Human development since week 2 to fetus



during development the fetus is connected to the placenta via the umbilical cord which is a long flexible stock that has two small arteries in 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 fetuses heart through the umbilical vein when the baby's born the umbilical cords no longer needed so it's cut off leaving the navel better known as the bellybutton in week two of development the blastocyst has two parts an inner part that's the embryo blast and an outer part called the trophoblast which develops into the saito trophoblast and since isseo trophoblast the embryo blast has two layers called the EPI blast which contains the amniotic fluid and the hypo blast which contains the yolk sac which is filled with vitalina that can nourish the embryo cells from the EPI blast layers start to differentiate into extraembryonic mesoderm cells named that because they're outside of the developing embryo these are some of the earliest mesoderm cells and they start to form even when the embryo blasts itself is a bilaminar disk these mesoderm cells line the inside of the saito trophoblast and syncytial trophoblast and form the chorion and a space starts to form that will eventually become the chorionic cavity the embryo blast is connected to the chorion by a short band of extraembryonic mesoderm called the body stock and this is the first of three structures that make up the umbilical cord now in Week three of development the embryo folds in two directions in the longitudinal plane there's a cranial and caudal fold so that it looks less like a pancake and more like a little shrimp the folding shapes part of the yolk sac into a gut tube with the remainder of the yolk sac remaining connected not at the cranial or caudal end but just in the middle the endoderm layer which derives from the epiblast forms the gut tube and suit and divides into three parts the foregut the mid gut and the hindgut the mid gut is open to the yolk sac via a connection called the vital induct which is the second structure in the umbilical cord around the same time the hindgut grows a little out pocketing called the allantois which grows toward the umbilical cord and drains the bladder the allantois is the third structure in the umbilical cord in week four of development the amniotic cavity folds down and around the embryo the body stock the vital induct the new and to us get pushed together and form the umbilical cord and they emerge out of the umbilical ring which is a fibrous ring of tissue that develops on the abdominal wall at the spot where they emerge from the fetus the cells lining the amniotic cavity which originally derived from the EPI blast cells start to generate a lot of amniotic fluid this causes the amnion to swell and take over most of the space in the chorionic cavity as the amnion expands like a big bubble it folds in and covers the body stock inviting duct forming an outer membrane for the umbilical cord now briefly around week six of development the fetal intestines grow so quick that a part of the intestine squeezes through the umbilical ring into the umbilical cord this is called physiological umbilical herniation Media happens normally and by the end of the third month of development the intestines withdraw completely back into the abdominal cavity so looking at the umbilical cord in cross-section there's the outer layer of amnion and then there's the vitalink duct which has its own arteries and veins as well as the yolk sac there's also the allantois and finally the two umbilical arteries in a single umbilical vein after the formation of the umbilical cord the vital induct anoke sac starts to shrink and eventually they disappear in rare cases the vital induct won't regress all the way and it'll leave behind a tiny remnant mid gut an outpouching of the intestines that's called a meckel's diverticulum the allantois continues developing into the bladder and leaves behind a remnant called the uric kiss in the fetus which is then renamed as the median umbilical ligament once the baby's born so all that remains in the umbilical cord is the umbilical vein the two umbilical arteries and a gelatinous substance called Wharton's jelly which protects the umbilical vessels after birth the umbilical vein becomes the livers round ligament and the umbilical arteries become the medial umbilical ligaments to help keep it straight think of the fact that there are two umbilical arteries so all of the umbilical arteries become medial ligaments whereas there's only an allantois which turns into a median ligament all right as a quick recap the umbilical cord is made up of three key structures the body stock the vital induct and the allantois there's also a normal process called physiological umbilical herniation where the intestines briefly go into the umbilical cord as well the vital induct regresses completely but in some cases will leave behind a meckel's diverticulum the end of the allantois nearest the umbilical cord turns into the median umbilical ligament in the umbilical vein becomes the round ligament of the liver while the two umbilical arteries become the medial ligaments you

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