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Human development days 1-4
Human development days 4-7
Human development week 2
Human development week 3
Development of the digestive system and body cavities
Development of the fetal membranes
Development of the placenta
Development of the umbilical cord
Development of twins
Hedgehog signaling pathway
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During development, the fetus is connected to the placenta via the umbilical cord, a long, flexible stalk that has two small arteries and a large vein.
Deoxygenated fetal blood flows through the umbilical arteries to the placenta where it picks up oxygen and glucose while dropping off carbon dioxide.
Oxygenated blood then heads back towards the fetus’s heart through the umbilical vein.
When the baby is born the umbilical cord is no longer needed, so it’s cut off, leaving the navel or belly button.
In week 2 of development, the blastocyst has two parts—an inner part called the embryoblast, and an outer part called the trophoblast, which develops into the cytotrophoblast and syncytiotrophoblast.
The embryoblast has two layers called the epiblast, which contains the amniotic fluid, and the hypoblast, which contains the yolk sac filled with vitelline fluid that can nourish the embryo.
Cells from the epiblast layer start to differentiate into extraembryonic mesoderm cells, so-named because they are outside of the developing embryo.
These are some of the earliest mesoderm cells, and they start to form even while the embryoblast itself is a bilaminar disc.
These mesoderm cells line the inside of the cytotrophoblast and syncytiotrophoblast and form the chorion.
As development progresses, a space called the chorionic cavity develops between the embryoblast and the chorion, and these two structures are connected by a short band of extraembryonic mesoderm called the body stalk.
The body stalk contains the umbilical vessels and is the first of three structures that make up the umbilical cord.
In week 3 of development, the embryo folds in two directions.
In the longitudinal plane, there is a cranial and caudal fold, so that the embryo now looks less like a pancake and more like a little shrimp.
The folding process shapes part of the yolk sac into a gut tube, with the rest of the yolk sac remaining connected not at the cranial or caudal end, but just in the middle of the fetus.
The umbilical cord is the conduit between the developing fetus and the placenta. It contains two arteries and one vein, allowing for nutrient and oxygen exchange and waste removal. The development of the umbilical cord starts at around week three and is fully formed by week 7 of gestation. At this time, it consists of the stalk, vitelline duct, and umbilical vessels.
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