Title: Mesoderm Is Required for the Formation of a Segmented Endodermal Cell Layer in the LeechHelobdella
Abstract: The homeobox geneLox3is expressed in a segmentally iterated pattern within the endoderm of the leechHelobdella.We use that expression here to study endoderm differentiation following experimental ablations of mesoderm.Lox3RNA was first detected byin situhybridization at the stage when a definitive cellular endoderm is formed from its syncytial precursor and was never observed in derivatives of other germ layers. Expression is initially distributed throughout the endoderm, but rapidly disappears from specific regions of the nascent gut wall so as to produce a pattern of segmental stripes. The stripe pattern differs markedly between midgut organs, with thin stripes ofLox3expression in the intercaecal constrictions of the crop and wide stripes ofLox3expression marking the caecal bulges of the intestine.Lox3expression in the rectum is not obviously segmental. Ablation of segmental mesoderm in the earlyHelobdellaembryo prevents the formation of definitive endoderm and the expression ofLox3RNA and leads to abnormalities in the morphogenesis of the gut tube. These endodermal deficits are precisely coextensive with the zone of mesodermal deficiency, suggesting that the mesoderm normally acts to promote the formation of the endodermal cell layer via local cell interactions. The segmental pattern ofLox3expression is largely unaffected in portions of the endoderm surrounding such deficits, suggesting that endodermal segmentation is not established by lateral interactions within that tissue layer. Rather, we propose that the segmental organization of the endoderm is imprinted by vertical interactions with the segmental mesoderm.