Effects of the Ay Gene on the Sensitive Periods of Hydrocortisone‐Induced Cleft Palate and Palatal Slit in Mice

The effects of the Ay gene on the sensitive periods of hydrocortisone‐induced cleft palate and palatal slit were investigated in C57BL/6 (a/a) and its congenic strain of C57BL/6‐Ay (Ay/a) mice by using the single‐dose administration method. Matings were conducted as follows (femalex male): Group I, a/a × a/a; Group n, a/a ×Ay/a; and Group III, Ay/a × a/a. Pregnant females were subcutaneously given a single dose of 200 or 400 mg/kg of hydrocortisone on day 11, 12, 13 or 14 of pregnancy and killed on day 18 for examination of palatal defects of live fetuses. Cleft palate was produced in all groups by treatment on days 11–13 of pregnancy at a dose of 200 mg/kg without any increase in fetal mortality. In Group I, the maximum incidence was observed on day 11 and followed by a rapid decrease thereafter. In Group U, in which half of the fetuses were expected to carry the Ay gene, the incidence still remained high on day 12, although it was decreased to the level of Group I by day 13. In Group HI, in which maternal mice as well as half of their fetuses carried the Ay gene, significantly higher incidences were observed on days 12 and 13 with the maximum value on day 12 when compared with those in Groups I and/or II. Palatal slit was also induced in all groups on each of treatment days at a dose of 200 mg/kg. Although the incidences in both Groups I and II were high on days 11 and 12 and gradually decreased thereafter, the value on day 13 was significantly higher in Group II than in Group I. In Group IE, the maximum incidence was found on day 12 and the values on days 13 and 14 were still significantly higher than those in Groups I and/or II. These results indicate that the sensitive periods of hydrocortisone‐induced cleft palate and palatal slit are prolonged into the later stage of embryonic development as a result of the Ay‐gene effects.