Chapter 26. Embryonic Folding and Flexion of The Embryo - Review of Medical Embryology Book

Embryonic folding: the flat trilaminar embryonic disk becomes a more cylindric embryo due to the longitudinal and transverse folding that occurs as a result of embryonic growth, especially of the neural tube. The foldings occur simultaneously and are not separate sequential events. Flexion, a process of curving, transforms the embryo into a sort of "tube" and isolates it from the embryonic membranes, to which it is eventually attached only by a thin stalk, the umbilical cord. The embryo increases rapidly in its long axis due to central growth being greater than peripheral growth, and the dorsal region of the embryo grows more rapidly than its ventral region, resulting in the embryo curving itself around the umbilical region. The dorsal region also thickens, especially in the midline, and the edges of the disk swing ventrally carrying the amnion with them. Thus, the embryo is surrounded by its amniotic cavity

LONGITUDINAL FOLDING produces both head- and tailfolds, or flexion, and creates a cranial and caudal region to the embryo

Headfold: neural folds (end of week 3) begin to develop into the brain and project dorsally into the amniotic cavity

The forebrain grows cranially beyond the oropharyngeal membrane and overhangs the primitive heart. At the same time, the septum transversum (a mass of mesoderm cranial to the pericardial coelom), the heart, the pericardial coelom, and the oropharyngeal membrane turn under onto the ventral surface

During folding, part of the yolk sac is incorporated as the foregut (between brain and heart, ending blindly at the oropharyngeal membrane). The membrane separates the foregut from the stomodeum or primitive mouth cavity

After folding, the septum transversum lies caudal to the heart and develops into a major portion of the diaphragm
Before folding, the intraembryonic coelom is a flattened horseshoe-shaped cavity. After folding, the pericardial coelom lies ventrally and the pericardioperitoneal canals run dorsally over the septum transversum to join the peritoneal coelom which, on each side, communicates with the extraembryonic coelom

The tailfold (caudal end) takes place later than the headfold and results from the dorsal and caudal growth of the neural tube

As the embryo grows, the tail region projects over the cloacal membrane which eventually comes to lie ventrally
During folding, part of the yolk sac is incorporated into the embryo as the hindgut, the terminal portion of which soon dilates and forms the cloaca, separated from the amniotic cavity by the cloacal membrane
Before folding, the primitive streak lies cranial to the cloacal membrane, but, after folding, lies caudal to it
The connecting stalk now attaches to the ventral embryonic surface, and the allantois is partly incorporated into the embryo

Each lateral body wall (somatopleure) folds toward the midline, rolling the edges of the embryonic disk ventrally to form a cylindric embryo
As lateral and ventral body walls form, part of the yolk sac is incorporated into the embryo as the midgut; simultaneously, the connection of the midgut with the yolk sac is reduced to a yolk stalk or vitelline duct
After folding, the area of the amnion attachment to the embryo is reduced to a narrow umbilicus on its ventral surface
As the midgut is separated from the yolk sac, it attaches to the dorsal abdominal wall via a thin dorsal mesentery
As the umbilical cord forms, the ventral fusion of the lateral folds reduces the area of communication between the intra- and extraembryonic coelom
As the amniotic cavity enlarges and obliterates the extraembryonic coelom, the amnion forms an outer covering for the umbilical cord