Differential expression sites of TGF-β isoforms in chicken limb buds during morphogenesis

TGF-β gene is expressed at various developmental stages and its principle role may be an involvement in organogenesis. The present study was performed to investigate the temporal expression of these TGF-β isoforms in the developing limb of White Leghorn Chicken, Gallus gallus (L., 1758). TGF-β isoforms were expressed in the developing limb as revealed by whole-mount in situ hybridization, but each showed a different pattern of expression. TGF-β2 was the dominant isoform compared with the other two isoforms. TGF-β2 first appeared along the proximodistal axis of the limb at stage 24 and condensed at the tip at stage 26. At stages 29-31, expression appeared in digits and then was extended to the interdigital spaces. A weak signal for TGF-β3 was first shown in the developing limb at stage 26, but there was no interdigital expression, unlike for TGF-β2. TGF-β4 was expressed in the developing limb at stage 26 and only in the interdigital spaces at stage 29. Reverse transcription - polymerase chain reaction analysis also showed that the transcript levels of TGF-β isoforms, especially TGF-β2, drastically increased at stage 29. These results suggest that TGF-β isoforms, with their patterns of expression, are specific regulatory factors that participate in limb development and digit morphogenesis.