into the filamentous stage. The presence of mRNA for the RAR alpha, RAR beta, and RAR gamma in elongating porcine conceptus provides evidence that RAR, particularly RAR alpha, may function as an important acceptor for retinoic acid and possibly regulate gene expression prior to and during rapid trophoblastic elongation in the porcine conceptus.