Studies of t6/t6 mouse embryos.

The phenocritical period of the t6/t6 genome extends from the late blastocyst substages through the elongated egg-cylinder stage. Most of the homozygous mouse embryos die during the short egg-cylinder stage. The viable egg-cylinder staged t6/t6 embryos can be distinguished from their phenotypically wild-type litter-mates at both the light and electron microscopic levels. The distinguishing characteristics of these embryos are aberrantly arranged entodermal cells, excessive cytoplasmic lipid and crystal-containing mitochondria. These same features are also characteristic of those mutant embryos which are developmentally arrested at both the short and elongated egg-cylinder stages. Over 50% of the t6/t6 embryos can be identified as early as the late blastocyst substages by the presence of large, electron-dense cytoplasmic lipid droplets.