Placental Development

How often does placenta previa occur?

In about 1 in 200 women; many cases resolve themselves over time.

What is the space where maternal blood circulates called?

Intervillous space.

At term, what are the visible differences b/w the maternal and fetal sides of the placenta?

Maternal side is dark red in colour due to maternal blood and b/c part of the decidua will have separated with it. The surface is arranged in about 20 lobes (cotyledons), which are separated by sulci (furrows), into which the decidua dips down to form septa (walls). The fetal surface has a shiny appearance due to the amnion covering it; branches of the umbilical vein and arteries are visible spreading out from the insertion of the umbilical cord, which is normally in the centre.

The trophoblasts have a potent invasive capacity. How is this checked?

Moderated by the decidua, which secretes cytokines and protease inhibitors that moderate trophoblast invasion.

Does the placenta act as a barrier?

Not a true barrier, more of a filter; as a general rule, if something is in maternal blood, it will likely end up in fetal blood. Exceptions include very large molecules and molecules that are metabolized by the placenta, such as insulin and heparin.

What is placenta increta?

Occurs when the placenta further extends into the myometrium, penetrating the muscle; about 20% of cases.

What is placenta accreta?

Placenta accreta is a severe obstetric complication involving an abnormally deep attachment of the placenta; an invasion of the myometrium which does not penetrate the entire thickness of the muscle. This form of the condition accounts for around 75% of all cases.

What is placenta previa?

Placenta previa is a complication of pregnancy in which the placenta grows in the lowest part of the uterus and covers all or part of the opening to the cervix.

What are spiral arteries?

Spiral arteries are small arteries which temporarily supply the endometrium of the uterus during the luteal phase of the menstrual cycle. Spiral arteries in the decidua are remodeled so that they become less convoluted and their diameter is increased. The increased diameter and straighter flow path both act to increase maternal blood flow to the placenta. The relatively high pressure as the maternal blood fills intervillous space through these spiral arteries bathes the fetal villi in blood, allowing an exchange of gases to take place.

What is the maternal contribution to the placenta?

The decidua, formerly the endometrium (uterine lining). In preparation for implantation, the uterine endometrium undergoes 'decidualisation'.

What is the fetal contribution to the placenta?

The extraembryonic mesoderm, which becomes the chorion.

What becomes the placenta?

The trophoblast becomes the placenta and chorion, while the embryoblast becomes the embryo, amnion and umbilical cord.

What happens by Day 8?

The trophoblast cells begin to make hCG, a hormone that ensures that the endometrium will be receptive to the implanting embryo.

What vessels does deoxygenated blood travel through on the fetal side?

The umbilical arteries (paired).

What vessel does oxygenated blood travel through on the fetal side?

The umbilical vein.

Around Week 3 after fertilization, chorionic villi form. What is their function?

The villi become most profuse in the area where the blood supply is richest, the decidua basalis. The villi erode the walls of maternal blood vessels as they penetrate the decidua, opening them up to form a lake of maternal blood in which they float. The maternal blood circulates slowly, enabling the villi to absorb food and oxygen and excrete waste (nutritive villi). Some villi are more deeply attached to the decidua and are called anchoring villi.

What is placenta percreta?

The worst form of the condition is when the placenta penetrates the entire myometrium to the uterine serosa (invades through entire uterine wall). This variant can lead to the placenta attaching to other organs such as the rectum or bladder; about 5% of cases.

How does the syncytiotrophoblast layer help to evade the maternal immune system?

Those cells do not express antigens, so can evade detection as foreign tissue by maternal immune system.

Why is hPL produced by the placenta?

- has a role in glucose metabolism in pregnancy - as levels of hCG fall, so the level of hPL rise and continue to do so throughout pregnancy

How is fetal blood different from adult blood?

- it contains larger and more numerous erythrocytes (RBCs) with a higher hemoglobin content, which maximizes their uptake of oxygen - fetal hemoglobin has a higher affinity for oxygen in the slightly more acid fetal environment

Where and why is progesterone secreted by the placenta?

- made in the syncytial layer in increasing quantities until immediately before the onset of labour when its level falls - inhibits uterine contractility; stimulates breast ductal growth; increases sensitivity to CO2; decreases cellular maternal immune responses

Objectives for this lecture.

1. Ensure an understanding of the development/origins of the human placenta, including maternal and fetal contributions. 2. Develop an understanding of the maternal/fetal blood separation. How the fetus is protected from the maternal immune system. 3. Awareness of the complications related to the growth of the placenta. 4. A beginning understanding of the endocrinology of the placenta and impact on the fetus and the mother.

How much of the maternal blood goes to the placenta?

20-25%

What often happens around the time of implantation?

A bleed, which looks like a menstrual flow.

What is the approx weight of the mature placenta?

About 480 g; at this point, it is 5x less than the baby and occupies about a third of the uterus.

At what point do the placenta and fetus weigh about the same?

Around 15 weeks; both under 100g.

How come maternal and fetal blood never mix?

Because four layers of tissue separate the maternal blood from the fetal blood making it impossible for the two circulations to mix unless any villi are damaged.

When is implantation complete?

By Day 9.

By what point is the placenta completely formed and functioning?

By week 10 after fertilization.

What are the basic steps in the development of the embryo?

Day 1-3: Zygote, traveling along uterine tube and undergoing mitotic cellular replication and division => Day 1: divides into 2 cells => Day 2: divides into 4 cells => Day 2.5: divides into 8 cells => Day 3: divides into 16 cells Day 3-4: The 16-cell zygote is now referred to as a Morula; continues to replicate and divide, forming an inner cavity, called a blastocele. Day 4-5: Blastocyst (blastula) is formed via blastulation; made up of an inner cell mass (embryoblast) and an outer cell mass (trophoblast); enters the uterus By Day 7.5: begin implantation

Describe the anatomy of a villus.

Each chorionic villus is a branching structure arising from one stem. Its centre consists of mesoderm and fetal blood vessels, and branches of the umbilical artery and vein. These are covered by a single layer of cytotrophoblast cells and the external layer of the villus is the syncytiotrophoblast.

How do oxygen and carbon dioxide pass from maternal to fetal blood (or vice versa)?

Via simple diffusion, mediated by concentration gradients.

Is it possible to expel the placenta with placenta accreta, increta or percreta?

With increta and percreta, it is not possible to expel the placenta and the uterus may rupture; usually involves a c-section, and possibly a hysterectomy.

What hormones are made by the placenta?

- hCG - estriol (placental estrogen) - progesterone - hPL

Why is estriol secreted by the placenta?

- estriol is a growth-stimulating hormone, which are secreted in large amounts throughout pregnancy - they are produced by the placenta as the activity of the corpus luteum declines - increases maternal sensitivity to CO2; promotes softening of the cervix; increases maternal fibrinogen and inhibits thrombolysis; increases uterine contractility

Where and why is hCG produced by the placenta?

- produced by the cytotrophoblast layer of the chorionic villi; - peak levels b/w 7th and 10th week; - function is to stimulate the growth and activity of the corpus luteum;