Formation and histological structure
Yolk sac formation is a process in which germ layers grow over the yolk, enclosing it in a specialized membrane. As a result, a fully functional extraembryonic organ is formed.
Histologically, the wall of this organ is always strictly composed of two layers:
- Inner layer — represented by the extraembryonic endoderm, which develops from the hypoblast.
- Outer layer — represented by the visceral layer of the extraembryonic mesoderm. An important feature of this layer is the active development of blood vessels within it, which are essential for supplying nutrients and oxygen to the embryo.
Features of topography and development in birds
In birds, the yolk itself is located directly inside the sac—this is the main part of the ovum that did not participate in cleavage.
The connection between the organ and the embryo's body undergoes significant changes during embryogenesis:
- Initially, the cavity of the yolk sac has a very wide communication with the cavity of the primitive gut.
- The boundary between them is formed by body folds.
- As the embryo's body folds and the intestinal tube closes, this wide communication begins to narrow.
- Eventually, the vitelline stalk is formed — a narrow communication channel connecting the embryonic gut and the yolk sac itself.
Specifics of the yolk sac in mammals
The main distinctive feature in mammals is that yolk is absent. Nevertheless, the organ still forms: extraembryonic endoderm cells (from the hypoblast) actively proliferate, simply enclosing the free space within the trophoblast.
The spatial arrangement (topography) at this stage is as follows: the flat embryo, represented by the epiblast, lies strictly between two vesicles — the amniotic vesicle (located above) and the yolk sac (located below).
There are two successive stages of organ formation in mammals:
- Primary yolk sac — it directly adjoins the trophoblast, but is short-lived and regresses fairly quickly.
- Secondary yolk sac — smaller in volume compared to the primary one, yet it is this sac that holds functional significance for subsequent embryogenesis. It is worth noting that this organ functions only during early developmental stages, after which it also undergoes regression.
Functions of the yolk sac
Despite its provisional nature, the yolk sac performs three vital tasks:
- Hematopoietic function. This is the first site in the body where blood formation begins. The process is localized in the mesoderm of the sac wall.
- Generative function. Gonocytes — the precursors of germ cells — are first discovered in the wall of the organ. Subsequently, these cells migrate to the developing gonadal primordia of the embryo.
- Trophic function. Most pronounced in birds, where the yolk (along with albumen) serves as the main source of nutrients and building material for the growing embryo. In mammals, however, this function is practically nonexistent.
Briefly: answers to topical questions
At what week of human intrauterine development does the yolk sac undergo complete regression?
The yolk sac undergoes complete regression by the 12th week of pregnancy.
Normally, this organ is clearly visualized on ultrasound at 6 weeks. Before its involution, the yolk sac performs several critical temporary functions:
- Hematopoietic — supplying primary blood cells;
- Synthetic — production of alpha-fetoprotein and transferrins;
- Excretory, metabolic, and immunoregulatory.
Premature regression of the yolk sac (before the maturation of the fetal liver, spleen, and reticuloendothelial system) is considered a prognostic sign indicating a risk of miscarriage or missed abortion.
Which fetal structure does the vitelline stalk become part of later on?
Later, the vitelline stalk becomes part of the connecting stalk (amniotic stalk), which subsequently transforms into the umbilical cord.
The vitelline stalk forms as a result of longitudinal and transverse folding of the embryo's body, when the wide communication between the yolk sac cavity and the primitive gut narrows. Along with the emerging allantois, it becomes enclosed within the amniotic stalk. In the fully formed umbilical cord, the vitelline stalk regresses, though its rudimentary remnants can sometimes be found upon histological examination of umbilical cord cross-sections.
To remember the functions of the yolk sac, use the acronym H-G-T: Hematopoiesis (blood), Gonocytes (generative), Trophics (nutrition).
Class and exam questions
From which germ layers is the yolk sac wall formed?
The inner layer is formed from the extraembryonic endoderm (derived from the hypoblast), while the outer layer is covered by the visceral layer of the extraembryonic mesoderm.
What is the vitelline stalk?
This is a narrow channel that forms as the embryo folds and the intestinal tube closes. It connects the cavity of the primitive gut with the yolk sac.
What is the main difference between the mammalian and avian yolk sac?
In mammals, it completely lacks yolk, so the trophic function is practically absent, whereas in birds it serves as the main source of nutrition.
What else to study on this topic
- The process of gonocyte migration from the yolk sac wall to the gonadal primordia
- Mechanisms of angiogenesis and initial hematopoiesis in the extraembryonic mesoderm
- Formation and role of the amniotic vesicle in the topography of the flat embryo
- Regression of the primary yolk sac and formation of the secondary one in mammals
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