Placenta

Question 1

What does the placenta do?

Answer 1

Carries out function the immature foetus cannot perform itself

Question 2

What are the 5 main functions of the placenta?

Answer 2

Respiration - fetal lungs are immature until late in pregnancy and do not fully function until delivery. The placenta absorbs oxygen and excretes products from fetal metabolism.

Excretion - waste products are removed from the fetal blood and excreted by the maternal organs

Nutrition - placenta absorbs the nutrients that he foetus needs and breaks them down using enzymes into simpler molecules for fetal use. Some nutrients are stored eg. Glucose stored as glycogen

Protection - placental barrier is limited but does prevent passage of most bacteria. Small microorganisms such as viruses can pass and affect the the foetus, some antibodies from maternal blood cross the placenta to protect the foetus in the first months after birth.

Hormone production - the placenta acts as a complex endocrine gland producing hormones. hCG is produced in the early villi to maintain the corpus luteum until the placenta is developed enough. Oestrogen and progesterone develop relevant maternal organs for pregnancy.

Question 3

What is the trophoblast?

Answer 3

Outer cell mass of the blastocyst

Question 4

What are chorionic villi?

Answer 4

Finger like projections

Question 5

What occurs to the blastocyst during implantation?

Answer 5

Forms chorionic villi from the trophoblast over this entire surface. The chorionic villi with the most abundant blood supply in the decides basalis proliferate and those with the lesser blood supply degenerate to form the chorionic membrane,

Question 6

Where does implantation usually occur?

Answer 6

Near to the fundus on either the anterior or posterior surface

Question 7

What is the function of the chorionic villi?

Answer 7

To penetrate the decides and erode the walls of maternal blood vessels allowing sinuses to form. Some villi burrow deeply into into the decides to act as anchorage for the placenta. The remaining villi float in the sinuses where the exchange of nutrients and gases takes place.

Question 8

What are sinuses?

Answer 8

Pools of blood

Question 9

What is the placental weight at term?

Answer 9

Roughly 1/6 of fetal weight

Question 10

Describe the maternal surface of the placenta

Answer 10

Attached to the decidua.
Arranged in 15-20 irregular lobes called cotyledons that are divided into groves called sulci.
Each cotyledon is divided into lobules which contains one chorionic villus.
The chorionic villus ok snugs from the fetal surface and divide many times to lie in maternal sinuses.
This creates a large surface area for nutrient and gas exchange.
Each villus contains fetal blood cells covered by a single layer of cytotrophoblast cells and the syncytiotrophoblast.

Question 11

Describe the fetal surface of the placenta

Answer 11

Covered by amniotic membrane.
Umbilical cord arises from the placenta and is covered with a continuation of the amnion.
Each cotyledon recieves it’s own branch of the umbilical artery and vein.

Question 12

Describe the umbilical cord

Answer 12

Connects to the placenta and foetus at the umbilicus
Formed by 5th week of pregnancy
Originates from the duct that forms between the amniotic sac and hold which transmits the umbilical cord vessels
Function is to transport oxygen and nutrients to the developing foetus and remove waste products
Contains two arteries and a vein which are continuous with the blood vessels in the chorionic villi of the placenta
Vessels are enclosed and protected by whartons jelly
Whole cord covered by layer of amnion
No nerves
If only 2 vessels in cord ? Renal malformation however can have no significance
Cord approx 50 cm long 1-2cm diameter allowing birth of baby without traction on placenta
Short cord defined as 40cm
If cord too long poses risk of wrapping around baby and occlusion of vessels
Cord has spiral twists which give it added strength

Question 13

How is the chorioamnion membranes formed?

Answer 13

Basal and chorionic plates come together and meet at the edges

Question 14

Describe the amnion

Answer 14

The inner membrane derived from the inner cell mass.
Single layer of epithelium with s connective tissue base.
Tough, smooth, translucent and continuos with outer surface of umbilical cord which moves over the chorion aided by mucus.
Contains amniotic fluid which is produced in small amounts as well as PGe2

Question 15

What is PGE2?

Answer 15

Prostaglandin E2

Question 16

Describe the chorion

Answer 16

The outer membrane continuous with placental edge comprised of mesenchyme, cytotrophoblasts and vessels from extended spiral arteries of decidua basalis.
Rough, thick, fibrous, opaque membrane which lines the decidua Vera during pregnancy.
Produces enzymes that can reduce progesterone levels and also produce prostaglandins, oxytocin and platelet activity factors to stimulate uterine activity.
Easily ruptured.

Question 17

Describe amniotic fluid

Answer 17

Clear alkaline, slightly yellow liquid contained by the amniotic sac.
Derived from the maternal circulation across the placental membranes.
Foetus contributes to the amniotic fluid through metabolism in small quantities and fluid from its lungs

Question 18

What are the functions of the amniotic fluid?

Answer 18

Distends amniotic sac allowing free movement and development of foetus.
Equalises pressure and protects foetus.
Maintains constant uterine temperature.
If membranes remain intact it protects placenta and cord from uterine pressure
Aids effacement and dilatation when presenting part is poorly applied (prostaglandins)

Question 19

What are the constituents of amniotic fluid?

Answer 19

99% water
1% dissolved solid matter eg. Food supplements and waste products 
Vernix caseosa and lanugo 
Meconium (abnormal)
500ml at term

Question 20

What are the two phases of implantation?

Answer 20

Prelacunar and lacunar

Question 21

Describe the prelacunar phase of implantation?

Answer 21

7 days post conception the blastocyst makes contact with the decidua (apposition)
Placentation begins
Implantation process involves chemical mediators, prostaglandins and protelytic enzymes released by the decidua and trophoblast
Maternal tissue is invaded
Nearby maternal blood vessels ensure optimum blood flow to the placenta
Cytotrophoblasts form. Double layer and differentiate into different types of syncytiotrophoblasts

Question 22

Describe the lacunar stage of implantation

Answer 22

Increasing number of syncytiotrophoblasts surround the blastocyst and small lakes (lacunae) form to become intervillous space.
Syncytiotrophoblasts bathe in the blood

Question 23

How are chorionic villi formed?

Answer 23

A layer of syncytiotrophoblasts and cytotrophoblasts

Question 24

How is blood supplied to chorionic villi?

Answer 24

New blood vessels develop from progenitor cells promoted by low levels of oxygen. Angiogenesis (further growth of vessels) produces a vascular network that connects the with the blood vessels that have developed in the embryo

Question 25

What is the difference between the vessels in the decidua basalis and decidua capsularis?

Answer 25

Over time the villi specialise and differentiate
The decidua basalis has a richer blood supply and vessels in this region anchor the placenta and become the make source of nutrition
Vessels in the decidua capsularis degenerate over time from lack of nutrition and eventually become the chorionic membrane

Question 26

How is placental circulation established?

Answer 26

Invading trophoblasts modify maternal spiral arterioles to accommodate a 10x increase in blood flow around the open circulation surrounding the chrorionic villi.
Blood flow slows around the villi and returns to the endometrial walls and maternal circulation

Question 27

How much maternal blood is in the intervillous spaces?

Answer 27

Approximately 150ml

Question 28

Describe the flow of fetal blood with regard to the umbilical cord

Answer 28

Deoxygenated blood is pumped by the fetal heart towards the placenta by the umbilical arteries.
When waste products have been removed and oxygen has been absorbed the blood returns to the foetus via the umbilical vein