FETAL MEMBRANES     (EXTRAEMBRYONIC ORGANS).
                                                             PLACENTA.   (=PL).
       PL is composed of 2 parts:   1) MATERNAL   portion      and       2) FETAL   portion.
                                                     FETAL  portion    of  PL:
           It is  presented  by organ  VILLOUS CHORION   (or chorionic plate).
        VILLOUS CHORION   is composed of  numerous   TERTIARY chorionic VILLI..
 Each tertiary chorionic villus is composed of :  1) CAPILLARies  with  fetal  blood, 
                          2) Loose irregular connective tissue,  forming   stroma (or core)  of villus,  
         3) CYTOtrophoblast layer,      4) SYNCYTIOtrophoblast layer  (it is multinucleated).
 These 4 parts of  tertiary chorionic villus also are called   “blood   placental   BARRIER”.
Syncytiotrophoblast  layer  secretes  hormones  CHORIONIC GONADOTROPIN and PROGESTERON.    Presence of chorionic gonadotropin  in  urine of women   confirms   
   diagnosis  “PREGNANCY”. 
                                                      MATERNAL portion    of PL:
  It  presented  by modified endometrium,  named  BASAL PLATE, or DECIDUA BASALIS.  
                       Endometrium – is tunica mucosa of UTERUS.
     Modifications in decidua basalis are:   1) LACUNAE  -  it are big cavities with maternal  blood, 
    2) Connective tissue placental SEPTAE.  It separate chorionic villi each against other.
    3)  DECIDUAL cells in lamina propria of endometrium.      It are big cells  with   a lot of   
         GLYCOGEN  in cytoplasm.
   Direction of O2 and nutrients diffusion is: from materal blood of lacuna into capillaries of  tertiary chorionic villus. O2 from capillaries outflows on  umbilical VEIN to fetus body.  Diffusion of CO2 happens from capillaries of  tertiary villus.into  endometrial lacunae. It means: gases exchange
  happens  THROUGH  “blood  placental  barrier”.
   3  main functions of PL : 1) RESPIRATORY funct. (gases exchange between mother and fetus),   
                                            2) ENDOCRINE funct.  ( secretion of hormones).
                                            3) TROPHIC funct. (PL supplies  fetus by nutrients).
  There are 2 fetal membranes with endocrine function :   1) Placenta,     2) Villous chorion
 
                                                                UMBILICAL CORD:
 Externally  it is covered by simple columnar amniotic epithelium.
        Inside it contains:   1) TWO umbilical ARTERIES,     2) ONE  umbilical VEIN, 
    3) WHARTONs  jelly  (or MUCUOUS  CONNECTIVE tissue) –   it is between vessels. 
 Whartons jelly contains too much  of  HYALURONIC acid.     Funct.:  it  prevents
 compression  of  umbilical vessels.  Funct.  of  umbilical VEIN –   it transports    OXYGENATed  blood from  placenta  to  fetus body.
                                                                     AMNION.
It is composed of :   1) amniotic CAVITY with amniotic FLUID,       2) WALL of amnion.
  Wall of amnion is composed of  3   layers :      1) Simple columnar EPITHELIUM,. 
  2) DENSE fibrillar connective tissue,           3) LOOSE  fibrillar connective tissue. 
Funct. of amniotic epithelium: it SECRETES  and reabsorbs   amniotic FLUID.
                                                                     YOLK SAC.
                                       Wall of yolk sac is composed  of  2  layers: 
                1) Extra-embryonic ENDODERM,             2) Extra-embryonic MESODERM   
 Funct.:A) Endoderm is a place of formation of primordial  sex cells, named  gonoblasts.
       B) Mesoderm is a place of formation of  FIRST red blood cells,  named MEGALOBLASTS;            and  FIRST blood VESSELS.   It is  HEMOPOIETIC  function of yolk sac.
                                      ALLANTOIS.  -   it  is finger-like projection (or diverticulum) of
 posterior wall of yolk sac.                 It is  composed of 2 layers: 
  1)    Extra-embryonic endoderm,      2)   Extra-embryonic mesoderm.
Funct.:allantois participates in formation of urinary bladder  (and may be involved in abnormalities of bladder development).