Spatial Formation and Divergence
The development of provisional structures is closely linked to the spatial divergence of tissue complexes. Directly behind the embryo's body folds, an important stage takes place—the divergence of extraembryonic parts of the germ layers.
This process proceeds in two opposite directions, forming two pairs of layers:
- The lower pair of layers moves downward. As a result of this directed cellular movement, the yolk sac is formed.
- The upper pair of layers, by contrast, moves upward. This spatial displacement leads to the formation of amniotic folds.
This separation allows the future organism to isolate itself from the extraembryonic parts that take over life-support functions during early development.
Comparative Characteristics: Birds and Mammals
To understand the evolutionary and histological specifics of provisional organs, comparative embryology is often used. The composition of these temporary structures varies slightly depending on the vertebrate class.
The extraembryonic organ complex in birds includes four elements:
- Yolk sac
- Amnion
- Serous membrane (serosa)
- Allantois
Features of the complex in mammals: Here, the composition of organs undergoes important changes. Mammals also possess a yolk sac, amnion, and allantois. However, a fundamental and key difference from birds is the complete absence of the serous membrane. Instead, mammals form a specialized villous membrane known as the chorion.
Histological Structure of Provisional Organ Walls
For convenience of study and systematization, each provisional organ is viewed as a structure whose wall always consists of exactly two tissue components.
As a general histological rule: extraembryonic mesoderm is present in the walls of absolutely all the listed organs. How it is positioned (externally or internally), as well as which germ layer acts as the second component (ectoderm or endoderm), directly depends on the origin and functional purpose of the specific organ.
| Organ Name | Inner Layer | Outer Layer |
|---|---|---|
| Amniotic sac | Extraembryonic ectoderm (amniotic) | Extraembryonic mesoderm |
| Chorion | Extraembryonic mesoderm | Extraembryonic ectoderm (trophoblast) |
| Yolk sac | Extraembryonic endoderm (hypoblast) | Extraembryonic mesoderm |
| Allantois | Extraembryonic endoderm | Extraembryonic mesoderm |
As seen from this classification, the ectoderm participates in the formation of the amnion and chorion, while the endoderm serves as the basis for the yolk sac and allantois.
Significance in the Embryology Course
The study of extraembryonic structures traditionally concludes the block dedicated to the early stages of embryogenesis. Understanding how the histogenesis and organogenesis of these temporary tissues occur lays a fundamental basis. This knowledge is essential for students to successfully master subsequent sections of special histology.
At the same time, the final summary of all early embryogenesis processes applied specifically to the human organism is usually examined at the concluding stages of the course.