General Process Overview
Immediately after gastrulation ends, or in parallel with its final stages (approximately starting from days 20–21 of development), a crucial stage begins: the separation of the embryo's body. This stage continues up to day 28.
The main goal of this period is to transform the flat embryonic disc into a fully spatial, volumetric structure. During this process, a clear separation of the future fetus's body from its surrounding extraembryonic elements occurs. This result 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 embryonic disc actively fold downward. As a result of these movements, trunk folds are formed.
The embryo essentially invaginates into the amniotic cavity while rising above the yolk sac. The flat disc gradually rolls up to form a closed cylindrical tube.
This is of particular importance for the development of the digestive system. During the rolling process, the embryonic endoderm closes, forming the primary intestinal tube. The trunk fold acts as the boundary separating the formed intestinal 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 changing the embryo's appearance.
The cranial (head) and caudal (tail) ends of the embryo bend intensively in the ventral direction—that is, 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 surface.
- Later, it is in the prechordal plate area that the membrane will rupture, leading to the formation of the oral opening.
Final results of 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: Initially, the wide communication between the embryo and the yolk sac narrows significantly. A thin, narrow stalk forms in its place. Subsequently, this stalk becomes an integral part of the amniotic stalk (which will later turn into the umbilical cord).
- Amnion development: The amniotic membrane actively proliferates, completely surrounding the embryo's body from all sides. The only area lacking amniotic coverage remains the attachment site of the amniotic stalk (the future umbilical cord).
- Nervous system: Simultaneously with the folding processes, rapid growth of the neural tube is observed. In its head (cranial) part, brain vesicles—large local expansions that will form the brain—are formed and clearly visualized.
Briefly: answers to topical questions
Which exact germ layers and membranes make up the trunk fold?
The trunk fold consists of all four germ layers. The following structures take direct part in its formation:
- Ectoderm
- Endoderm
- Visceral layer (part of the mesoderm)
- Parietal layer (part of the mesoderm)
This composition ensures the separation of the embryo's body from extraembryonic elements. As a result of trunk fold formation, each germ layer is divided into two functional parts: the embryonic part, which is located inside the folds and forms the embryo's body, and the extraembryonic part, located outside.
To visualize the process, imagine a flexible sheet of paper. First, its right and left edges roll downward to form a tube (lateral folding). Then, the upper and lower ends of this tube bend toward each other (longitudinal folding), forming the letter "C".
Class and exam questions
From which germ layer does the intestinal tube form during folding?
The intestinal tube is formed from the embryonic endoderm. During lateral folding of the disc, it closes into a tube and separates from the yolk sac.
What happens to the prechordal plate?
During longitudinal folding, it shifts from the dorsal surface to the ventral surface. Later, the oral opening forms at this site.
How does the embryo's connection with the yolk sac change?
The wide communication narrows down to a narrow stalk. This stalk then becomes part of the amniotic stalk, from which the umbilical cord is formed.
At what timelines does the separation of the embryo's body occur?
The process begins concurrently with the end of gastrulation, around days 20–21 of embryogenesis, and is fully completed by day 28.
What else to study on this topic
- Gastrulation process and formation of the embryonic disc
- Differentiation of the embryonic endoderm
- Formation of the amniotic stalk and umbilical cord
- Development of the neural tube and brain vesicles
- Transformation of the prechordal plate
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