MESODERM IS REQUIRED FOR THE FORMATION OF A SEGMENTED ENDODERMAL CELLLAYER IN THE LEECH HELOBDELLA

The homeobox gene Lox3 is expressed in a segmentally iterated pattern within the endoderm of the leech Helobdella. We use that expression he re to study endoderm differentiation following experimental ablations of mesoderm. Lox3 RNA was first detected by in situ hybridization at t he stage when a definitive cellular endoderm is formed from its syncyt ial precursor and was never observed in derivatives of other germ laye rs. Expression is initially distributed throughout the endoderm, but r apidly 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 of Lox3 expression i n the intercaecal constrictions of the crop and wide stripes of Lox3 e xpression marking the caecal bulges of the intestine. Lox3 expression in the rectum is not obviously segmental. Ablation of segmental mesode rm in the early Helobdella embryo prevents the formation of definitive endoderm and the expression of Lox3 RNA and leads to abnormalities in the morphogenesis of the gut tube. These endodermal deficits are prec isely coextensive with the zone of mesodermal deficiency, suggesting t hat the mesoderm normally acts to promote the formation of the endoder mal cell layer via local cell interactions. The segmental pattern of L ox3 expression is largely unaffected in portions of the endoderm surro unding such deficits, suggesting that endodermal segmentation is not e stablished by lateral interactions within that tissue layer. Rather, w e propose that the segmental organization of the endoderm is imprinted by vertical interactions with the segmental mesoderm. (C) 1997 Academ ic Press.