AbstractThis study examines development of rat, mouse, and human embryonic palates in submerged, serum‐free organ culture. The concentration‐response profiles for retinoic acid (RA), triamcinolone (TRI), hydrocortisone (HC), dexamethasone (DEX), and 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD) were examined and the mechanisms of clefting in vitro were compared to observed in vivo responses. Craniofacial regions were dissected on gestational day (GD) 12 for mice and GD 14 for rat, and cultured for 3‐4 days in Bigger's BGJbmedium in flasks flushed with 50% O2, 45% N2, 5% CO2. Growth and fusion of secondary palates were scored under a dissecting microscope. In serum‐free control medium, mouse and rat palatal fusion occurred within the 4‐day culture period. Supplementing with fetal bovine serum (FBS) in excess of 1% interfered with growth and fusion in control medium. RA significantly inhibited fusion of mouse and rat palates at 5 × 10−9and 1 × 10−10M, respectively, with RA‐induced clefting related to abnormal proliferation and differentiation of medial epithelia. In contrast, glucocorticoid‐induced clefting was due to concentration‐dependent inhibition of shelf growth. TRI significantly inhibited fusion at 4 × 10−5M, and 1 × 10−4M DEX or HC, inhibited fusion of 19 and 42% of shelves, respectively. The response rate for DEX in the presence of 1% FBS was increased (42% unfused). TCDD clefting was due to altered medial epithelial differentiation and 1 × 10−8M TCDD affected 36% of CD‐1 mouse, 23% of C57BL/6N mouse, and 47% of F344 rat palates. When the medium was supplemented with 1% FBS, selenium, transferrin, and additional glutamine, the response of C57BL/6N embryos increased to 75%. This rate is similar to that reported for Trowell's‐type cultures with IMEM:F12 medium and 1% FBS. The increased responsiveness to DEX or TCDD in the presence of serum suggests that an unknown factor in serum may be required for full activity. Three human embryonic palatal explants (GD 52 or 53) were cultured for 3‐6 days and fused during culture. The present study demonstrates that serum‐free organ culture supports development of mouse, rat, and human palatal explants. The present study demonstrates the capacity of this organ culture system to model palatogenesis for several species, and to distinguish between various mechanisms of clefting as presented through selected model compounds. This model should be useful for exploring mechanisms o