34. Amnion, Allantois, and Yolk Sac

  1. The amnion is the membrane around the fetus. Amniocentesis is the transabdominal aspiration of fluid from the amniotic sac (the innermost of the membranes enveloping the embryo in utero). The amnion's epithelial cells have microvilli which may play a role in fluid transfer
    1. ORIGIN OF FLUID: may initially be secreted by amniotic cells but most comes from the maternal blood
      1. The fetus also contributes by excreting urine into the amniotic fluid
      2. By late pregnancy, one-half a liter of fetal urine is added daily
    2. VOLUME OF FLUID increases slowly to about 30 ml at 10 weeks, 350 ml at 20 weeks, and 1000 ml by 37 weeks. It then decreases very sharply
      1. Low volume (about 400 ml) results in oligohydramnios, a consequence of placental insufficiency and decreased blood flow
      2. Polyhydramnios (excess fluid) may occur when the fetus does not drink its usual amount of fluid and is often associated with malformations of the CNS
    3. FLUID EXCHANGE: water in the amniotic fluid changes every 3 hours
      1. Fluid is normally swallowed by the fetus, up to 400 ml/day near term, and absorbed back into the fetal circulation via the fetal gastrointestinal tract
    4. COMPOSITION OF FLUID: it is really a suspension consisting of desquamated fetal epithelial cells and equal parts of organic and inorganic salts in 98-99% water
      1. Changes occur as fetal excreta is added with pregnancy development
    5. SIGNIFICANCE OF FLUID: embryo floats freely in the fluid
      1. Permits symmetric external growth of the embryo
      2. Prevents adherence of the amnion to the embryo
      3. Cushions the embryo against some trauma
      4. Helps control embryonic body temperature
      5. Helps fetus move freely, aiding in skeletomuscular development
  2. The allantois
    1. SIGNIFICANCE: blood formation occurs in its walls during weeks 3-5, and its blood vessels become the umbilical arteries and vein
    2. FATE: its intraembryonic portion runs from the umbilicus to the urinary bladder with which it is continuous
      1. As the bladder enlarges, it involutes to form the urachus
      2. After birth, the urachus becomes a fibrous cord, the median umbilical ligament
  3. The yolk sac: nonfunctional as yolk storage site in the human
      1. Role in transfer of nutrients in the embryo during weeks 2-3
      2. Blood development occurs in its walls beginning in week 3 and continues to form there until the hematopoietic activity begins in the liver at about week 5
      3. During week 4, its dorsal part is incorporated into the embryo as an entodermal tube, the primitive gut, and gives rise to the epithelium of the trachea, bronchi, lungs, and digestive tract
      4. Primordial germ cells appear in the yolk sac wall in week 3 and migrate to the area of developing sex glands (gonads) where they become germ cells (oogonia and spermatogonia)
    2. FATE: by week 12, small yolk sac lies in chorionic cavity between amnion and chorionic sac; it shrinks as pregnancy proceeds and eventually gets smaller and solid and may even persist through pregnancy but is of no significance
      1. The yolk stalk usually detaches from the gut by the end of week 5, but in 2% of cases, the proximal intra-abdominal part of the yolk stalk persists as a diverticulum of the ileum, Meckel's diverticulum

amnion, allantois, and yolk sac: image #1