DNA Synthesis and pronucleus development in pig zygotes obtained in vivo: An autoradiographic and ultrastructural study

Abstract

Porcine zygotes flushed from oviducts 48,52,56,60, or 64 hr after hCG were incubated 30 min in ³H‐thymidine, transferred to nonradioactive medium for 2 hr, and incubated for 30 min with ¹⁴C‐thymidine. After this procedure, ova were prepared (i.e., at 51,55,59,63, or 67 hr after hCG) for autoradiography and ultrastructural observations, respectively. The first autoradiographic labelling, i.e., DNA synthesis, was observed at 56–56.5 hr after hCG, while the latest labelling was seen at 60–60.5 hr. At 51 hr after hCG, formation of the pronuclear envelope was observed, while no nucieolus precursor bodies or prestages to these structures were found. At 55 hr a few clusters of small electron‐dense granules were observed, together with condensed chromatin in the pronuclei. At 59 hr the apposed regions of both pronuclei contained nucleolus precursor bodies and condensed chromatin, in close contact with both clusters of small granules and clusters of an additional category of large granules and the nuclear envelope. Additionally, large accumulations of the small granules were found in the vicinity of similarly sized accumulations of the large granules without chromatin association. At 63 hr the spherical accumulations large granules on some occasions presented a central vacuole, and condensed chromatin and clusters of small granules were attached to its periphery. Within the vacuole, electrondense material was found. It is concluded that (1) the S‐phase in porcine zygotes is initiated around 56 hr post‐hCG injection and is of a duration of 4.5–7.5 hr and (2) the progress of the S‐phase is paralleled by the appearance of and complex interaction between different granules in the nucleoplasm.