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.
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 (decidualization): implantation → thickening and structural changes of the endometrium → formation of decidua
Function of decidua:
- Histiotrophic nutrition: storage of fat and glycogen causes cellular enlargement → secretion of lytic enzymes by syncytiotrophoblast during invasion of the decidua → nutrient uptake by syncytiotrophoblast
- Immune privilege: tight junctions separate the conceptus from adjacent endometrial tissue
- Preparation for placental circulation: under the influence of progesterone, decidual vessels transform into a network of anastomosing spiral arteries (uterine vascular remodeling)
- The decidua has three distinct parts, which are distinguished according to their relation to the site of implantation:
- Placentation refers to the development of the placenta. The embryonic portion of the placenta is derived from cells of the trophoblast and the maternal portion from the decidua basalis.
Early placental development:
- Until approx. day 9 of embryogenesis
- From approx. day 9 of embryogenesis
- Lacunae form in syncytiotrophoblast, these are separated by thin syncytiotrophoblast trabeculae.
- Lytic enzymes of syncytiotrophoblast eventually erode the spiral arteries of the decidua, causing maternal blood to fill the lacunae.
- Lacunae merge to form the
- Hemotrophic nutrition: nutrient supply from maternal blood
Early villous stage:
- From approx. day 13–28 of embryogenesis
- Prelacunar stage:
- During the course of placental development, the composition of chorionic villi changes.
Chorionic villi development and maturation:
- Primary villi
- Secondary villi
Tertiary villi (terminal villi): connect to the umbilical cord vessels during week 3 of development
- Tertiary villi develop through vascularization
- Terminal villi develop after the 4thmonth → cytotrophoblast cells begin to disappear → only isolated cytotrophoblast cells (Langhans cells ) remain
- Structure of terminal villi
- Basal plate (decidual basalis) (mainly maternal component)
- Intervillous space and villous trees (fetomaternal zone)
- Chorionic plate (fetal component)
Basal plate (decidual basalis)
- Mainly maternal component of the placenta, abuts the uterine wall
- Structure: maternal decidua with several ingrown embryonal cells (cytotrophoblast, syncytiotrophoblast, and extravillous trophoblast cells )
- Supplied by uterine spiral arteries
Intervillous space and villous trees
- Contact zone between maternal and fetal placental structures (site of fetomaternal gas and nutrient exchange)
- Filled with maternal blood
- Contains protruding villous trees
Villous trees: placenta is composed of 30–50 branching villous trees
- Stem villi: basal region of villous trees with fetal arteries and veins
- Intermediate villi: region of villous trees with fetal arterioles, venules, and capillaries
- Terminal villi: tertiary villi that float freely in the intervillous space and are directly involved in gas and nutrient exchange
- Anchoring villi: anchor the villous trees to the decidua
- Fetal component of the placenta
- Structure: formed by the syncytiotrophoblast, the cytotrophoblast, and the somatic layer of the extraembryonic mesoderm
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).
- Structure until the 4th month (from maternal to fetal)
- Structure from the 4th month (from maternal to fetal)
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!
- Site of production: Syncytiotrophoblast
- Function of hormones
- The most important hormones are HCG, HPL, CRH, estrogen, and progesterone
|Hormone||Site of production||Effect(s)||Course during pregnancy|
|hCG (human chorionic gonadotropin)|| |
|hPL (human placental lactogen)|
|Progesterone|| || |
|Thyroid hormones|| |
|Oxytocin|| || |
Gas and nutrient exchange
- Passive transport
- Active transport: amino acids, peptides, hormones, vitamins, fatty acids, inorganic ions
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 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
- The umbilical cord contains 3 blood vessels that carry fetal blood:
Early stage in the development of the umbilical cord
- Connecting stalk: precursor of the mature umbilical cord
- Allantois (small sac-like structure that protrudes into the connecting stalk)
Vitelline duct: joins the midgut to the yolk sac
- Obliteration during 7th week
- A failed obliteration leads to vitelline fistula (no obliteration), or meckel diverticulum (partial obliteration)
Late stage in the development of the umbilical cord
- Ground substance: gelatinous connective tissue (Wharton jelly)
- Cover: amniotic epithelium
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.
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
Amnion: inner amniotic membrane
- Develops from the embryoblast and secretes amniotic fluid
Chorion: middle amniotic membrane
- Develops from the cytotrophoblast
Decidua: outermost membrane
- Develops from the decidua capsularis, which lies above the site of implantation
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 by the amniotic epithelium
- The fetus swallows approx. 400 mL/day of amniotic fluid per day, which is excreted through the kidneys.