Classification of extraembryonic structures
In the early period of human development, four main structures are laid down 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 the allantois (allantois). They perform their vital tasks exclusively in the early stages of embryogenesis, after which they undergo regression quite rapidly.
The second group comprises permanent organs. This includes the chorion (villous membrane) and the amnion, which originates from the amniotic vesicle. These structures function right up to childbirth. Together with the tissues of the uterine endometrium, 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 (curling of the embryo), this structure maintains a connection with the forming gut only via a narrow channel, the so-called vitelline stalk.
At the same time, the allantois appears. The vitelline stalk and the allantois become enclosed within the body of the connecting stalk. Subsequently, it is the connecting stalk that undergoes transformation, turning into a fully developed umbilical cord.
The active phase of functioning of these temporary organs is limited to the first weeks of embryonic development (approximately up to the 8th week inclusive). By the time the umbilical cord is fully formed, they are already in the stage 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 kind of "guiding vector". Along it, blood vessels grow that ultimately connect the body of the embryo 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 is formed 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. Through this dynamic process, an optimal internal fluid environment necessary for the harmonious development of the fetus is created and maintained.
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 structures of the uterus and the embryo. That part of the endometrium that forms the placenta on the maternal side is called the decidua basalis (decidua basalis). On the fetal side, the chorion frondosum (chorion frondosum) is 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 chorion laeve (chorion laeve). The remaining free uterine cavity is lined by the decidua parietalis (decidua parietalis).
Briefly: answers to topical questions
What functions does the yolk sac perform in the human embryo?
The yolk sac in the human embryo performs hematopoietic and generative functions.
- Hematopoietic function — blood formation begins for the first time in the wall of the sac (the first foci of hematopoiesis and vasculogenesis).
- Generative function — serves as a temporary residence and formation site for primordial germ cells (gonocytes) that have migrated from the epiblast.
Trophic function is practically absent in humans.
What functions does the allantois perform in human embryogenesis?
In human embryogenesis, the allantois acts as a "guiding vector" for blood vessels. Along it (more precisely, along its mesodermal cord), fetal blood vessels sprout toward the chorionic villi, connecting the embryo to the developing placenta. Through these vessels, the allantois indirectly supports the respiratory, nutritive, and excretory functions of the placenta.
What structures are part of the mature umbilical cord?
The mature umbilical cord includes the following structures:
- Amniotic membrane — covers the umbilical cord externally (simple columnar epithelium and a thin lamina propria).
- Mucous connective tissue ("Wharton's jelly") — mucous connective tissue that makes up the stroma and contains mucocytes.
- Two umbilical arteries — carry venous blood from the fetus to the chorion.
- One umbilical vein — carries arterial blood from the placenta to the fetus.
- Nerve endings — are located along the vessels.
Also, rudimentary remnants of the allantois and the yolk stalk may be found in cross-sections.
What hormones does the placenta secrete?
The placenta secretes three groups of hormones and biologically active substances:
- Pituitary hormone analogues — human chorionic gonadotropin (hCG), placental lactogen (prolactin/hPL), adrenocorticotropic hormone (ACTH), chorionic somatomammotropin, as well as analogues of GH, TSH, and melanocyte-stimulating hormone.
- Female sex hormones (steroids) — estrogens and progesterone (progestins).
- Relaxins (produced by decidual cells).
The placenta also synthesizes hypothalamic releasing and inhibiting factors (CRH, TRH, somatoliberin, somatostatin, prolactoliberin).
How to remember parts of the placenta: Placenta = basalis + frondosum (decidua basalis + chorion frondosum). Membranes = capsularis + laeve (decidua capsularis + chorion laeve).
Class and exam questions
Where do the first foci of embryonic hematopoiesis appear?
The first foci of hematopoiesis, as well as primordial germ cells, are initially established in the yolk sac during the first weeks of development.
How is the umbilical cord formed?
The connecting stalk, inside which the yolk stalk and allantois are located, gradually transforms and forms the umbilical cord.
What is the function of the amniotic membrane?
The amnion performs a crucial specific function: it secretes and resorbs amniotic fluid, creating an internal fluid environment for fetal development.
What are the maternal and fetal parts of the placenta made of?
The maternal part is formed from the endometrium (decidua basalis), while the fetal part is formed predominantly from the chorion (chorion frondosum) and is lined on the inside by the amnion.
What else to study on this topic
- Mechanisms of yolk sac regression
- Formation and topography of the connecting stalk
- Dynamics of amniotic fluid secretion
- Histological structure of the decidua
- Interaction of maternal and fetal circulatory systems
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