Placental anatomy + development Flashcards
Placenta - significance in pregnancy
growth restriction - many due to placental insufficiency spontaneous abortion, miscarriage nutrition/oxygenation prematurity hemorrhage preeclampsia
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
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)
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)
Decidua regions
Decidua parietalis
Decidua capsularis
Decidua basalis
decidua parietalis/capsularis later join to form decidua vera as embryo grows
Decidua parietalis
non-implantable site
Decidua capsularis
thin capsule of endometrium covering developing embryo
Decidua basalis
implantation site
eventually becomes maternal portion of developing placenta
Implantation of placenta
usually placenta implants in fundus
can be ectopic
placenta previa
implantation over cervix
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
stages of implantation
Apposition
Adhesion
Invasion
Apposition - implantation
microvilli on trophoblast interdigitate wiht pinopodes on apical surface of uterine epithelium
embryonic pole is oriented towards uterine epithelium
Adhesion - implantation
increased physical interaction between blastocyst and uterine epithelium
Invasion - implantation
blastocyst penetration of uterine epithelium
Trophoblasts
outer cell mass of blastocyst
destined to become 2 cell types:
cytotrophoblasts
Syncytiotrophoblasts
Cytotrophoblasts
progenitor cells
villus cytotrophoblasts
extravillus cytotrophoblasts
Syncytiotrophoblasts
giant, multinuclear cells formed by fusion of cytotrophoblasts
terminally differentiated
invasion of endometrium
Inner cell mass differentiation
into 2 layers
Epiblast
Hypoblast
Epiblast
extra-embryonic ectoderm
contribute to formation of amnion - forms amniotic cavity that surrounds entire embryo + fetus
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
Chorion origin
mesoderm + cytotrophoblast
contributes to placenta vascularization
Umbilical cord origin
Allantois (from hindgut endoderm) + chorion
Amnion origin
extra-embryonic ectoderm
Yolk sac origin
parietal endoderm
Fetal components of the placenta
umbilical cord amnion lacunae fetal placental vasculature chorionic plate basal plate villi cotyledon
Maternal components of placenta
decidua
maternal placental vasculature (spinal arteries)
intervillous space filled with maternal blood
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
Villi in early pregnancy
maternal and fetal blood separated by 3 cell layers:
- syncytiotrophoblasts
- cytotrophoblasts
- fetal capillary endothelium
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
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
Shallow cytotrophoblast invasion
increased risk of pre-eclampsia, abruptio placenta
maternal vessels not opened properly
fetal risks: hypoxia, malnutrition, growth restriction, stillbirth, prematurity
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
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%)
Uterine blood flow at term
700 ml/min
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)
Stem villi
support structure with central arteries + veins
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
Anchoring villi
extend to maternal surface, spread laterally to meet and form cytotrophoblast shell/basal plate
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
Placental components
umbilical cord
placental membranes (chorion, amnion)
placental disc - fetal surface, maternal surface, parenchyma (villus tissue)
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%
Central insertion of umbilical cord
more common
run protected in Wharton’s jelly until insertion into fetal surface of placental disc
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
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
Placental membrane
Amnion + chorion
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
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
Normal placenta
should appear clear and non-cloudy
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
Placental membrane histology
cuboidal epithelium
basement membrane
compact fibroblast/spongy layers
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)
Fetal surface of placenta
umbilical cord insertion
arborizing pattern, spreading out form/draining to umbilical cord
may identify fetal surface vessel thrombosis or hematoma
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
Maternal surface of placenta - histology
maternal vessels within decidua
may yield information about maternal well-being
Increased surface area for exchange
increases in cellular content and surface area until 36 weeks gestational age
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
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
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
First trimester histology of placenta
large villi, covered in 2 layers of cells (cytotrophoblast, syncytiotrophoblast) with few centrally located vessels and abundant loose stroma
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
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
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
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
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
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
