Development of the folds of the embryo

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 * It is related to the rapid development of the embryo, especially brain and spinal cord.
 * Occurs in both the median and horizontal planes.
 * The growth of the embryo circumference lags behind the growth of the embryo in the longitudinal axis, resulting in folds and narrowing of the transition of the embryo into the yolk sac.

Head crease

 * At the beginning of the 4th week, they acquire neural crests, form the basis of the brain and extend into the amnion, then outgrow the oropharyngeal membrane, thereby reaching the septum tendineum, heart, pericardium and oropharyngeal membrane on the ventrum of the embryo, at the same time it splits from endoderm yolk sac foregut (is between brain and heart) which is separated by OFM from stomodeum.

Caudal fold

 * It arises from the growth of the distal part of the spinal cord, the hindgut splits, which later expands at the cloacal membrane in the cloaca (the base of the bladder and anus), the germinal shaft (primordium of the umbilical cord) is attached to the ventrum of the embryo and the gastrointestinal tract [ [allantois]] is partially incorporated into the embryo.

Side fold

 * Formed by rapid growth of the spinal cord and somites, the bases of the ventrolateral body wall bend towards the midline and wrap around the edges of the germinal plate, with this a part of the yolk sac endoderm is taken into the embryo as the midgut, the folds eventually interrupt the communication of the EXE and INE chlomata, the umbilical cord is covered amnion.
 * Yolk sac - at first it communicates widely with the midgut, but due to the growth of the lateral folds, it is soon reduced to just the stalk of the yolk sac.
 * Amnion – communication with the embryo has shifted to the ventrum due to the growth of the caudal fold, it is reduced to umbilical landscape.

Related Articles

 * Stages of embryo and fetal development: First week of human development • Second week of human development • Third week of human development • Fourth to eighth week of intrauterine development