Sp. Brinsko et al., INITIATION OF TRANSCRIPTION AND NUCLEOLOGENESIS IN EQUINE EMBRYOS, Molecular reproduction and development, 42(3), 1995, pp. 298-302
The time of activation of the embryonic genome (maternal-embryonic tra
nsition) in equine embryos was investigated by assessing incorporation
of H-3-uridine and nucleolar development. In Experiment 1, embryos we
re recovered from the oviduct (n = 15) and the uterus (n = 3). Recover
ed embryos were assessed for morphologic development and quality score
. Recovered embryos with less than 8 cells (two cells, n = 4; four cel
ls, n = 5; five cells, n = 2) were incubated with H-3-uridine (560 mu
Ci/ml) for 10 hr, while eight-cell embryos (n = 2), morulae (n = 2), a
nd blastocysts (n = 3) were incubated with 280 mu Ci/ml for 0.5-1 hr.
At the end of incubation, embryos were washed twice in PBS with 10% FB
S and incubated for 30 min with 2.5 mg/ml of unlabelled uridine. Embry
os were spread onto glass slides, dipped into emulsion, and exposed fo
r 8 d, then developed and counterstained with Giemsa and propidium iod
ide. Embryos at the blastocyst, morula, eight-cell, and five-cell stag
es incorporated H-3-uridine into their cell nuclei as detected by auto
radiography. In a second experiment, nucleologenesis in equine embryos
was examined by transmission electron microscopy. Nucleoli or nucleol
ar precursors were found in 12 of 23 embryos examined. Most embryos in
the four- to six-cell stage (n = 7) had nucleolar precursor bodies (n
pb) consisting of homogeneous fibrillar structures. Two five- to six-c
ell embryos also possessed reticulated nucleoli with both fibrillar an
d granular components as did all eight-cell embryos (n = 3). Nucleoli
in one morula and one blastocyst were reticulated with prominent granu
lar components, fibrillar components, and apparent fibrillar centers.
These results indicate that incorporation of H-3-uridine and the forma
tion of functional nucleoli with typical fibrillar and granular compon
ents occurs between the four- to eight-cell stage in equine embryos. (
C) 1995 Wiley-Liss, Inc.