TlWh (Rb (5; 19)lWh), an albino strain of mice not previously used in the investigation of cleft palate (CP), is homozygous for a (5; 19) Robertsonian translocation which provides conspicuous chromosomal markers. Preliminary studies in this laboratory revealed that CP is induced in all the offspring of TlWh females treated with 2 · 5 mg of cortisone acetate on days 11–14 of pregnancy and that palatal closure is accomplished at a later morphological and chronological age than in C57BL/6J strain mice. The latter strain had a CP frequency of 42 % under similar experimental conditions.

Eight- to 16-cell embryos were aggregated in the following combinations: TlWh ↔ TlWh, C57BL/6J ↔ C57BL/6J and TlWh ↔ C57BL/6J. The resulting allophenic blastocysts were transferred to pseudopregnant C57BL/6J recipients which were given the above cortisone treatment and killed on the 18th day of pregnancy. CP was found in all of the 48 TlWh ↔ TlWhfetuses, 4 (20 %) of 20 C57BL/6J ↔ C57BL/6J fetuses and in 17 (26-2 %) of 65 TlWh ↔ C57BL/6J fetuses. Analysis of chromosome preparations from the palates of the TlWh ↔ C57BL/6J fetuses demonstrated that there is a positive correlation between the presence of CP and the percentage of susceptible cells in the palate (P < 0·02). Other data suggest a possible growth advantage in favor of C57BL/6J cells in the palates of TlWh ↔ C57BL/6J allophenic mice.

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