mortality/aging
• blastomeres in homozygous null embryos arrested at preimplantation (morula) stages of development
• morulae were recovered at non-Mendelian frequencies: approximately 50% of homozygous null embryos arrested at an even earlier developmental stage
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embryo
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• overall, homozygous null morulae appeared similar to wild-type embryos with evenly sized blastomeres and well-defined cell-cell borders
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• mutant blastomeres failed to proliferate past morula stages of development, and appeared disorganized and unevenly sized
• at 3.5 dpc, electron microscopy revealed that blastomeres of homozygous null embryos failed to form well-differentiated nucleoli, although electron dense material (presumably remnants of the nucleolus precursor body) and electron dense spheres located near the nuclear periphery were readily identifiable
• high magnification revealed the presence of ribosomes in control embryos but not in homozygous null embryos: mutant embryos displayed a striking 23-fold reduction in the number of ribosomes relative to wild-type
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• all homozygous null embryos examined at 3.5 dpc contained only 8-16 cells (i.e. the number of cells normally observed in wild-type morulae at 2.5 dpc), and none of the homozygous null embryos had formed blastocysts
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