CLEFT BEAK INDUCED BY HYDROCORTISONE IN THE CHICK IS PREVENTED BY INCREASED CELL-DIVISION AFTER EXPERIMENTAL REDUCTION OF AMNIOTIC-FLUID

Hypoplasia of the medial nasal process has been reported in chick embr yos on embryonic day (ED) 5, 24 h after their exposure to hydrocortiso ne (HC). As a result, the cleft beak occurs in 80-100% of specimens on ED 9. In order to analyze its influence on cell proliferation, HC was injected intra-amniotically into embryos on ED 4, and the mitotic ind ex and number of BrdU-positive cells were evaluated 24 h later, both i n the epithelium and mesenchyme of the medial nasal processes, on seri al frontal histological sections. Two hours after BrdU administration, there were 50% of labeled mesenchymal cells in the embryos exposed to HC and only 23% in the control group. The mitotic index of mesenchyma l cells was significantly lower in the HC group than in the controls. The epithelium showed no significant difference. HC seemed to prevent the mesenchymal cells from entering mitosis. The cleft beak in the emb ryos exposed to HC on ED 4 was totally eliminated by tearing open the amnion (amniotomia) and allowing fluid to leak out on ED 5. In some of specimens exposed to HC, the mitotic index was investigated at six ti me intervals from 15 to 120 min after amniotomia. A significant increa se in the mitotic index was detected in the mesenchymal cells of the m edial nasal processes during the first hour after amniotomia. Such a p rompt increase of the mitotic activity may be hypothetically explained by release of the HC from its receptor binding as a consequence of ou tflow of the amniotic fluid together with the HC pool, and freeing of the mesenchymal cells, blocked in the G(2) phase, to enter mitosis. As a result, the hypoplasia of the medial nasal process might be compens ated and the development of the cleft beak prevented.