Initiation of transcription and nucleologenesis in equine embryos

The time of activation of the embryonic genome (maternal‐embryonic transition) in equine embryos was investigated by assessing incorporation of ³H‐uridine and nucleolar development. In Experiment 1, embryos were recovered from the oviduct (n = 15) and the uterus (n = 3). Recovered embryos were assessed for morphologic development and quality score. Recovered embryos with less than 8 cells (two cells, n = 4; four cells, n = 5; five cells, n = 2) were incubated with ³H‐uridine (560 μCi/ml) for 10 hr, while eight‐cell embryos (n = 2), morulae (n = 2), and blastocysts (n = 3) were incubated with 280 μCi/ml for 0.5–1 hr. At the end of incubation, embryos were washed twice in PBS with 10% FBS and incubated for 30 min with 2.5 mg/ml of unlabelled uridine. Embryos were spread onto glass slides, dipped into emulsion, and exposed for 8 d, then developed and counter‐stained with Giemsa and propidium iodide. Embryos at the blastocyst, morula, eight‐cell, and five‐cell stages incorporated ³H‐uridine into their cell nuclei as detected by autoradiography. In a second experiment, nucleologenesis in equine embryos was examined by transmission electron microscopy. Nucleoli or nucleolar precursors were found in 12 of 23 embryos examined. Most embryos in the four‐ to six‐cell stage (n = 7) had nucleolar precursor bodies (npb) consisting of homogeneous fibrillar structures. Two five‐ to six‐cell embryos also possessed reticulated nucleoli with both fibrillar and granular components as did all eight‐cell embryos (n = 3). Nucleoli in one morula and one blastocyst were reticulated with prominent granular components, fibrillar components, and apparent fibrillar centers. These results indicate that incorporation of ³H‐uridine and the formation of functional nucleoli with typical fibrillar and granular components occurs between the four‐ to eight‐cell stage in equine embryos. © 1995 wiley‐Liss, Inc.