VARIABILITY OF EMBRYONIC-DEVELOPMENT AMONG 3 INBRED STRAINS OF MICE

We examined the relationships between litter size, embryonic growth, d ays of gestation, onset and duration of morphological stages and devel opment of the first arch skeleton in three inbred strains of mice - C5 7BL/6, CBA/J and C3H/He. Detailed embryonic staging was based on crani ofacial development between 11 and 18 days of gestation. Considerable intra-and interlitter variation of morphological stages of embryonic d evelopment exists in all three inbred strains. The relationship of mor phological stages to days of gestation reveals that each stage has a d ifferent 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 r each 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 g estation 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, wh ereas in C3H/He an even number of embryos tends to be implanted in bot h horns. Timing of the development of Meckel's cartilage differs betwe en the three inbred strains: both condensation and onset of matrix dep osition begin one stage earlier in C57BL/6 than in CBA/J and C3H/He. O n the other hand, alkaline phosphatase, one of the earliest markers fo r bone development, is expressed at the same time in all three inbred strains. Differences in timing of skeletal development between the str ains may be attributed in part to the genealogical closeness OF CBA/J and C3H/He mice.