Placenta and Fetal Membranes (Dennis) Flashcards
What are the 4 fetal membranes?
- chorion
- amnion
- umbilical vesicle
- allantois
What is the purpose of the placenta?
- the placenta is a fetomaternal organ that allows for exchange of nutrients/O2 between maternal and fetal bloodstreams (maternal/fetal blood has minimal mixing)
- vessels in the umbilical cord connect the placental circulation w/ the fetal
What are the 2 components of the placenta?
1) fetal part = chorionic sac (outermost fetal membrane)
2) maternal part = dervied from endometrium
- functional layer of the endometrium that separates the remainder of the uterus after parturition
- establishes the maternal component of the placenta
decidua
What are the 3 components of the decidua?
1) decidua basalis: deep to conceptus, forms the maternal part of the placenta (baby/placenta)
2) decidua capsularis: superficial and overlies conceptus (chorion)
3) decidua parietalis: remaining parts of the decidua (‘walls’ of the uterus)
- cellular and vascular changes that occur as the blastocyst implants cause accumulation of pale-staining cells w/ glycogen and lipids
- provides nutrients for the developing embryo
decidual reaction
What does the trophoblast differentiate into?
1) cytotrophoblast: inner layer
2) syncytiotrophoblast: outer layer > invades and displaces decidual cells of the endometrium
(syncytium: multinucleated mass of cells that band together and lose cell membranes)
When does trophoblast differentiation occur and what events does this lead to?
- trophoblast differentiation occurs late 2nd week (~10 days post-fert) as blastocyst embeds into endometrium
- lacunae (air-pockets) appear in syncytiotrophoblast layer and fill w/ blood/uterine secretions
- lacunar networks form by fusion of adjacent lacunae
- establishes primordial uteroplacental circulation: O2/nutrients pass to embryo via diffusion through lacunar network
What are the components of the extraembryonic mesoderm?
(EM derived from hypoblast, surrounds amnion and primary umbilical vesicle)
- extraembryonic somatic mesoderm: lines trophoblast and covers the amnion
- extraembryonic splanchnic mesoderm: surrounds umbilical vesicle
- extraembryonic coelom: fluid-filled cavity that surrounds amnion and umbilical vesicle
- structure composed of extraembryonic somatic mesoderm + cytotrophoblasts + syncytiotrophoblasts
- contributes to fetal portion of the placenta and fetal circulation
chorionic sac
What is the amnion and how does it develop?
- amnion (amniotic sac): thin, tough membrane that surrounds the embryo/fetus and amniotic fluid
- development:
amnioblasts separate from the epiblast > enclose developing amniotic cavity > amnion eventually obliterates chorionic cavity and forms epithelial covering umbilical cord
What is the purpose of amniotic fluid and what is it comprised of?
- purpose: cushions fetus, permits fetal movements, regulates fetal body temp
- comprised of organic compounds (proteins, carbs, fats, enzymes, hormones) and inorganic salts, ions, glycophospholipids, and steroid hormones
How does amniotic fluid develop?
- initially derived from maternal tissue and interstitial fluid
- eventually, fetal urinary, respiratory, and GI systems contribute
- volume increases as gestation progresses: 10 wks, 30 mL > 20 wks, 350 mL > 37 wks, 700-1000 mL
- excessive amniotic fluid; too much prod or not removed effectively
- genetic and/or CNA anomalies, or blockage of GI tube
- clinical sx: abd pain, significant swelling/bloating, breathlessness
- uterus can grow too large, premature rupture of membranes (PROM) risk
(poly)hydramnios
- insufficient amniotic fluid (<400 mL)
- etiology: placental abnormality or maternal high blood pressure
- causes: anomalies w/ fetal development (renal agenesis, pulmonary hypoplasia, inhibited growth such as club foot)
- decreased fluid does not provide enough cushion to fetus and umbilical cord
oligohydramnios
What structures give rise to the placenta?
- formation of villous chorion and chorionic villi from chorionic sac
- reorganization of the decidua basalis
(fetal part = villous chorion
maternal part = decidua basalis)
Describe the process of placental attachment to the uterine wall:
cytotrophoblasts (of chorionic sac) will proliferate at chorionic plate:
1) cells from cytotrophoblast extend through syncytiotrophoblasts, to attach to decidua basalis
2) the outer boundary of the cytotrophoblast cells that have invaded decidua basalis forms the cytotrophoblastic shell, surrounding the chorionic sac
3) the space between the chorionic sac and the cytotrophoblastic shell creates and encloses the intervillous space
(as the cells from the cytotrophoblast invade, they pass through lacunar networks and form the intervillous space)
How do cotyledons develop?
- decidual erosion produces placental septa > divides villous chorion into irregular convex areas (cotyledons)
- cotyledons contain 2+ main stem villi and multiple branch villi
How is maternal blood incorporated into the placenta and fetus?
- intervillous space of the placenta contains maternal blood (11-14th week)
- blood delivery to fetus is pulsatile to avoid overwhelming the placenta
- villous chorion (fetal) and decidua basalis (maternal) are fused, chorionic villi span intervillous space and are exposed to maternal blood
- <20 weeks blood supply consists of: syncytiotrophoblast, cytotrophoblast, CT within villi, and endothelium of fetal capillaries
- >20 weeks: syncytiotrophoblast, CT within villi, and endothelium of fetal capillaries
How do primary chorionic villi develop?
