Inductive signals from the somatopleure mediated by bone morphogenetic proteins are essential for the formation of the sternal component of avian ribs

The posterior five pairs of avian ribs are composed of vertebral and sterna l components, both derived from the semitic mesoderm. For the patterning of the rib cartilage, inductive signals from neighboring tissues on the semit ic mesoderm have been suggested to play critical roles. The notochord and s urface ectoderm overlying the semitic mesoderm are essentially required for the development of proximal and distal regions of the ribs, respectively. Involvement of the somatopleure in rib development has already been suggest ed but is less understood than those of the notochord and surface ectoderm. In this study, we reinvestigated the role of the somatopleure during rib d evelopment. We first identified the chicken homologue of the mouse Mesenchy mal forkhead-1 (cMfh-1) gene based on sequence similarities, cMfh-1 was obs erved to be expressed in the nonaxial mesoderm, including the semitic mesod erm, and, subsequently, in cartilage forming the ribs, vertebrae, and appen dicular skeletal system. In the interlimb region, corresponding to somites 21-25 (or 26), cMfh-1-positive semitic mesoderm was seen penetrating the so matopleure of E4 embryos, and cMfh-1 was used as a molecular marker demarca ting prospective rib cartilage. A series of experiments affecting the penet ration of the semitic mesoderm into the somatopleure was performed in the p resent study, resulting in defects in sternal rib formation. The inductive signals emanating from the somatopleure mediated by BMP family proteins wer e observed to be essentially involved in the ingrowth of the semitic mesode rm. BMP4 alone, however, could not completely replace inductive signals fro m the somatopleure, suggesting the involvement of additional signals for ri b formation, (C) 2001 Academic Press.