The Placenta

Early Development | Main Anatomy Index | Embryonic Folding

This page was contributed by David Boshell

The Placenta

• The human placenta is of chorioallantoic type, consisting of 2 components:
  1. The chorion, or outer layers of the chorionic cavity, including:
     1. The syncytiotrophoblast
     2. The cytotrophoblast
     3. The extraembryonic mesoderm
  2. The allantois, containing the umbilical vessels and 3 foetal blood vessels: 2 arteries and 1 vein.
• It also comes under haemochorial classification, with circulation derived from maternal blood and the chorion and other solid parts being derived from the foetus.

• The foetal trophoblast invades the endometrium, breaks down the endometrial blood vessels into lacunae, and forms villi around them.

• Thus, the placenta is a materno-foetal unit, with:
  1. The foetal portion formed by the chorion (chorionic plate)
  2. The maternal portion formed by the decidua basalis (basal plate).

• The foeto-maternal interface consists of these opposed layers:
  1. The chorion frondosum
  2. Fibrin
  3. The decidua basalis

Back to top

The decidual cell reaction of the endometrium

• After implantation, all but the deepest layer of endometrium proliferates, and is called the decidua (decidua graviditas).
• Stromal cells, under influence of the invading trophoblast, differentiate into large, rounded, glycogen filled decidual cells, whilst the uterine glands enlarge, becoming more coiled, before thinning later in pregnancy.
• There are 3 regions of the decidua:
  1. The decidua basalis, underlying the implantation site.
  2. The decidua capsularis, a thin portion between between the implantation site and the uterine lumen, surrounding the chorion.
  3. The decidua parietalis, including the remaining endometrium of the uterus.

• As the embryo grows and folds, the amnion rapidly expands, until the chorionic cavity is obliterated, and its surrounding decidua capsularis eventually fuses with the decidua parietalis, obliterating the uterine cavity by the 3rd month.

Back to top

The uteroplacental circulation

• The placenta begins to develop on day 9, with trophoblastic lacunae opening in the syncitiotrophoblast that anastomose with nearby maternal sinusoids.

• Chorionic villi begin to develop with extensions of cytotrophoblast, covered with syncitiotrophoblast, growing out into the lacunae between days 11-13, called primary stem villi.
• On day 16, extraembryonic mesoderm penetrates the cytotrophoblast core of the primary stem villi, transforming it into secondary stem villi.
• Anchoring villi will reach the maternal endometrium, whilst floating villi will not, floating in the lacunae.
• By the end of the third week, the anchoring villi contain differentiated blood vessels, and are called tertiary stem villi, thus establishing the uteroplacental circulation.
• The cytotrophoblast within the anchoring villi grows towards the maternal endometrium, branching to form a trophoblastic shell from neighbouring villi.
• Villi on the decidua capsularis degenerate after 8 weeks, producing the thin chorion laeve, whilst the villi adjacent to the decidua basalis grow and branch, with this thick region becoming known as the chorion frondosum (villous chorion).

• There are 2 types of cytotrophoblast cells:
  1. Darker syncitiotrophoblast cells that produce hCG
  2. Langhans trophoblastic cells in the villi
• Cells of the villi themselves include:
  1. Connective tissue fibroblasts (mesoblasts)
  2. Hofbauer cells - large cells with vacuoles that act like macrophages, acting to reshape the villi.

• There is unusual immunological tolerance of the placenta by the mother; blebs of syncitiotrophoblast break off from the villi into the lacunae, where they travel via maternal veins to the lungs where they are taken up by Dust cells, but there is no immune reaction to these.

• Circulation through the embryo and the villi starts at about day 21, with the intervillous space as the site of exchange of nutrients and waste products between the maternal and foetal circulatory system.
• Foetal and maternal blood do not mix, separated by the placental barrier of foetal tissues.

Formation of the umbilical cord

• The umbilicus is formed when the connecting stalk and the vitelline duct are bound together by the expanding amnion, and consists of 3 umbilical blood vessels with the allantois (of the connecting stalk), enclosed in mesenchyme.
• Deoxygenated foetal blood enters the placenta via 2 umbilical arteries, derived from the superior vesical arteries,with a star shaped lumen formed by 3 smooth muscle layers:
• Foetal blood runs through capillaries in the villi, where gases and metabolites are exhanged across the placental barrier with maternal blood supplied by spiral endometrial arteries.
• Oxygentated foetal blood returns from the placenta via the umbilical vein.

• Oxygen, water, carbon dioxide, hormones, vitamins and antibodies can all cross the placental barrier.
• Many harmful substances can also cross, including drugs like nicotine and alcohol, and viruses like Rubella, CMV and HIV.
• Untreated diabetes in the mother, leading to hyperglycemia, leads to high blood glucose across the placental barrier, thus inducing the foetal pancreas to secrete insulin, elevating foetal blood insulin and causing enourmous growth of the foetus.

• The placenta must be removed after birth and the uterus allowed to contract, otherwise the spiral arteries will not close, leading the bleeding and infection.

Back to top

The placenta as a major endocrine gland

• The placenta takes over the corpus luteum from the role of producing the steroid hormones oestrogen and progesterone as pregnancy proceeds; synthesising these hormones in the syncytiotrophoblast.
• The placenta also secretes several peptide hormones, including:
  1. Human chorionic gonadotropin (hCG) - produced by the syncytiotrophoblast, with synthesis beginning around day 6 and peaking in the 1st trimester; it is used to detect pregnancy.
  2. Human chorionic somatomammotropin (hCS) or Human placental lactogen (hPL) - synthesised by the syncitiotrophoblast, promotes general growth, glucose metabolism, and late in pregnancy, stimulates mammary duct development.
  3. Insulin-like growth factors I and II (IGF-I, IGF-II) - produced by and stimulate differentiation of the cytotrophoblast.
  4. Endothelial growth factor (EGF) - synthesised by the cytotrophoblast, stimulates differentiation of the trophoblast until week 6, after which its systhesis is shifted to the syncytiotrophoblast.

Back to top

• The pO₂ of foetal blood leaving the placenta is less than 1/3 of maternal pO₂, although it is never short of oxygen.
• This is because foetal haemoglobin (2 alpha, 2 gamma chains), does not bind 2,3-DPG as well as adult haemoglobin (2 alpha, 2 beta chains), resulting in a higher affinity for oxygen.
• So, at any given partial pressure, foetal blood has a higher oxygen saturation than the mother, and it will tend to "pull off" oxygen from the adult haemoglobin.

Back to top