The placenta, umbilical cord, and amniotic sac

Abstract

The placenta, umbilical cord, and amniotic sac protect and provide nutrients to the fetus. The placenta is a fetomaternal organ that enables selective transfer of nutrients and gases between mother and fetus. The placental barrier limits direct contact between the embryo and maternal blood, thus protecting both mother and child from potentially harmful substances (e.g., blood cell antigens of the unborn, bacteria from the mother). In addition, the placenta produces hormones that mediate maternal adaptation to pregnancy and maintain pregnancy. Establishing uteroplacental circulation involves several steps, including endovascular trophoblast invasion and uterine vascular remodeling. The 50–70 centimeter long umbilical cord connects the placenta with the fetus and contains one umbilical vein for oxygenated, nutrient-rich blood supply and two umbilical arteries for returning deoxygenated blood to the maternal circulation. The amniotic sac surrounds the fetus and contains the amniotic fluid providing mechanical protection to the developing fetus.

Development of uteroplacental circulation

Following implantation, the endometrial lining is transformed (decidual reaction). The decidua provides nourishment to the embryo until the definitive placenta forms. Approximately on day 12 of embryonic development, fetal blood vessels (through opening of the maternal vessels) come into contact with maternal blood, forming a region of fetal-maternal exchange.

Decidual reaction

Placentation

The placenta

Placental structure

At the end of pregnancy, the mature placenta weighs approx. 500 g, is about 2 cm thick, and has a diameter of 15–20 cm. It consists of three parts:

Basal plate (decidual basalis)

Intervillous space and villous trees

Chorionic plate

Placental barrier

Maternal and fetal circulation are separated by the placental barrier. The placental barrier controls the gas and nutrient exchange. Until the fourth month of development, the placental barriers consists of five layers. After the fourth month, the cytotrophoblast disappears from the villous wall, leaving only the isolated cytotrophoblast cells (Langhans cells).

After birth, the placenta must be inspected to ensure it has detached completely from the uterine wall. If this does not occur, there is a risk of postpartum hemorrhage. The check is performed by inspecting for the completeness of all placental cotyledons. On the fetal side, the placenta should be covered by the amnion!

Placental function

Hormone production

Hormone Site of production Effect(s) Course during pregnancy
hCG (human chorionic gonadotropin)
  • Continuation of pregnancy
    • Formation and maintenance of the corpus luteum graviditatis
    • Stimulation of progesterone and estrogen synthesis of the corpus luteum graviditatis
hPL (human placental lactogen)

CRH (corticotropin-releasing hormone)

  • Is thought to play a role in determining the duration of gestation, in maturation of the fetal lung, and in surfactant production
  • Increases during pregnancy or maternal stress
Estrogen
  • Increases continuously until birth
Progesterone
  • Optimizes transport of zygote along the fallopian tube and subsequent implantation

  • Maintains pregnancy

  • Transformation of endometrium (proliferative function → secretory function)

  • Prevents menstruation (degradation of endometrium)

  • Closure of the cervix and increased viscosity of cervical secretion

  • Inhibits uterine contractions

  • Stimulates the expression of uterine oxytocin receptors prior to delivery

  • Inhibits further differentiation of breast tissue in the second half of pregnancy

  • Following delivery, rapid decline of progesterone triggers the secretory activation lactogenesis phase of lactogenesis marked by copious milk production

  • Increases continuously until birth
Thyroid hormones
  • Vital for fetal neurologic development (e.g., myelination in the CNS)
  • Rises continuously until birth

    • The level of free, active hormone does not increase since the thyroid-binding globulin also increases

Oxytocin
  • Maternal hypothalamus
  • Induces uterine contractions
  • Promotes bonding between mother and child
Prolactin

Gas and nutrient exchange

Fat-soluble vitamins (A, D, E, K), immunoglobulins (except IgG), and most proteins are either unable to cross the placental barrier or have an only limited ability to do so. Vitamin K is an important cofactor for blood coagulation and should be administered to the newborn infant directly after birth!

Anti-D antibodies from the Rhesus system (= IgG antibodies) are able to cross the placental barrier. In contrast, isoagglutinins of the ABO system are mainly IgM antibodies, which means that they are unable to cross the placental barrier!

The umbilical cord

The umbilical cord connects the fetus with the fetal part of the placenta (chorionic plate). It typically attaches centrally to the chorionic plate of the placenta. Development of the umbilical cord begins at approx. the 3rd week of embryogenesis. By the end of pregnancy, the umbilical cord is approx. 50–70 cm long.

Formation and structure of the umbilical cord

Early stage in the development of the umbilical cord

Late stage in the development of the umbilical cord

The umbilical arteries carry deoxygenated blood, whereas the umbilical vein carries oxygenated blood!

Physiological umbilical hernia

Due to their rapid growth, there is a short period of time during which there is not enough space for the abdominal within the embryonic abdominal cavity. As a result, sections of the gut herniate into the extraembryonic coelom of the future umbilical cord from the 6th–10th week of development.

Amnion and amniotic cavity

Amniotic cavity

The amniotic sac is formed very early in pregnancy and surrounds the embryo as a protective shell. As the fetus grows, the amniotic cavity expands, which eventually results in the obliteration of the chorionic cavity and the uterine cavity.

  • Development: 2nd week of development through migration of epiblast cells
  • Components
    • Lined with amniotic epithelial cells
    • Filled with amniotic fluid, which is produced by amniotic epithelial cells

Amniotic sac

The amniotic sac is composed of maternal (decidua) and fetal components (chorioamniotic membranes) which surround the fetus and provide mechanical protection.

  • Amnion: inner amniotic membrane
    • Develops from the embryoblast and secretes amniotic fluid
  • Chorion: middle amniotic membrane
  • Decidua: outermost membrane

Amniotic fluid

Protective fluid within the amniotic sac that cushions the fetus, prevents adherence of the fetus to the amnion, and serves as a transport medium for nutrients and metabolites.

  • Composition: initially a clear liquid
    • Amount: approx. 850–1500 mL by the end of pregnancy (the amniotic fluid is completely exchanged every 3 hours)
    • pH: 7–7.5 (slightly alkaline)
    • Proteins, glucose, urea
    • Hair, dead skin, sebum
    • Fetal urine
    • Vernix: a milky-white, lipid-rich substance that consists of fetal dermal cells and sebaceous gland secretions. It covers the fetus's skin (especially in the third trimester).
  • Reabsorption
    • Reabsorption by the amniotic epithelium
    • The fetus swallows approx. 400 mL/day of amniotic fluid per day, which is excreted through the kidneys.

Clinical significance

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last updated 11/18/2018
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