Classification of Extraembryonic Structures
During early human development, four main structures form that are not part of the embryo's own body. They are generally divided into two groups depending on how long they function in the organism.
The first group consists of temporary (regressing) organs. These include the yolk sac (saccus vitellinus) and allantois (allantois). They perform their vital tasks exclusively during the early stages of embryogenesis, after which they undergo regression quite rapidly.
The second group consists of permanent organs. This includes the chorion (villous membrane) and the amnion, which originates from the amniotic vesicle. These formations function right up until childbirth. Together with the maternal endometrial tissues, they form the placenta and fetal membranes, supporting fetal viability throughout pregnancy.
Yolk Sac and Allantois
Initially, the yolk sac is located directly beneath the embryonic disc. As longitudinal folding of the embryo's body occurs (embryonic folding), this structure maintains a connection with the developing gut only through a narrow channel called the vitelline duct.
At the same time, the allantois appears. The vitelline duct and allantois become enclosed within the body of the connecting stalk (amniotic stalk). Later on, the connecting stalk transforms into the fully developed umbilical cord.
The active phase of these temporary organs is limited to the first weeks of embryonic development (approximately up to the 8th week inclusively). By the time the umbilical cord is fully formed, they are already in a state of regression. However, during this short time, they manage to perform critically important functions:
- The yolk sac acts as a temporary residence for gonocytes (primordial germ cells). Furthermore, it is here that the very first foci of hematopoiesis are established.
- The allantois serves as a specific "guiding vector." Blood vessels grow along it, ultimately connecting the embryo's body with the developing placenta.
Fetal Membranes and Their Functions
The developing fetus is almost entirely surrounded by three specialized membranes that provide reliable mechanical protection.
This complex includes:
- Decidua capsularis (decidua capsularis) — the outer layer of maternal origin, as it forms from the uterine endometrium.
- Chorionic membrane — the middle layer, which is a derivative of the chorion.
- Amniotic membrane — the inner layer, a derivative of the amnion.
In addition to general mechanical protection, the inner layer has a specific task. The amnion is responsible for the continuous secretion and resorption of amniotic fluid. This dynamic process creates and maintains the optimal internal fluid environment necessary for the harmonious development of the fetus.
Placenta: Sources of Development and Structure
The placenta is an organ that provides a functional connection between the maternal and fetal circulatory systems. Unlike the fetal membranes, it has a mixed origin, meaning it is formed by the fusion of maternal and fetal tissues.
Three different sources participate in the formation of the placenta:
- Endometrium: gives rise to the maternal part of the placenta.
- Chorion: forms the bulk of the fetal part of the organ.
- Amnion: covers the fetal part from the inside, facing the amniotic cavity.
It is important to clearly understand the logic of interaction between the uterine and embryonic structures. That part of the endometrium which forms the placenta on the maternal side is called the decidua basalis (decidua basalis). On the fetal side, the villous chorion (chorion frondosum) lies adjacent to it.
The remaining areas form the outer and middle fetal membranes: on the maternal side, this is the decidua capsularis (decidua capsularis), and on the fetal side, the smooth chorion (chorion laeve). The remaining free uterine cavity is lined by the decidua parietalis (decidua parietalis).