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Organ system development
Development of the cardiovascular system
Development of the ear
Development of the eye
Development of the face and palate
Pharyngeal arches, pouches, and clefts
Development of the digestive system and body cavities
Development of the gastrointestinal system
Development of the teeth
Development of the tongue
Development of the integumentary system
Development of the axial skeleton
Development of the limbs
Development of the muscular system
Development of the nervous system
Development of the renal system
Development of the reproductive system
Development of the respiratory system
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During week 3, the embryo is a flat disc made up of three germ layers: the endoderm, the mesoderm, and the ectoderm.
From the endoderm, which you can think of as being the belly side of this three-layered embryo pancake, a fluid filled bubble called the yolk sac forms and grows alongside the developing embryo.
The digestive system starts forming when the embryo folds along its vertical and horizontal axes—so it rolls up on itself, and turns into a tubular structure that looks a bit like a little shrimp.
Folding also pinches the yolk sac, sort of like squeezing a balloon through a ring, so a part of it goes inside the embryo and forms the primitive gut tube.
The primitive gut tube is initially sealed off at both ends—the buccopharyngeal membrane at the top separates the tube from the primitive mouth, and the cloacal membrane at the bottom separates it from the primitive anus.
This tube is divided into three parts, based on the arterial blood supply.
The first portion is the foregut, and it’s nourished by the celiac artery.
The middle portion, the midgut, is nourished by the superior mesenteric artery.
For a short while, the midgut communicates with the yolk sac through the vitelline duct, which is eventually incorporated into the umbilical cord.
Finally, there’s the last portion, the hindgut, which is nourished by the inferior mesenteric artery.
The hindgut ends with the cloaca, which is the primitive common drainage site for the urinary, genital, and digestive systems.
The gastrointestinal system starts to develop around week 3 of prenatal life. The earliest indication of gastrointestinal development is a thickening in the midline of the embryo that will become the gut tube. This thickening begins to form a groove along its length, and by week 5 of development this groove has divided into 3 sections: foregut, midgut, and hindgut. Each section will give rise to different parts of the gastrointestinal system.
The gastrointestinal system develops from all three germ layers (ectoderm, mesoderm, and endoderm). The ectoderm gives rise to the enteric nervous system; mesoderm gives rise to the connective tissue, including the wall of the gut tube and the smooth muscle, whereas the endoderm gives rise to the epithelial lining of the digestive tract, as well as to all of the associated glands and organs such as the liver, gallbladder, and the pancreas.
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