The Digestive Tract and Derived Primordia Differentiate by Following a Precise Timeline in Human Embryos Between Carnegie Stages 11 and 13
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The precise mechanisms through which the digestive tract develops during the somite stage remain undefined. In this study, we examined the morphology and precise timeline of differentiation of digestive tract-derived primordia in human somite-stage embryos. We selected 37 human embryos at Carnegie Stage (CS) 11-CS13 (28-33 days after fertilization) and three-dimensionally analyzed the morphology and positioning of the digestive tract and derived primordia in all samples, using images reconstructed from histological serial sections. The digestive tract was initially formed by a narrowing of the yolk sac, and then several derived primordia such as the pharynx, lung, stomach, liver, and dorsal pancreas primordia differentiated during CS12 (21-29 somites) and CS13 (≥ 30 somites). The differentiation of four pairs of pharyngeal pouches was complete in all CS13 embryos. The respiratory primordium was recognized in≥26-somite embryos and it flattened and then branched at CS13. The trachea formed and then elongated in≥35-somite embryos. The stomach adopted a spindle shape in all≥34-somite embryos, and the liver bud was recognized in≥27-somite embryos. The dorsal pancreas appeared as definitive buddings in all but three CS13 embryos, and around these buddings, the small intestine bent in≥33-somite embryos. In≥35-somite embryos, the small intestine rotated around the cranial-caudal axis and had begun to form a primitive intestinal loop, which led to umbilical herniation. These data indicate that the digestive tract and derived primordia differentiate by following a precise timeline and exhibit limited individual variations.
- The Anatomical Record
The Anatomical Record 299(4), 439-449, 2016-04
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