Establishment of Pregnancy in the Pig: II. Cellular Remodeling of the Porcine Blastocyst During Elongation on Day 12 of Pregnancy12

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Abstract
Morphology of pig blastocysts changes dramatically just preceding initial attachment of the trophoblast to the uterine epithelium. Blastocysts undergo a rapid transition from spherical to tubular and elongated filamentous forms between days 10 to 12 of pregnancy. Rates of blastocyst development and possible mechanism(s) involved with changes in blastocyst morphology were investigated. Estimates of blastocyst growth were determined by removing the uterine horns from each animal at differnet intervals. Estimates of blastocyst growth were 0.25 mm/h for development of spherical blastocysts from 4 to 9 mm in diameter; blastocysts elongated at a rate of 30-45 mm/h after reaching a diameter of 10 mm. Although elongation from 10 to 150 mm occurred within a few hours, an increase in cellular hyperplasia (mitosis) was not detected. Although blastocyst content of DNA and RNA did increase between days 10 and 16 of pregnancy (P < 0.01), total DNA was not different between spherical (9-10 mm), tubular (11-50 mm) and early filamentous (> 100 mm) blastocysts (day 12). The mitotic index of tubular blastocysts was reduced (P < 0.01) compared to that for spherical blastocysts, which also suggested that cellular hyperplasia is not involved in initial elongation of pig blastocysts. Morphological changes in cellular organization of both trophectoderm and extraembryonic endoderm were observed when blastocysts developed to 10 mm in diameter. At this stage, a dense band of cells (elongation zone) composed of both endoderm and trophectoderm was visualized extending from the inner cell mass to the tip of the ovoid blastocyst. Scanning and transmission electron microscopy indicated ultrastructural changes in endodermal and trophectodermal cells associated with formation of the elongation zone. Blastocyst elongation occurs rapidly after formation of the elongation zone through alterations in microfilaments and junctional complexes of trophoectodermal cells and formation of filapodia by endodermal cells.