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Flashcards in Placental anatomy + development Deck (66):
1

Placenta - significance in pregnancy

growth restriction - many due to placental insufficiency
spontaneous abortion, miscarriage
nutrition/oxygenation
prematurity
hemorrhage
preeclampsia

2

Implantation of placenta

requires 2 synchronous processes
- uterine preparation (decidual reaction)
- embryo development and ability to interact with endometrium

50% of all conceptions fail at this critical time

3

Uterine preparation for implantation

Proliferative phase: estrogen secreted by ovarian follicles--> endometrium proliferates and remodel

Secretory phase: thickening of endometrium and formation of glandular structures, increased vasculature
(increased levels of estrogen + progesterone)

4

Uterus - receptivity to implantation

limited time
8-10 days after LH surge = 6-7 d after ovulation
correlated to highest circulating levels of progesterone ("pro-gestation)

5

Decidua regions

Decidua parietalis
Decidua capsularis
Decidua basalis

decidua parietalis/capsularis later join to form decidua vera as embryo grows

6

Decidua parietalis

non-implantable site

7

Decidua capsularis

thin capsule of endometrium covering developing embryo

8

Decidua basalis

implantation site
eventually becomes maternal portion of developing placenta

9

Implantation of placenta

usually placenta implants in fundus
can be ectopic

10

placenta previa

implantation over cervix

11

Blastocyst - implantation

day 5: blastocyst absorbs fluid, develops a fluid-filled cavity
inner cell mass: compacts to one side to form embryonic pole
fluid-filled cavity: blastocoele
outer cell mass: trophoblast

12

stages of implantation

Apposition
Adhesion
Invasion

13

Apposition - implantation

microvilli on trophoblast interdigitate wiht pinopodes on apical surface of uterine epithelium
embryonic pole is oriented towards uterine epithelium

14

Adhesion - implantation

increased physical interaction between blastocyst and uterine epithelium

15

Invasion - implantation

blastocyst penetration of uterine epithelium

16

Trophoblasts

outer cell mass of blastocyst
destined to become 2 cell types:
cytotrophoblasts
Syncytiotrophoblasts

17

Cytotrophoblasts

progenitor cells
villus cytotrophoblasts
extravillus cytotrophoblasts

18

Syncytiotrophoblasts

giant, multinuclear cells formed by fusion of cytotrophoblasts
terminally differentiated
invasion of endometrium

19

Inner cell mass differentiation

into 2 layers
Epiblast
Hypoblast

20

Epiblast

extra-embryonic ectoderm
contribute to formation of amnion - forms amniotic cavity that surrounds entire embryo + fetus

21

Hypoblast

primitive endoderm
spreads to line inner surface of trophoblast
parietal endoderm gives rise to primary yolk sac
parietal endoderm together with later contributions from embryo --> extraembryonic mesoderm

hindgut endodermal cells migrate towards placenta to form allantois

22

Chorion origin

mesoderm + cytotrophoblast
contributes to placenta vascularization

23

Umbilical cord origin

Allantois (from hindgut endoderm) + chorion

24

Amnion origin

extra-embryonic ectoderm

25

Yolk sac origin

parietal endoderm

26

Fetal components of the placenta

umbilical cord
amnion
lacunae
fetal placental vasculature
chorionic plate
basal plate
villi
cotyledon

27

Maternal components of placenta

decidua
maternal placental vasculature (spinal arteries)
intervillous space filled with maternal blood

28

Invasion of trophoblasts

1) invasion of endometrium by syncytiotrophoblasts and cytotrophoblast columns
2) lacunar spaces develop within syncytiotrophoblast layer (intervillus space)
3) cytotrophoblast columns extend to maternal spinal arteries
- extravillus cytotrophoblast advance into spinal arteries --> endovascular trophoblast
4) columns extend laterally and meet one another to form cytotrophoblast shell/basal surface
5) second wave of extravillus cytotrophoblast invasion into inner 1/3 of myometrium

29

Villi in early pregnancy

maternal and fetal blood separated by 3 cell layers:
- syncytiotrophoblasts
- cytotrophoblasts
- fetal capillary endothelium

