Lecture 7 - Implantation and Placentation Flashcards
The early conceptus
from zygote to hatched blastocyst
Fertilized oocyte
ovum
-cyte at end of germ cell says
it’s haploid
-ium at end of germ cell says
diploid
- polar body (finished meiosis because fertilized, extra DNA) on the left, inside
- it’s been fertilized = 2n but not yet diploid nucleus
- male and female pro nuclei that haven’t fused
- ovum has 2 pro-nuclei
- fuzzy bit around the outside = zona pellucida
- still around oocyte even though fertilized
- hardens as block to polyspermy
- at edges of zona are cumulus cells that haven’t left yet
- eg at bottom
- 2 pro nuclei in center – once fuse get proper diploid cell, get mitosis
Zona pellucida
- hardens to block polyspermy
- cumulus cells at the edges
Once the 2 pro-nuclei fuse
get a proper diploid cell, mitosis
At the 2, 4, 8 cell conceptus
- zona pellucida still there so can’t grow
- when cells divide they’re smaller because restricted by zona pellucida unless stretch
The morula
- zona starts to thin, start to get expansion of the “embryo” inside
- embryonic growth - not yet an embryo
- every cell (blastomere) in there is totipotent
- can become anything
- takeo ne out and not influence what’s left behind (totipotent will divide and replace it)
- (pluripotent = many, has the potential to become several things)
Blastomeres in the morula are
totipotent
What is the function of the zona pellucida?
- hardens to stop polyspermy
- keeps the cells together
- once this has a good structure can lose the zona
- most mammals are polytocus
- several embryos without zona around each = may mix
- end up with chimeric, with blastomeres from one embryo to the other
- zona prevents embryonic chimerism
Blastocyst
(picture)
Blastocyst
- if a bowl of cells is dividing, will reach point where the sphere is bigger than the exponential series
- a cavity will open up in the middle = blastocoel
- this is now a blastocyst with cyst being a fluid-filled structure
- growing cyst
- see differentiation
- eg morula totipotent will cells identical
- cells areound the outside is trophoblast
- tropho indicating growth
- = growing sphere
-
pluriblast (inner cell mass ICM)
- pluriblast makes up the embryo
- so trophoblast goes on to make the placenta (contributes to)
Morula (b) vs. Blastocyst (e)
(picture)
Morula v Blastocyst
- morula has identical blastomeres
- totipotent - each one can make an embryo
- blastocyst is a fluid-filled cavity, outside is trophoblast
- cells don’t look the same
- trophoblast has villi
- later for attachment
- pluriblast gives rise to layers of the embryo
Blastocyst derivatives
(picture)
Blastocyst has
- trophoblast on the outside
- pluriblast on the inside (inner cell mass)
(blastocyst → trophoblast and pluriblast
trophoblast → cytotrophoblast and syncytiotrophoblast
pluriblast → hypoblast and epiblast)
Trophoblast divided into
cytotrophoblast and syncytiotrophoblast
(blastocyst → trophoblast and pluriblast
trophoblast → cytotrophoblast and syncytiotrophoblast
pluriblast → hypoblast and epiblast)
Cytotrophoblast
- individual trophoblast cells
(blastocyst → trophoblast and pluriblast
trophoblast → cytotrophoblast and syncytiotrophoblast
pluriblast → hypoblast and epiblast)
Syncytiotrophoblast
- fused cells
- individual cytotrophoblast cells fuse to give syncytium (large multinucleate cell)
- placenta with 1 cell covering the whole thing
(blastocyst → trophoblast and pluriblast
trophoblast → cytotrophoblast and syncytiotrophoblast
pluriblast → hypoblast and epiblast)
Together the cytotrophoblast and the syncytiotrophoblast contribute to the
chorion
(placental membrane)
(blastocyst → trophoblast and pluriblast
trophoblast → cytotrophoblast and syncytiotrophoblast
pluriblast → hypoblast and epiblast)
Pluriblast can be divided into
hypoblast (small growing cells) and epiblast (the ones on the outside)
(blastocyst → trophoblast and pluriblast
