Variability of embryonic development among three inbred strains of mice.

We examined the relationships between litter size, embryonic growth, days of gestation, onset and duration of morphological stages and development of the first arch skeleton in three inbred strains of mice--C57BL/6, CBA/J and C3H/He. Detailed embryonic staging was based on craniofacial development between 11 and 18 days of gestation. Considerable intra- and interlitter variation of morphological stages of embryonic development exists in all three inbred strains. The relationship of morphological stages to days of gestation reveals that each stage has a different duration, being shortest at Theiler's stage 18 and longest at stage 21 in all three inbred strains. Embryos of CBA/J mice tend to reach each stage later than do embryos of the other two strains, i.e., morphological development is slowest in CBA/J. The greatest length, a measurement of embryonic growth, increases at a constant rate during gestation in all three strains. In C57BL/6 and CBA/J, more embryos tend to be implanted in the right horn of the uterus than in the left, whereas in C3H/He an even number of embryos tends to be implanted in both horns. Timing of the development of Meckel's cartilage differs between the three inbred strains: both condensation and onset of matrix deposition begin one stage earlier in C57BL/6 than in CBA/J and C3H/He. On the other hand, alkaline phosphatase, one of the earliest markers for bone development, is expressed at the same time in all three inbred strains. Differences in timing of skeletal development between the strains may be attributed in part to the genealogical closeness OF CBA/J and C3H/He mice.