Electron microscope visualization of giant polysomes in sea urchin embryos

Chromatin spreading techniques have been applied to the electron microscopic visualization of polysomes in sea urchin (Strongylocentrotus purpuratus) eggs and embryos. Polysomes of giant size are commonly found after the 8-cell stage. The largest seen, from an early gastrula, was 13.6 μm in length, carried 277 ribosomes, with a message calculated to contain 6.49×10⁴ nucleotides and potentially to encoded 2.38×10⁶ daltons of peptide. Polysomes are rare and very large ones absent from lysates of unfertilized eggs. Giant polysomes appear in 4- to 8-cell stages and are common in 16-cell stages and thereafter. They are of two forms: a compact form with no spacing between ribosomes characteristic of stages through early mesenchyme blastulae, and an extended form found only after late mesenchyme blastulae. Both have potential for massive informational content. Some of each type have ribosome-free tails at one end, as long as 733 Å in the compact forms, and 7,890 Å in the extended ones. Occasionally they have a single array of fibrous material increasing from one end of a polysome to the other, interpreted to be nascent peptide chains. Polysomes are not found after brief, mild exposure of lysates to RNase A, or from embryos treated with puromycin. Very large polysomes are present in lysates of blastulae exposed since fertilization to actinomycin D, cycloheximide, or cordycepin. They appear in parthenogenetically activated or fertilized enucleate merogones, but are absent from unactivated merogones, demonstrating that egg masked messages can generate them. A potential embryological significance of giant, potentially polycistronic polysomes is suggested.