Formation of placenta and extra-embryonic membranes

1. Formation of placenta and extra-embryonic membranes

2. Topics • Extra embryonic membranes • Placentation • Functions of the placenta • Types of placenta

3. Extra-embryonic membranes • Chorion • Yolk sac • Amnion • Allantois

4. Chorion • Form from trophoblastic tissues on day 12 • Grows and forms a space in the decidua • Chorionic invasion will destroy mother’s capillaries and fill the space with blood • Placenta villi from chorionic fingers will enter this bloody pool • The placental villi contains fetal capillaries and they are separated from the maternal blood by a thin chorionic tissue layer for exchange of nutrients • Forms part of the placenta on the fetal side

5. Yolk sac • A primary structure for digestive and respiratory process • Becomes small by the 6th week to from a yolk stalk on the umblical cord • Functions as a transporter of nutrients at 2nd to 3rd week • First site for formation of blood cells before function taken over by the liver at week 5 • Contains primordial germ cells that will enter the gonads to form either spermatogonia or oogonia

6. Amnion • Also called the water bag • A tough, thin and transparent membrane • The amniotic space contains fluid rich in discarded epithelial cells, proteins, carbohydrates, fats, enzymes, hormones, pigment, fetal excretory products and towards the end of pregnancy will have amniotic stem cells • Suspends embryo/fetus in a shock-free environment, prevents embryo from sticking to the membrane and become malformed, permits embryo to move freely and also maintains a suitable temperature for embryo/fetus

7. Allantois • Part of the hindgut • Assist in formation of the urinary bladder • It’s blood vessels becomes the veins and arteries of the umblical cord • Becomes the median umblical ligament yang descends from the urinary bladder to the navel

8. Placentation • Placenta is form from trophoblastic and decidual tissues • Chorions binds to endometrium and grows rapidly until 5 months of gestation • Placenta on fetal side is smooth while on the maternal side is full of grooves to increase surface area for gaseous exchange • Placenta is fully functional by week 4 and 5 • Blood supply to fetus enters via umblical artery and leaves via umblical vein (both in umblical cord) • At week 5, the umblical cord also consists of yolk stalk, body stalk and chorion • Food and oxygen enters fetus and carbon dioxide and excretory products enters body via umblical cord

9. Functions of the placenta • Acts as a barrier or filter • Functions for gaseous exchange • Apart from food, can also bring in toxic substances like HIV virus, bacteria, nicotine, drugs and alcohol • Protects fetus from immunologic rejection • Produces hormones (HCG- Human Chorionic Gonadotrophin, HCS Human Chorionic Sommatotrophin/Human Placental Lactogen, E and P4) – hormones levels produced depends on the stage of pregnancy • Gives passive immunity

10. Types of placenta • Histology Attachment Species Mother’s tissue • 1. Epitheliochorial Diffuse Horse, pig Blood • 2. Synepitheliochorial Cotyledon Cow, sheep Epithelium • 3. Haemonochorial Discoid Human Blood • Endotheliochorial Zonary Dogs, cats Endothelium capillary • Haemodichorial Discoid Rabbit Blood • 6. Hamotrichorial Discoid Rat/mice Blood • 7. Haemonochorial Bi-discoid Rhesus monkey Blood

11. Amniotic sac Amniotic fluid 5 month old fetus Umblical cord Placenta