Whole mount in situ hybridization studies were performed to investigate the expression pattern of the homeobox gene Hex (also known as Prh) during early stages of chick embryogenesis. At the time of laying, cHex transcripts are detected in Koller's sickle and the forming hypoblast. During gastrulation (HH stage 4), cHex is expressed in anteriorly-displaced hypoblast cells. At stage 6, cHex transcripts are observed within endoderm in an anterior are that overlaps the cardiogenic region. Later cHex expression is observed within pharyngeal endoderm immediately adjacent to the forming myocardium, in the endocardium and in the liver and thyroid gland primordia. cHex transcripts are also detected within blood islands beginning at stage 4, and in extraembryonic and intraembryonic vascular endothelial cells as vessels form.

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