amnion

(noun)

The innermost membrane of the fetal membranes of reptiles, birds, and mammals; the sac in which the embryo is suspended.

Related Terms

Examples of amnion in the following topics:

  • Amnion Development

    • The amnion contains the fluid that cushions and protects the fetus.
    • The amnion is a closed sac appearing in the inner cell mass as a cavity.
    • When first formed, the amnion is in contact with the body of the embryo, but by about the fourth or fifth week, amniotic fluid (liquor amnii) begins to accumulate within it.
    • This fluid increases in quantity, causing the amnion to expand and ultimately to adhere to the inner surface of the chorion so that the extra-embryonic part of the coelom is obliterated.

  • Bilaminar Embryonic Disc Development

    • The embryonic disc is derived from the epiblast layer, which lies between the hypoblast layer and the amnion.
    • The epiblast forms the floor of the amniotic cavity and is continuous with the amnion.

  • Development of Metabolism

    • Diagram showing the expansion of amnion and delimitation of the umbilicus.

  • Yolk Sac Development

    • It is situated between the amnion and the chorion and may lie on or at a varying distance from the placenta.

  • Tissue Development

    • The epiblast in this region moves towards the primitive streak, dives down into it, and forms a new layer, called the endoderm, pushing the hypoblast out of the way (this goes on to form the amnion).

  • Muscle Development

    • The cells of the inner cell mass (embryoblast), known as human embryonic stem cells (hESCs), differentiate to form four structures: the amnion, the yolk sac, the allantois, and the embryo itself.