Embryonic Folding

As development progresses, the embryo begins to fold in a manner than it transitions from a flat trilaminar disc (as seen in week three of development) to a curved three dimensional structure.

This folding occurs in two planes:

1. Longitudinal folding

   • Increased growth of the aminotic cavity without growth of the yolk sac pushes the growing embryo of the edge of the yolk sac so it can instead come together underneath it

   • Folding in the cranial region brings the buccopharyngeal membrane to the site of the future mouth

   • The region of the neural tube that gives rise to the brain lies cranial to the buccopharyngeal membrane

   • The future heart tube gets pulled down to the thoracic region

   • Further caudal folding brings the cloacal membrane underneath the embryo and the connecting stalk towards the umbilical region

   • The connecting stalk will maintain the connection between the embryo and the placenta

2. Lateral folding

   • The sides of the yolk sac get pushed further in together as the right and left sides of the folding embryo come together

   • As the right and left sides meet, their counterparts on either side are able to fuse together

     • For example, the ectoderm layer of cells from the right and left meet to form one continuous outer layer of the embryo

By the fifth week of development, the embryo appears to look like a tube within a tube. This is due to the outer tube of ectoderm enveloping the inner tube of endoderm.

Clinical Notes:

1. Gastroschisis - this occurs when abdominal contents herniate out of the anterior abdominal wall.