title: Initiation of transcription and nucleologenesis in equine embryos.
abstract: 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 microCi/ml) for 10 hr, while eight-cell embryos (n = 2), morulae (n = 2), and blastocysts (n = 3) were incubated with 280 microCi/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 counterstained 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.
authors.affiliation: Department of Clinical Sciences, Cornell University, Ithaca, NY 14853, USA.
authors.forename: S P
authors.initials: SP
authors.lastname: Brinsko
authors.forename: B A
authors.initials: BA
authors.lastname: Ball
authors.forename: G G
authors.initials: GG
authors.lastname: Ignotz
authors.forename: P G
authors.initials: PG
authors.lastname: Thomas
authors.forename: W B
authors.initials: WB
authors.lastname: Currie
authors.forename: J E
authors.initials: JE
authors.lastname: Ellington
mesh_terms: D000818:Animals; D002466:Cell Nucleolus / Q000502:physiology* / Q000648:ultrastructure; D046508:Culture Techniques; D004622:Embryo, Mammalian / Q000502:physiology*; D005260:Female; D018507:Gene Expression Regulation, Developmental; D006736:Horses; D008297:Male; D015533:Transcriptional Activation; D014316:Tritium / Q000378:metabolism; D014529:Uridine / Q000378:metabolism
chemical_list: D014316:Tritium; D014529:Uridine