6mm Pig Embryo - approximately Human day 32 (Carnegie Stage 13/14 embryo)
The gastrointestinal tract (GIT) develops primarily from the endoderm, with inputs from the mesoderm (splanchnic mesoderm) and the ectoderm (neural crest cells form its primary nerve plexus). After the endoderm is folded into a blind-ended tube in week 4, it begins to develop into the primary lining of the GIT, while the mesoderm provides its muscular wall and connective tissues components. The GIT is typically partitioned into 3 parts, based on their separate blood supplies: foregut, midgut and hindgut, with the allantois coming off the hindgut. A bud coming off the tract forms the liver; eventually buds appear that form gallbladder and pancreas. The rostral end of the tube ends at the buccopharyngeal membrane, where it interacts with ectoderm directly (no mesoderm in between). This area forms the future mouth. A similar endodermal-ectodermal interaction occurs at the tail – the cloacal membrane. The section of the GIT here is called the cloaca. During week 4, the tube begins to dilate in a certain region, with the dorsal border growing more rapidly than the ventral. This establishes the greater and lesser curvatures of the stomach. The stomach also rotates 90o along the longitudinal axis. On day 22, a small endodermal thickening is visible on the ventral side of the duodenum – this becomes the hepatic diverticulum that forms the liver over the next few days. The liver has an important in blood formation. As early as the fourth week, red blood cells production moves to the liver, as opposed to blood islands in the extraembryonic mesoderm.
The respiratory system is endodermal in origin, initially “budding off” the foregut during week 3. This bud forms the respiratory diverticulum, at the level of the glottis between the adult oesophagus and trachea. It continues to bud in week 4, forming a pair of lung buds.
The kidneys develop from intermediate mesoderm, which lies between the lateral plate mesoderm and the somites. The kidney develops in multiple stages, which occur in a rostrocaudal sequence. The earliest structure to form is the pronephros, in week 4, featuring a pronephric duct with associated nephrogenic mesenchyme. This degenerates early on, leaving only the duct system running down to the cloaca – this becomes known as the mesonephric duct, in the embryo. The next stage is the formation of the mesonephros, also in week 4. Its differentiation is induced by the pronephros. However, the mesonephros is also a transient structure. It provides a template for the adult metanephros, beginning on day 35-37.
This page is part of the UNSW Medical degree Independent Learning Project (ILP) prepared by Aashish Kumar (2006).
Please email Dr Mark Hill if you wish to make a comment about this current project.
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