In vitro studies of mouse embryos bearing mutations in the T complex: t6.

Cultured blastocysts homozygous for the t6 mutation lose their inner cell mass within a few days of attachment to the culture dish. At about the same time it becomes apparent that putative t6-mutant trophoblast cells and their nuclei fail to enlarge to the degree of their normal counterparts. These abnormalities in mutant embryos are reflected by an abrupt drop on the seventh equivalent gestation day in the rate of increase of beta-glucuronidase activity. The failure of t6/t6 trophoblast nuclei to enlarge normally appears to be due partially to endoreduplication at a slower rate than normal trophoblast nuclei and partially to premature cessation of DNA synthesis. Analyses indicate that this abnormality is not reversed when mutant embryos are placed in chimeric association with normal ones. Trophoblast outgrowths from mutant and normal trophectodermal vesicles are similarly distinguishable by differences in outgrowth and nuclear size as well as DNA content and synthesis. Despite the fact that t6/t6 embryos and trophectodermal vesicles are phenotypically different from normals from early times in culture, the trophoblast cells in the mutant structures acquire and continue to produce two enzymes characteristic of trophoblast differentiation, delta 5, 3 beta-hydroxysteroid dehydrogenase and plasminogen activator. On the basis of these and previous observations, we propose that the primary effect of the t6 mutation is to cause a metabolic lesion which kills inner cell mass cells relatively quickly but which has a more gradual effect upon trophoblast cells. The fact that phenotypically recognizable t6/t6 trophoblast cells can survive for several days before dying makes this a potentially useful system in which to search for the t6 gene product(s).