Genome reactivation in developing early pig embryos: an ultrastructural and autoradiographic analysis.

The onset of RNA synthesis in developing early pig embryos from 1-cell to 8-cell and morula stages was studied using high-resolution autoradiography of (5-3H)uridine incorporation. No transcriptional activity was detected in nuclei of 1- and 2-cell stage embryos with this technique. In these embryos nucleolus-like bodies (NLB) consist of sharply delineated, round, electron dense fibrillar masses. In the 4-cell stage embryos, the first uridine-3H incorporation in the nucleoplasm was detected and localized mainly near the regions of condensed chromatin. The first signs of reticulation and chromatin association were observed at the periphery of NLBs. In the next cell cycle (5- to 8-cell embryos) uridine-3H labelling was detected in the nucleoplasm and nucleoli. In these embryos, nucleoli consist of a central dense fibrillar mass without any transcriptional activity and fibrillo-granular cortex over which label was localized. The degree of functional restructure of nucleoli was variable within one blastomere or among different blastomeres, some nucleoli being more reticulated and showing more transcriptional activity than others. Fully developed nucleoli were present in early morulae. Electron dense unidentified structures described here as small dense round-shaped bodies (RDB) often surrounded by blocks of large chromatin granules were observed in intact 2-cell and alpha-amanitin treated 4-cell stage embryos. These structures did not show any transcriptional activity.