- cytotrophoblasts grow into syncytiotrophoblast layer (end of week 2), is essentially a core of cyto covered by synctio
- eventually form syncytial columns (through week 8)
What is the difference between smooth and villous chorion?
- villi a/w decidua capsularis become depressed > degenerate > smooth chorion
- villi a/w decidua basalis branch profusely > form area of chorionic sac > villous chorion
How do secondary chorionic villi develop?
extraembryonic mesoderm grows within primary villi (3rd week):
- extraembryonic mesoderm (core)
- cytotrophoblasts
- syncytiotrophoblasts
How do tertiary chorionic villi develop?
extraembryonic mesoderm differentiates into capillary and blood cells:
- capillaries and CT core (EEM)
- cytotrophoblast layer (middle)
- syncytiotrophoblast layer (outer)
- secondary villi do not have capillaries but tertiary villi have capillaries
- late in pregnancy, capillary network grows more extensive and some of the cytotrophoblasts atrophy
What are the different types of villi?
- main stem villi: extend from chorionic plate
- branch villi: extend from main stem villus, increase surface area, main exchange between mother/fetus occurs here
- anchoring villi: villi that attach to the maternal tissues through the cytotrophoblastic shell
What changes occur to the decidua during fetal growth?
1) villous chorion forms at placental site; remaining capsularis forms smooth chorion
2) embryo/fetus grows, decidua capsularis bulges into uterine cavity
3) capsularis will degenerate; amnion and chorion fuse = amniochorionic membrane
4) amniochorionic membrane fuses w/ decidua parietalis on opposite wall
What role do branch chorionic villi play in blood supply to fetus?
- branch chorionic villi are bathed in maternal blood and are the main site of exchange of nutrients
- provide large surface area for material exchange across placental membrane
What substances DO NOT cross the placenta?
- protein hormones (i.e. pituitary hormones)
- bacteria
- drugs w/ amino acid like structures (i.e. methyldopa)
What substances CAN cross the placenta?
- nutrients: H2O, glucose, electrolytes, amino acids, vitamins
- hormones: testosterone, progestins, thyroxin, triiodothyronine
- IgG antibodies and insulin (insulin in negligible amnts)
- waste products: urea, uric acid, conjugated bilirubin
- drugs (most cross): alcohol, cocaine, heroine, labor management drugs
- infectious agents: cytomegalovirus, rubella, varicella, measles, herpes, Treponema pallidum (syphilis), T**oxoplasma gondii, Listeria monocytogenes
What are the constiuents of the umbilical cord?
- comprised of mucous tissue (Wharton’s jelly)
- 2 umbilical arteries (deoxygenated fetal blood)
- 1 umbilical vein (80% oxygenated fetal blood)
- allantois proper
- attachment to placenta is usually near center of fetal surface
- agenesis or degeneration of one of the two umbilical arteries
- 1 in 100 singletons; 5 in 100 multiples
- can be a/w chromosomal and fetal anomalies (20%)
- can impact perinatal and 3rd stage labor outcomes
- detected before birth by ultrasonography
absence of umbilical artery
- placenta implants in lower uterine segment/cervix
- 4 types: total, partial, marginal, and low lying
- often leads to serious 3rd trimester bleeding
- most common type of abnormal placentation (0.5%)
- predisposing factors are having the condition prior (60% of cases) and history of previous cesarean section
- 20% of all cases of pregnancy bleeding are due to this condition
placenta previa
- replacement of normal villi by dilated or hydrophobic (edematous) translucent vesicles
- portion of villi are edematous, capillaries can be seen in the villi
- fetal tissue commonly found
- normal ovum fertilized w/ two sperm
- triploid (69, XXY), or tetraploid (92, XXXY) karyotype
partial hydatidiform mole
- replacement of normal villi by dilated or hydrophobic (edematous) translucent vesicles
- all/most villi are enlarged, covered w/ trophoblast invasion
- no fetal tissue
- fertilization of blighted ovum > all DNA is paternal
- fertilized by two sperm or duplication of single sperm
- frequent karyotype of 46,XX or 46,XY
complete hydatidiform mole
What types of differences in membranes can there be between dizygotic twins?
(dizygotic twins originate from 2 zygotes (2 amnions and 2 chorions), fraternal twins)
- relationships of amnions/chorions are dependent on how blastocysts implant
- separate implantation: 2 chorions and 2 amnions
- adjacent implantation: 2 amnions, 2 fused chorions, 2 placentas
What types of differences in membranes can there be between monozygotic twins?
(monozygotic twins originate from 1 zygote)
- membranes are dependent upon timing of division
- the earlier the split > the more separate the membranes and placentas will be
- 65% of monozygotic twins develop from 1 zygote by division of the embryoblast of the blastocyst, producing 2 embryoblasts (2 amniotic sacs, 1 chorionic sac, and 1 placenta will develop)
- 35% of monozygotic twins develop from 1 zygote where separation of the blastomeres may occur anywhere from 2-cell stage to morula stage, producing 2 identical blastocysts (each embryo subsequently develops its own amniotic and chorionic sacs)
monozygotic twins
- staging: 2-8 cell stage
- time of cleavage:
- nature of membranes:
monozygotic twins
- staging: 2-8 cell stage
- time of cleavage: 0-72 hr
- nature of membranes: diamniotic, dichorionic
monozygotic twins
- staging: implanted
- time of cleavage:
- nature of membranes:
monozygotic twins
- staging: implanted
- time of cleavage: 9-12 days
- nature of membranes: monoamniotic, monochorionic
