General Characteristics of the Process
Immediately after gastrulation or concurrently with its final stages (approximately starting on days 20–21 of development), a critical stage begins—the separation (delimitation) of the embryo's body. This stage continues until day 28.
Main goal of this period is to transform the flat germ disc into a fully spatial, volumetric structure. Simultaneously, a clear separation occurs between the future fetus's body and the surrounding extraembryonic elements. This is achieved through two synchronously occurring folding processes:
- Lateral (or transverse) folding.
- Longitudinal (or craniocaudal) folding.
Mechanism of Lateral Folding
Transverse folding determines the formation of a cylindrical body shape. During this period, the edges of the germ disc begin to fold downward actively. As a result of these movements, lateral body folds are formed.
The embryo seems to invaginate into the amniotic cavity while rising above the yolk sac. The flat disc gradually rolls up, forming a closed cylindrical tube.
This is of particular importance for the development of the digestive system. During the rolling process, the embryonic endoderm closes to form the primitive gut tube. The body fold acts as a boundary separating the newly formed gut tube from the yolk sac, although a physical connection between them persists for some time.
Longitudinal Folding and Topographic Changes
Longitudinal folding proceeds strictly in parallel with lateral rolling, radically altering the embryo's external appearance.
The cranial (head) and caudal (tail) ends of the embryo bend intensely in a ventral direction—toward the yolk sac. A longitudinal section clearly demonstrates how the embryo acquires its typical C-shape.
In addition, important topographic rearrangements take place:
- The prechordal plate shifts from the dorsal surface of the body to the ventral side.
- Later, a membrane rupture will occur precisely in the prechordal plate zone, leading to the formation of the oral cavity (stomodeum).
Outcomes of Embryonic Folding
The combination of longitudinal and transverse folding leads to global changes in the structure of the embryo and its connection with provisional organs.
- Connection with the Yolk Sac: The initially wide communication between the embryo and the yolk sac narrows significantly. In its place, a thin, narrow stalk is formed. Subsequently, this stalk becomes an integral part of the connecting stalk (body stalk) (which later transforms into the umbilical cord).
- Amnion Development: The amniotic membrane actively grows, completely surrounding the embryo's body from all sides. The only area lacking amniotic coverage remains the site of origin of the connecting stalk (future umbilical cord).
- Nervous System: Concurrently with folding processes, rapid growth of the neural tube is observed. In its cranial part, brain vesicles—large local expansions that will form the brain—are formed and clearly visualized.