Synthesis and Release of Polypeptides by Pig Conceptuses During the Period of Blastocyst Elongation and Attachment12

Pig blastocysts, cultured in a modified minimal essential medium in the presence of radioactive L-leucine, released radiolabeled macromolecules into the medium at a linear rate for at least 24 h. Proteins released into the medium by blastocysts during the period of expansion through attachment were identified by two-dimensional polyacrylamide gel electrophoresis and fluorography. Spherical (4–9 mm), tubular (10–50 mm) and early filamentous blastocysts (>100 mm length), recovered between Days 10.5 and 12 of pregnancy, all released, as their major products, a group of low molecular weight (Mn) acidic polypeptides (Mr 20,000–25,000; p1 5.6–6.2). By Day 13 the pattern of protein synthesis changed markedly and the major proteins detected between Days 13 and 16 were basic and in the molecular weight range of 35,000–50,000. In addition, a high molecular weight glycoprotein of Mr >650,000 was released. The major basic polypeptide (Mr ≃45,000) and the high molecular weight glycoprotein have been isolated from Day 16 blastocyst incubations. After Day 18, a distinctive group of new polypeptides in the 50,000–70,000 molecular weight range were synthesized. These correspond in electrophoretic mobilities to fetal serum proteins, including transferrin, α-fetoprotein and fetuin, and appear to be primarily products of the embryo and/or yolk sac. Explant cultures of Day 25 and 30 chorioallantois cultured without embryonic tissue released three major polypeptides into the medium which were distinct from any synthesized during the preattachment period (Days 10.5–16). Although little is known about the physiological role of conceptus proteins produced early in pregnancy, the identification and isolation of these products is a necessary first step in determining their function.