30

Villi in later pregnancy

villi become more branched and vascular
fetal vessels move to or eccentric location
cytotrophoblasts degenerate so maternal and fetal blood separated by 2 cell layers (syncytiotrophoblast, endothelium)
decreased distance between maternal and fetal circulations

31

Spiral arter invasion

process of spiral artery invasion critical to development of maternal circulation in placenta
Remodelling of spiral arteries:
- converts tight, thick-walled muscular vessel into an open, capacitance vessel that can accommodate tremendous increase in maternal blood flow required to adequately nourish placenta + fetus

32

Shallow cytotrophoblast invasion

increased risk of pre-eclampsia, abruptio placenta
maternal vessels not opened properly
fetal risks: hypoxia, malnutrition, growth restriction, stillbirth, prematurity

33

Deep invasion of cytotrophoblasts - types

Placenta accreta (75-85%)
Placenta increata
Placenta percreta - invaded through serosa of uterus
Normal (decidua)
fetal risks of bleeding and prematurity
maternal morbidity of bleeding; may require hysterectomy at labour

34

Pathologic deep invasion risk factors

Prior C-section/uterine surgery (sar becomes site of uncontrolled invasion)
Placenta previa - lower uterine segment implantation more likely to have deep invasion
Previa + prior uterine surgery:
- 1 prior C-section and current previa = 25% risk
- 2 prior C-section and current previa = 40% risk

Accreta often undiagnosed until delivery when you are unable to deliver placenta --> postpartum hemorrhage, hysterectomy, death (maternal death up to 7%)

35

Uterine blood flow at term

700 ml/min

36

Placental maturation

extensive branching of villi
increased surface area available for exchange
closer approximation of maternal + fetal blood flow
increased uterine blood flow - 10x increase overn on-pregnant uterus (700ml/min at term)

37

Stem villi

support structure with central arteries + veins

38

Terminal villi

final branch of villus tree, comprising 50% of villus surface area
extensive capillary network
major site of maternal-fetal exchange
bathed in well-oxygenated maternal blood that enters intervillous space from spinal arteries in decidua basalis
Maternal blood propelled into intervillous space in jet-like streams traveling upward to chorionic plate befoer percolating down through villi towards maternal venous drainage

39

Anchoring villi

extend to maternal surface, spread laterally to meet and form cytotrophoblast shell/basal plate

40

Fetal placental vasculature

blood from fetus enters placenta from 2 umbilical arteries that arise from fetal internal iliac arteries
Umbilical arteries + veins protected by Wharton's jelly
Umbilical cord cord contacts chorionic plate in centre (usually) and then spread radial branches from umbilical arteries over fetal surface of placenta
Branches then divide vertically into stem and intermediate villi, ending in capillary network in terminal branches

41

Placental components

umbilical cord
placental membranes (chorion, amnion)
placental disc - fetal surface, maternal surface, parenchyma (villus tissue)

42

Umbilical cord

2 arteries + veins in Wharton's jelly - provide cushion
Vein deliver oxygenated blood; artery deoxygenated back to placenta
Usually coiled with increasing cord length as pregnancy progresses
may insert centrally or eccentrically into disc (90%)
insertion within 1 cm of disc margin - 7%

43

Central insertion of umbilical cord

more common
run protected in Wharton's jelly until insertion into fetal surface of placental disc

44

Velamentous insertion

inserts into placental membranes
2% of pregnancies
3 component vessel of cord run final distance to disc through membrane, unsupported by Wharton's jelly
associated with reduced fetal growth/risk of rupture of fetal arteries

Vessels located over maternal cervix = vasa previa

45

Vasa previa

umbilical vessels over maternal cervix
significant risk of fetal hemorrhage at labour and membrane rupture
if diagnosed antenatally, recommendation is for C-section for delivery prior to labour onset/rupture of membranes

46

Placental membrane

Amnion + chorion

47

Amnion

develops from inner cell mass
eventually covers umbilical cord, fetal surfaces of placenta and creates amniotic sac around fetus
multilayered with cuboidal epithelium lying on well-defined basement membrane
- deep to basement membrane are compact fibroblast and spongy layers

