Placenta and its functions

1. Placenta and its functions

2. Remarkable organ • Undergoes differentiation and maturation during the short life span

3. DEVELOPMENT OF THE PLACENTA

4. Fertilization Zygote ( diploid cell with 46 chromosomes) undergoes cleavage Morula ( 16 cell stage) Enters the uterine cavity 3 days after fertilization gradual accumulation of fluid between the cells of morula ( early blastocyst) 58 cell blastocyst outer cells – trophoblast inner cells - form the embryo

6. Trophoblast • Helps in implantation • Important role in the nutrition of the conceptus • Endocrine organ

7. Blastocyst implantation 3 steps • Apposition – initial adhesion of the blastocyst to the uterine wall • Adhesion – increased contact between the blastocyst and uterine epithelium • Invasion – penetration and invasion of the syncitiotrophoblast and cytotrophoblast into the endometrium , inner third of the myometrium and uterine vasculature

9. Trophoblastic differentiation 8th day postfertilization , after initial implantation … differentiates into • Outer multinucleated syncytium – primitive syncytiotrophoblast • Inner layer of primitive mononuclear cells – cytotrophoblast

11. After the implantation is complete… Trophoblast further differentiates into • Villous trophoblast give rise to chorionic villi - transports nutrients and oxygen between the mother and the fetus • Extravilloustrophoblast migrates into decidua and myometrium and penetrates into the maternal vasculature endovascular trophoblast interstitial trophoblast

13. Interstitial trophoblast • invades the decidua and penetrates the myometrium to form the placental bed giant cells • Surrounded by spiral arteries Endovascular trophoblast • Penetrates the lumen of the spiral arteries

14. 10 th day • Gentle erosion between the epithelial cells of the surface of endometrium , the invading trophoblasts burrow deeper and the blastocysts become totally encased within the endometrium

15. Blastocyst wall • facing the uterine lumen – single layer of flattened cells • Opposite side – thicker wall trophoblast embryo forming inner cell mass primitive ectoderm underlying endoderm (Hypoblast) (Epiblast)

17. A small cavity develops within the epiblast forming the amniotic cavity • Epiblast + hypoblast  embryonic disc • Cells rising from the hypoblast form an exocoelomic membrane ( Heuser’s membrane) that lines the inner part of the cytotrophoblast and together with the cytotrophoblast forms the exocoelomic activity or the primitive yolk sac

19. The syncytium between the cytotrophoblastic shells and the decidua degenerates and is replaced by a fibrinoid material called the NITABUCH’S MEMBRANE • This limits the penetration of the trophoblast in to the myometrium • Absence can lead to various degrees of adherent placenta

21. Day 12 after conception • The syncytiotrophoblast of the trophoblastic shell is permeated by system of intercommunicating channels called the trophoblastic lacunae • The lacunae are separated for each other by trabaculae • The lacunae communicate with each other and eventually a large space is formed . Now each trabeculae is surrounded by this lacunar space

23. The syncytiotrophoblast grows into the endometrium • The endometrium is eroded and blood fills up in the lacunar spaces • Cytotrophoblast begin to multiply and grow into each trabeculus • PRIMARY VILLI

24. Extra – embryonic mesoderm invades the centre of each primary villus • SECONDARY VILLI • Angiogenesis • TERTIARY VILLI

26. Development of chorion and decidua • As the blastocyst with its surrounding trophoblast expands into the decidua, • one pole extends outward toward the endometrial cavity • Opposite side will form placenta from villous trophoblasts and anchoring cytotrohoblast

27. Chorionic villi in contact with deciduabasalis proliferate to form chorionfrondosum (fetal component of placenta) • As the growth of embryonic and extra-embryonic tissue continues , the blood supply of the chorion facing the endometrial cavity is restricted • Deciduacapsularis cease to grow and degenerate

28. This portion of chorion becomes avascular fetal membrane that abuts the deciduaparietalis End of third month , chorionic laeve is separated from the amnion by exocoelomic cavity Intimate contact to form avascularamniochorion

29. Invasion of spiral arteries • Modification of spiral arteries are carried out by 2 populations endovascular trophoblast • Interstitial trophoblast(penetrate spiral artery lumen) (surrounds the spiral arteries)

30. Interstitial trophoblast • Penetrates decidua and myometrium • Aggregates around the spiral arteries • Functions: vessel preparation for endovascular trophoblastic invasion

31. Endovascular trophoblast • enters the lumen of the spiral arteries and forms cellular plugs Destroys the vascular endothelium by apoptosis Invades and modifies the vascular media with fibrinoid material Spiral arteries later regenerate the endometrium • Invading endovascualrtrophoblast can extend several cms along the lumen and migrate against arterial flow • Vascular changes are not observed in deciduaparietalis

