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27.3C: Amnion Development

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  • Page ID
    8273
  • The amnion contains the fluid that cushions and protects the fetus.

     

    LEARNING OBJECTIVES

     

    Describe the development of the amnion

     

    KEY TAKEAWAYS

    Key Points

     

    • In humans, the amnion is present in the earliest observed embryonic stage, appearing as a cavity within a mass of cells.
    • The amniotic cavity is roofed in by a single stratum of flattened, ectodermal cells, called the amniotic ectoderm, while the floor is composed of the prismatic ectoderm of the embryonic disk.
    • Outside the amniotic ectoderm, a thin layer of mesoderm is connected by the body stalk with the mesodermal lining of the chorion.
    • By approximately the fourth to fifth week, amniotic fluid (liquor amnii) begins to accumulate in the amnion, which increases in quantity and expands to contact the chorion.
    • During the later stages of pregnancy, the amniotic fluid allows easier and more fluid movements of the fetus and diminishes the risk of injury.

     

    Key Terms

     

    • amniotic fluid: In placental mammals, a fluid contained within the amnion membrane that surrounds a developing embryo or fetus (also called liquor amnii).
    • embryonic disk: The floor of the amniotic cavity is formed by the embryonic disk (or disc), which is composed of a layer of prismatic cells.
    • amnion: The innermost membrane of the fetal membranes of reptiles, birds, and mammals; the sac in which the embryo is suspended.
    • chorion: One of the membranes that exist during pregnancy between the developing fetus and mother.

    This is a cutaway drawing showing a human fetus in a womb. The fetus is seen enclosed within the amnion.

     

    Amnion: The human fetus is enclosed within the amnion.

    The amnion is a closed sac appearing in the inner cell mass as a cavity. This cavity is roofed in by a single stratum of flattened, ectodermal cells called the amniotic ectoderm. Its floor consists of the prismatic ectoderm of the embryonic disk.

    The continuity between the roof and the floor is established at the margin of the embryonic disk. Outside the amniotic ectoderm is a thin layer of mesoderm (continuous with that of the somatopleure), which is connected by the body stalk with the mesodermal lining of the chorion.

    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.

    This increase continues up to the sixth or seventh month of pregnancy, after which it diminishes somewhat. At the end of pregnancy, it amounts to about one liter.

    The amniotic fluid allows some free movement for the fetus during the later stages of pregnancy and also diminishes the risk of injury. It contains less than two percent solids, and consists mainly of urea and other extractives, inorganic salts, a small amount of protein, and, frequently, a trace of sugar.

    Note: That some of the liquor amnii is swallowed by the fetus is proved by the fact that epidermal debris and hairs have been found among the contents of the fetal alimentary canal.