Islet-1 marks the early heart rudiments and is asymmetrically expressed during early rotation of the foregut in the chick embryo.

Islet-1 (Isl-1), the LIM domain homeobox gene, is a well-known early marker of neuronal specification. Here, we show its spatial and temporal patterns of expression during early heart and gut development in the chick embryo. Isl-1 transcripts are first detected in the early cardiac progenitors and underlying endoderm at late stage 4. By stages 5-6, it is also expressed in the prechordal plate. From stage 6 onward, transcripts are also detected in the endoderm forming the anterior intestinal portal and floor of the caudal foregut. With progressive rostrocaudal fusion of the paired cardiac rudiments, Isl-1 expression is maintained in the cardiac mesoderm and associated endoderm. By the onset of heart beating, transcripts become restricted to the dorsal mesocardium and more caudal medial splanchnic mesoderm flanking the open gut. Within the foregut, Isl-1 is expressed in the endoderm of the oral membrane, thyroid rudiment, and second pharyngeal pouches, as well as within the second branchial grooves adjacent to the secondary pouches. Interestingly, with the onset of gut rotation, Isl-1 expression is detected unilaterally in the splanchnic mesodermal wall of the future greater curvature of the caudal stomach/rostral duodenum. Thus, Isl-1 is a novel and useful marker of the early cardiac rudiments and of the original left side of the rotating foregut. During early organogenesis, Isl-1 is also expressed in several other discrete domains as reported previously. Additionally, it is expressed at the interface between the hind limbs and trunk. Copyright 2000 Wiley-Liss, Inc.