33. Placental growth • In the first trimester the placental growth is more rapid than the fetus • By term , placental weight = 1/6 th of fetal wt

34. Maternal surface: • Slightly elevated convex areas ( lobes) – varies from 10-38 • Lobes are incompletely separated by grooves • Grossly visible lobes are referred to as cotyledons • Total number of lobes remain the same throughout gestation and individual lobes continue to grow

35. Fetal surface • Covered by smooth amnion with umbilical cord attached at or near the centre • Branches of the umbilical vessels are visible beneath the amnion as they radiate from the insertion of the cord

36. Maternal circulation • Arterial circulation : • About 120-200 spiral arteries open into the intervillous spaces by piercing the basal plate • In 12 weeks of pregnancy , there is a cytotrophoblastic invasion into the spiral arteries upto the intra-decidual portion • Endothelial and the muscular layer are replaced by fibrinoid material • Between 12-16 weeks, there is a secondary invasion of the trophoblast extending upto the radial arteries within the myometrium • Spiral arteries are converted to uteroplacental arteries

37. Circulation of the intervillous spaces • The arterial blood enters the space under pressure • Lateral dispersion occurs after it reaches the chorionic villi • Villi pulsation with uterine contraction – help in the migration of the blood towards the basal plate • During uterine contraction, the veins are occluded and the arterial blood is forced into the intervillous space, while uterine relaxation facilitates venous drainage

39. Fetal circulation • 2 umbilical arteries carry impure blood from the fetus enter the chorionic plate underneath the amnion , supplying each half of the placenta Artery breaks into smaller branches which enter the stems of chorionic villi Divides into primary , secondary and tertiary vessels of the corresponding villi • Maternal and fetal blood flow side by side but never in opposite direction • Fetal blood flow through the placenta – 400 mL/min

40. Placental barrier • Partition between the fetal and maternal circulation • In later part of pregnancy the membrane thickness reduces from 0.025mm to0.002mm • In early pregnancy • Syncytiotrophoblast • Cytotrophoblast • Basement membrane • Stromal tissue • Endothelium of the fetal capillary wall with its basement membrane

41. At term , there is attenuation of the syncytial layer • Sparse cytotrophoblast and distended fetal capillaries fill the villus

42. Teratogenicity • Diethylstilbestrol – vaginal adenosis , cervical hoods , uterine hypoplasia of the female offspring • Lithium – Ebstein’sanomaly,fetal diabetes insipedus , polyhydraminos, neonataalgoitre • Sodium valprovate – increased neural tube defects • Aminoglycosides – nephrotoxicity , ototoxicity in preterm infants • Phenetoin- cleft lip/palate, hypertelorism , finger nail hyperplasia

43. PLACENTAL FUNCTIONS • Transfer of nutrients • Respiratory , excretory , nutritive • Endocrine function – produces steroid and peptide hormones • Immunological function • Barrier function

44. Respiratory function: • No gaseous exchange • Intake of O2 and output of CO2 takes place by simple diffusion • Pressure gradient – driving force • O2 supply to the fetus – 8mL/kg/min and this is achieved with cord blood flow of 165-330mL/min • Excretory function: • Waste products such as urea , uric acid and creatinine are excreted by simple diffusion

45. Nutritive function • Glucose – glucose transporters GLUT1 by facilitated diffusion, glucose level : fetus > maternal • Lipid –transferred or synthesized , triglycerides and fatty acids are transported in early pregnancy , later synthesised , cholesterol – direct transfer • Amino acids- active transport, amino acid concentration : fetus > mother

46. Water and electrolytes • Sodium , potassium and chloride – by simple diffusion • Calcium, phosphorus and iron – by active transport ( fetal > maternal) • Water soluble vitamins – by active transport • Fat soluble vitamins – transferred slowly • Hormones • Insulin , steroids from the adrenals, thyroid , chorionic gonadotropin , or placental lactogen cross at a very slow rate

47. PLACENTAL GRADING • Grade 0 -Smooth chorionic plate, homogenous echotexture of substance of placenta. Most common in 1st trimester and early 2nd trimester (8-20weeks).   

48. Grade I -Small intraplacental calcification randomly dispersed within the substance of a placenta. May appear as early as 14 weeks, and is most common until 34 weeks. 

49.  Grade II -Calcification of basal plate. Does not usually appear until after 30 weeks.   

50. Grade III - Calcified indentation of placenta extending from the basal plate to the chorionic plate (placental cotyledons ). Usually not seen until 35 weeks. Found in 30% of term placentas