48

Chorion

develops from chorionicerus opposite i villi on side of chorionic sac that is expanding into uterus
villi degenerate leaving smooth chorion that eventually expands to fuse with decidua parietalis on side of uterus opposite implanted embryo
multilayered comprising of cellular and reticular layer, pseudomembrane and trophoblast

49

Normal placenta

should appear clear and non-cloudy

50

Stained membranes

greenish black staining = meconium (fetal stool) passage in utero related to fetal stress/loss of sphincter tone
greenish-yellow staining may suggest ascending infection

51

Placental membrane histology

cuboidal epithelium
basement membrane
compact fibroblast/spongy layers

52

Placental disc

normal placenta increases in size throughout gestation
remains larger than fetus until ~16 weeks
at term: normal placenta weighs ~500 g, is 2-3 cm thick and 15-20 cm in diameter
placental disc divided into:
-fetal surface
-maternal surface
-parenchyma (villus tissue)

53

Fetal surface of placenta

umbilical cord insertion
arborizing pattern, spreading out form/draining to umbilical cord
may identify fetal surface vessel thrombosis or hematoma

54

maternal surface of placenta - gross

interfaces with uterus
normally a beef red colour
does not normally display organized clot
divided into concrete lobules or cotyledons that should be assessed for completeness at time of delivery

55

Maternal surface of placenta - histology

maternal vessels within decidua
may yield information about maternal well-being

56

Increased surface area for exchange

increases in cellular content and surface area until 36 weeks gestational age

57

Closer approximation of maternal + fetal circulation

distance separating maternal and fetal blood diminished during gestation by:
- decrease in cross sectional area of villi
- movement of fetal vessels from a central villus location to mor eccentric one
- decrease in thickness of syncytiotrophoblast
- development of "vasculosyncytial membranes": focal attenuation of trophoblast in close approximation to vessel wall

58

Increased uterine blood flow

as pregnancy progresses, greater proportion of blood flow directed away from endometrium/myometrium and towards placental cotyledons
Near term: >90% of uterine blood flow to placenta

59

Uterine contraction post-delivery

prevention of maternal hemorrhage
after delivery of fetus + placenta
normal vaginal delivery: mother might lose up to 500 ml of blood, but if uterus fails to contract can take only minutes at flow rate of 700ml/min to exsanguinate

60

First trimester histology of placenta

large villi, covered in 2 layers of cells (cytotrophoblast, syncytiotrophoblast) with few centrally located vessels and abundant loose stroma

61

Third trimester histology fo placenta

villi become smaller and highly vascular
fetal vessels moev to more eccentric location
cytotrophoblasts degenerate leaving single syncytiotrophoblast layer in close proximity to fetal capillary endothelium
fusion of fetal capillaries + syncytiotrophoblast occur in 3rd trimester --> vasculosyncytial membrane
In some areas syncytiotrophoblast draw up into "syncytial knots" --> significantly decrease distance between maternal/fetal circulations

62

Factors that can modify placental maturation

Maternal nutrition
altitude
exercise
maternal disease - HTN, DM, ethanol, nicotine
Pregnancy progressing beyond term
--> reaches max size/surface area at 37 wks; beyond term met with decrease in placental function
- if fetus continues to grow, placenta-fetus ratio decreases
- increased incidence of perinatal morbidity and mortality

63

Placental examination

every placenta should be examined grossly after delivery
thorough history of previous OB Hx, labour/delivery, early neonatal outcome
statement of indications
detailed gross exam, adequate microscopic exam and appropriate use of ancillary studies if required

64

umbilical cord examination

length and diameter
insertion and vessel count
varicosity, false and true knots
areas of engorgement,torsion or deficiencies of Wharton's jelly and changes in colouration

65

Placental membrane examination

colouration
point of rupture
- position important in relationship to placenta/blood vessels, especially if some blood vessels travel on membranes unprotected
Damage to one of fetal blood vessels could lead to loss of fetal blood and potentially hypovolemia/shock/death

66

Placental disc examination

contour
accessory lobes
dimensions, trimmed weight
Fetal surface - vascular pattern, thrombi, cysts, plaques
Maternal surface - completeness, fibrin, calcification, infarction
Cut surface: infarcts, fibrin, gross abnormality