د. منى زهير Lec. 4 Fetal membranes and placenta

د. منى زهير

Lec.4

Fetal membranes and placenta

Fetal membranes: are structures have developed from the zygote but don't form part of the embryo (i.e. extra embryonic structures). They:

• 
  assist in the protection of the embryo.
  
• 
  they may provide respiration , nutrition and excretion for the baby.
  
• 
  they include : amnion , chorion , yolk sac and allantois.
  

Placenta is a compound organ and it is included with these structures.

Further development of villi and decidua :

• 
  During the 2^nd month , more tertiary villi are formed. in the following months small extensions appear extending from these villi to the adjacent lacunar or inter villous spaces.
  
• 
  By the beginning of the 4^th month the cytotrophoblast and some connective tissue (mesodermal) cells disappear from these villi, and so the only layers that left separating the fetal and maternal circulations are :
  

1. 
   Syncytium.
   
2. 
   Endothelial wall of the blood capillaries.
   

• 
  Villi on the embryonic pole continue to grow forming the chorion frondosum.
  
• 
  Villi on the abembryonic pole degenerate and the chorion become smooth and called the chorion laeve.
  

• 
  Decidua over the chorion frondosum called decidua basalis consisting of a compact layer of polygonal cells with abundant amount of lipids and glycogen called decidual cells.
  
• 
  Decidua over the abembryonic pole is called the decidua capsularis.
  
• 
  Due to continuous growth and increase the size of the chorionic villi, the decidua capsularis stretched and degenerates, thus the chorion laeve comes into contact with the decidua parietalis (the remainder part of the uterine wall) and fuse, obliterating the uterine lumen .similarly the fusion of the amnion and chorion to form the aminochorionic membrane obliterates the chorionic cavity.
  
• 
  The chorionic frondosum with decidua basalis makes up the placenta.
  

Structure of the placenta:

• 
  It is fully developed during the 4^th and 5^th months , it is formed of two components :
  

1. 
   Fetal part : formed by the chorionic frondosum.
   
2. 
   Maternal part : formed by the decidua basalis.
   

• 
  The placenta bordered by the chorionic plate on the fetal side, and by the decidual plate of the decidua basalis on its maternal side , between the two plates are the inter villous spaces are filled with maternal blood.
  
• 
  During the 4^th and 5^th month , the decidua forms a number of septa called decidual septa, which projects into the inter villous spaces but not reach the chorionic plate, as a result of this septal formation the placenta is divided into a number of compartments called cotyledons.
  

• 
  As pregnancy continue, the placenta increase in surface area and thickness, occupying about 25-30 % of the internal surface of the uterus. (the increase in thickness is due to elongation and branching of the existing villi).
  

Full term placenta :

• 
  At full term the placenta has a discoid shape, a diameter of 20 cm, (2.5-3) cm. thickness, and 500–600 gm. weight, the placenta flattens out at the periphery where it becomes continuous with fetal membranes.
  
• 
  On examining the placenta (after its expulsion from the uterus):
  

• 
  Maternal surface is rough , darker in color , oozes blood from the torn vessels and about 15-20 slightly bulging areas appear representing the cotyledons and covered by a thin layer of decidua basalis.
  
• 
  Fetal surface is smooth, shiny covered by the aminochorionic membrane and the chorionic vessels are seen to converge toward the umbilical cord.
  
• 
  The attachment of the umbilical cord is usually eccentric and occasionally marginal. rarely the umbilical cord insert into the amino chorionic membrane outside the placenta (velamentous insertion).
  

Circulation of the placenta :

• 
  The oxygenated blood enters the inter villous spaces via the spiral arteries, the blood bathes the numerous small extensions of the villous tree, where the exchange of (O2, nutrients) and (CO2, waste product) take place, then the blood flow flows back from the chorionic plate to the decidua where it enters the endometrial veins, into the maternal circulation.
  
• 
  The capacity of the inter villous spaces is large, they contain about 150 – 175 CC. of blood, which renewed about 3-4 times per minute.
  

• 
  Before the fourth month the fetal blood is separated from the maternal blood by the placental barrier at which the placental exchange take place and are composed of :
  

1. 
   Endothelial wall of blood capillaries.
   
2. 
   Mesodermal or Connective tissue core.
   
3. 
   Cytotrophoblast.
   
4. 
   Syncytium.
   

• 
  In mature placenta (i.e. fourth – fifth month and on) the placental barrier are composed of :
  

1. 
   Very thin syncytium.
   
2. 
   Endothelial wall of blood capillaries.
   

Functions of the placenta :

1. 
   Respiration : exchange of gases such as O2 , CO2 , CO (as lungs in adult).
   
2. 
   Exchange of nutrients and electrolytes such as amino acids , fatty acids.
   
3. 
   Excretion of waste products (as kidneys in adult)
   
4. 
   Production of hormones :
   

1. 
   Progesterone : to maintain pregnancy.
   
2. 
   Estrogenic hormones (estradiol and estrogen) : may help in parturition.
   
3. 
   Human chorionic gonadotropin : has similar function to LH (luteinizing hormone), its secretion begins early before the fourth month preventing the degeneration of corpus luteum.
   
4. 
   Human placental lactogen : gives the fetus priority on maternal blood glucose and makes the mother somewhat diabetic (it is a growth hormone).
   

5. 
   Protection :
   

1. 
   Act as a barrier preventing the entering of many foreign bodies like bacteria, but many viruses like rubella, varicella and measles can pass the placental barrier.
   
2. 
   Detoxification of some drugs.
   
3. 
   Transmission of maternal antibodies: the fetus requires maternal antibodies of (IgG) type , and obtain passive immunity against diphtheria, small pox, and measles but not against chicken pox and whooping cough.
   

RH incompatibility :

It occur if the mother is RH^-(negative), the fetus is RH⁺ and when fetal RBCs. enter the maternal circulation. this will stimulate antibodies formation against the fetal RBCs. in the mother. these antibodies will then return to the fetus and cause a breakdown of the RBCs. of the fetus , a case known as erythroblastosis fetalis or hemolytic disease of the new born which may lead to intrauterine death.

Placental changes at the end of pregnancy :

Many changes occur in the placenta which may be an indication of a reduced exchange between the two circulations , these change include :

1. 
   The deposition of fibrinoid on the surface of the villi.
   
2. 
   An increase of the fibrous tissue in the core of the villus.
   
3. 
   An increase in the thickness of the basement membrane Of the villous capillaries.
   
4. 
   Obliterative changes in the small capillaries of the villi.