The ultrastructure of implanted trophoblast cells of the yellow agouti mouse.

An electron microscope study of ultrathin sections of trophoblast cells of implanting blastocysts resulting from the crossing of heterozygous yellow agouti mice has been conducted. Several conditions for successful preparation of the blastocysts have been described. These include avoidance of the use of sucrose in the buffers, reduction of specimen washing times and exclusion of propylene oxide from the dehydration procedure. Electron micrographs of cells from a blastocyst considered, on the basis of light microscopy, to be 'abnormal' revealed a cytoplasm with many 'empty' areas and numerous 'vacuolated' mitochondria. Electron micrographs of cells from blastocysts considered to be 'normal' revealed, in one instance, a preponderance of cells with very few of these 'abnormal' features, and, in the other instance, a preponderance of cells with many abnormalities. Both 'normal' blastocysts revealed cellular features usually regarded as either primitive or pathological, namely tight junctions, vacuolated mitochondria and mitochondria with cristae oriented parallel to their long axis. It was generally concluded that the appearance of the blastocysts, in the light microscope, at 100-105 hours of development, could not be used as a valid criterion of normality or abnormality, for, although a blastocyst classified as 'abnormal' under the light microscope appeared 'abnormal' also in the electron microscope, those classified as 'normal' under the light microscope appeared either 'normal' or 'abnormal' in the electron microscope. This disturbing situation may indicate that the electron microscope is diagnosing 'abnormality' before the light microscope. The presence of immature forms of organelles (e.g., vacuolated mitochondria) indicates either a failure to mature or regression. Failure to mature could be benign or of sinister import.