What type of epithelium lines the wall of the amnion at late stages of prenatal development?

The wall of the amniotic membrane is lined by a simple columnar (or low cuboidal/prismatic depending on species) 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 amniotic ectoderm cells; the amnion functions to create and maintain the fluid environment. Maternal diabetes mellitus can lead to polyhydramnios associated with fetal polyuria and the response of the amniotic epithelium to high glucose levels in the amniotic fluid. Specific resorption pathways include absorption across the amnion into the fetal/maternal circulation, with impaired blood supply to the amnion potentially reducing fluid resorption.

Which structure gives rise to the amnion in birds?

From the amniotic folds, specifically from their inner (lower) limbs.

Do amniotic folds participate in the development of the amnion in mammals?

No, in mammals the amnion forms via cleavage (cavitation) of the epiblast into the embryonic epiblast and the amniotic ectoderm.

What is the connecting stalk?

It is the structure connecting the embryo to the chorion. It is the only area where the fetus is not surrounded by amniotic fluid.

Home › Histology › Amnion

Amnion

*Amnion*

For medical students2 min readUpdated 2026-10-10

The amnion (amniotic sac) is a provisional extraembryonic membrane and the innermost fetal membrane surrounding the embryo. It forms a closed fluid-filled reservoir, providing the necessary aqueous environment for embryonic development and performing protective functions.

Tissue compositionExtraembryonic ectoderm (inner) and extraembryonic mesoderm (outer)

EnvironmentCreates and maintains an aqueous environment for the embryo

Mammalian differenceForms without amniotic folds, via cavitation of the epiblast

Connecting stalkThe only site where the fetus is not bathed in fluid; connects the embryo to the chorion

Origin and Histological Structure

The amnion and the serosa (serosal membrane) develop from the same embryonic material — the amniotic folds. Two germ layers participate in their formation:

Extraembryonic ectoderm.
Parietal layer of the extraembryonic mesoderm.

In its final form, the amnion is a closed sac. Histologically, its wall has a strict 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 layer orientation is reversed (inversion): the mesoderm faces inward, and the ectoderm faces outward (toward the eggshell).

Developmental Mechanism in Birds

In birds, the amnion forms through the classic fold mechanism:

Tissues rise to form a fold — the amniotic fold — which begins to grow over the embryo from both sides.
The fold itself is divided into 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 inside of the shell.

Developmental Features in Mammals

In mammals, the amnion is laid down at earlier stages — as early as 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 surround the ectodermal vesicle externally. As a result, the final structure of the mammalian amnion becomes identical to that in birds.

Position Dynamics and Functions

At early developmental stages, the embryo lies beneath the amniotic cavity, which serves as a sort of "roof." However, as the embryo grows and folds, it becomes completely submerged inside the cavity. Consequently, the amniotic fluid bathes the fetus from all sides, making the amnion the innermost fetal membrane.

The only exception is the connecting stalk (funiculus umbilicalis precursor) — the region connecting the embryo to the chorion. There is no fluid here, and umbilical vessels will eventually pass through this area.

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).

Mnemonic

To remember the layer arrangement of the amnion and serosa, use the "mirror" rule: the amnion faces the embryo with its ectoderm, whereas the serosa faces away from it, with its ectoderm on the outside.

Frequently asked questions

What type of epithelium lines the wall of the amnion at late stages of prenatal development?: The wall of the amniotic membrane is lined by a simple columnar (or low cuboidal/prismatic depending on species) 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 amniotic ectoderm cells; the amnion functions to create and maintain the fluid environment. Maternal diabetes mellitus can lead to polyhydramnios associated with fetal polyuria and the response of the amniotic epithelium to high glucose levels in the amniotic fluid. Specific resorption pathways include absorption across the amnion into the fetal/maternal circulation, with impaired blood supply to the amnion potentially reducing fluid resorption.
Which structure gives rise to the amnion in birds?: From the amniotic folds, specifically from their inner (lower) limbs.
Do amniotic folds participate in the development of the amnion in mammals?: No, in mammals the amnion forms via cleavage (cavitation) of the epiblast into the embryonic epiblast and the amniotic ectoderm.
What is the connecting stalk?: It is the structure connecting the embryo to the chorion. It is the only area where the fetus is not surrounded by amniotic fluid.

Go deeper

Formation of the serosal membrane and its role in gas exchange
Histogenesis of the amniotic ectoderm
Process of epiblast cavitation in mammals
Structure of the amnio-serous junction