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Embryonic folding is the process of converting the embryo from a flat disc, into a cylinder.
This cylinder consists of three main layers, derived from the trilaminar embryonic disc: the endoderm in the center, the ectoderm on the outside, and the mesoderm, which is found between the two layers.
During the fourth week of development, folding of the embryo occurs as a result of the differential rates of growth of the embryonic structures. The embryonic disc and amnion grow rapidly, however, growth of the yolk sac is much slower.
Embryonic folding occurs in two planes: the horizontal plane, which results in the development of two lateral body folds, and the median sagittal plane, which causes the longitudinal cranial (head fold) and the caudal (tail fold) of the embryo to develop.
As folding continues simultaneously in both planes, the result is a very rapid development of the embryo.
The endoderm of the trilaminar disc is mainly responsible for the formation of the epithelial lining and glands of the gastrointestinal tract. As the lateral, cranial and caudal folds of the endoderm fold towards the midline, they fuse, incorporating the dorsal part of the yolk sac to create the primitive gut tube.
The gut tube is divided into three main parts: the foregut, midgut, and hindgut.
The foregut can be seen at the cranial end of the embryo and is temporarily closed by a membrane, called the oropharyngeal membrane. At the end of the fourth week of development, the oropharyngeal membrane ruptures to connect the future oral cavity with the pharynx.
Until the fifth week of development, the midgut is connected to the yolk sac, which narrows into a stalk known as the vitelline duct, as embryonic folding continues. Eventually, the yolk sac constricts and detaches from the midgut, and the midgut seals.
The caudal end of the hindgut is also temporarily closed by a membrane, called the cloacal membrane, which separates the upper and lower parts of the anal canal.
The cloacal membrane ruptures during the seventh week of development to form the urogenital and anal openings.
The ectoderm covers the entire outer surface of the developing embryo, except for the eventual umbilical region.
This germ layer, along with the dermatomes, lateral plate mesoderm, and neural crest cells formed during neurulation, eventually form the skin, as well as several other vital structures, including the central and peripheral nervous systems.
The mesoderm germ layers organize into somites, which eventually give rise to muscle tissue, cartilage, bone, and subcutaneous tissue of the skin.
As a result of embryonic folding, the major body plan becomes established, and the three germ layers continue to differentiate, giving rise to their own tissues and organ systems.