Origin and histological structure
The amnion and serosa develop from the exact same material—amniotic folds. Two germ layers take part in their formation:
- Extraembryonic ectoderm.
- Parietal layer of extraembryonic mesoderm.
In its final form, the amnion is a closed sac. Histologically, its wall has a distinct layer orientation: the ectoderm always faces inward, directly toward the fetus and the amniotic cavity, while the mesoderm faces outward. Interestingly, in the serosa, which also forms from these folds, the orientation of the layers is opposite (inverted): the mesoderm faces inward, and the ectoderm faces outward (toward the eggshell).
Mechanism of development in birds
In birds, the amnion forms in the classic manner via folds:
- The tissues rise and form a bend—the amniotic fold—which begins to creep over the embryo from both sides.
- The fold itself comprises inner (lower) and outer (upper) limbs.
- The folds meet above the embryo and fuse, forming the amnio-serous junction.
- The inner limbs form the amnion itself, while the outer limbs form the serosa lining the inner surface of the shell.
Developmental features in mammals
In mammals, the amnion is laid down at much earlier stages—already during the first phase of gastrulation. The main difference is that amniotic folds do not form here.
Instead, cavitation (delamination) of the epiblast occurs. It splits into two layers: the embryonic epiblast and the amniotic ectoderm. An amniotic cavity appears between them due to fluid secretion. Initially, the roof and lateral walls of the amnion consist solely of ectoderm. Later, mesenchymal cells (extraembryonic mesoderm) migrate from the embryonic disc and envelop the ectodermal vesicle from the outside. As a result, the final structure of the mammalian amnion becomes identical to that of birds.
Positional dynamics and functions
In early developmental stages, the embryo lies beneath the amniotic cavity, which serves as a sort of "roof" for it. However, as the embryo grows and folds, it becomes completely immersed inside the cavity. Consequently, the amniotic fluid bathes the fetus from all sides, making the amnion the innermost membrane.
The only exception is the amniotic stalk—the region connecting the embryo to the chorion. There is no fluid here, and umbilical vessels will later pass through this site.
Key functions of the amnion:
- Secretion of amniotic fluid and maintenance of the aqueous environment.
- Cushioning and mechanical protection of the developing fetus.
- Trophic function (providing nutrition).
Briefly: answers to topical questions
What type of epithelium lines the wall of the amnion in late intrauterine development?
The wall of the amniotic membrane is lined by a simple columnar epithelium resting on a basement membrane. This epithelial layer is of ectodermal origin and forms directly from the extraembryonic ectoderm. Beneath the epithelium lies the lamina propria, consisting of dense fibrous connective tissue derived from the extraembryonic mesoderm.
What are the specific sources of production and pathways of resorption of amniotic fluid?
Amniotic fluid is produced through secretion by the amniotic ectoderm cells; the amnion functions to create and maintain the fluid environment. The cited sources also indicate that in maternal diabetes mellitus, polyhydramnios is associated with fetal polyuria and the reaction of the amniotic epithelium to high glucose levels in the amniotic fluid. Specific pathways of amniotic fluid resorption are not detailed in the sources; it is only noted that impaired blood supply to the amnion may be accompanied by decreased resorption of amniotic fluid.
To remember the layers of the amnion and serosa, use the "mirror" rule. The amnion faces the embryo with its ectoderm, while the serosa faces away from it—with its ectoderm outward.
Class and exam questions
From the amniotic folds, specifically from their inner (lower) limbs.
From the amniotic folds, specifically from their inner (lower) limbs.
Do amniotic folds participate in amnion development in mammals?
No, in mammals the amnion forms by splitting the epiblast into the embryonic epiblast and amniotic ectoderm.
What is the amniotic stalk?
This is the structure connecting the embryo to the chorion. It is the only site where the fetus is not surrounded by amniotic fluid.
What else to study on this topic
- Formation of the serosa and its role in gas exchange
- Histogenesis of the amniotic ectoderm
- The process of epiblast cavitation in mammals
- Structure of the amnio-serous junction
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