trophoblast → cytotrophoblast and syncytiotrophoblast
pluriblast → hypoblast and epiblast)
Hypoblast
goes on to form the yolk sac
(blastocyst → trophoblast and pluriblast
trophoblast → cytotrophoblast and syncytiotrophoblast
pluriblast → hypoblast and epiblast)
Epiblast
gives layers of our embryo (endo meso ectoderm)
- will also give rise to ectoderm and extraembryonic mesoderm
- amniotic tissue around ICM
- extraembryonic tissue around the trophoblast
(blastocyst → trophoblast and pluriblast
trophoblast → cytotrophoblast and syncytiotrophoblast
pluriblast → hypoblast and epiblast)
The amniotic ectoderm and the extraembryonic mesoderm form
the amnion
The endoderm produces
allantois
Blastocyst hatching
(picture)
Blastocyst hatching
- has to hatch to leave zona pellucida behind
- the blato needs to hatch to be able to attach
Embryo attachment
(picture)
Embryo attachment
- apposition - the embryo physically touches endometrium
- is reversible
- adhesion then irreversible
- can then invade
- compares apposition adhesion invasion in atherosclerosis (WBC through clogged artery)
Embryo attachment
2 phases
- apposition
- reversible
- coming together with endometrium
- followed by adhesion/adherence
- stickiness
- molecular mechanisms differ between species and include redundancy (several molecules fulfill the same functions)
- redundant because it’s not allowed to go wrong
Embryo attachment
need to down-regulate
endometrial molecules which would block/deter adherence
- can repel or attract embryo
- so need to get rid of anything to deter embryo
- eg MUC1 gets in the way so must get rid of it
- has sugars attached through serine and threonine residues
- eg MUC1
- encodes O-linked glycoprotein
- up-regulated by progesterone
- down-regulated by blastocyst
- encodes O-linked glycoprotein
- gets rid of protective layer
Embryo attachment
need to up-regulate molecules which facilitate
adherence (/attraction)
eg LIF1
- estrogen-induced cytokine with paracrine roles in
- decidualization of endometrium
- change in endometrium that makes it ready for the embryo
- up-regulation of EGF family
- decidualization of endometrium
- differences in mammals but MUC1 important and LIF1 important, also need upregulation of EGF family
Embryo attachment
EGF family members include
heparine-binding EGF-like growth factor (HB-EGF)
- activates EGF receptors plus heparine sulphage glycoproteins (HSPG) on TB to dissolve ZP
- HB-EGF responsible through EGF receptors for dissolving zona pellucida
- here blastocyst needs to hatch before attaches to endometrium
- upregulate LIF1 then this to dissolve zona pellucida so embryo can hatch
Embryo attachment
other proteins implicated in embryo apposition include
- integrins (on the TB)
- bind selectins on endometrium cells
- selectins (on the endometrium)
- ECM molecules (eg laminin, fibronectin, collagen)
- for binding
- trophinin-bystatin-tastin complex
- specific complex for embryo attachment
- Frizzled-Notch interactions
- Frizzled is the ligand
- each of these pairs will cause to stick but have each of these to make sure they stick/implant
- majority (80%) of embryos don’t pass here, even with this redundancy
Timing of embryonic events
(post ovulation)
- conceptus enters uterus at 3 days
- 3-4.5 days to form blatocyst
- moust, rat as soon as it forms in tattaches
- all within a week of ovulation the embryo is attached (not necessarily implanted)
- time to form blastocyst is slightly slower
- attachment is longer for embryo to attach (free-flating in uterine cavity)
- becuase not attached - how does it get nourishment
- normally go for nearest blood vessel but doesn’t here - floats for a bit
Embryo implantation
blastocyst implantation can be
- invasive
- non-invasive
Invasive blastocyst implantation
breach endometrial epithelium to invade the underlying stroma
(eg primates, carnivores, and rodents)
Non-invasive blastocyst implantation
endometrial epithelium intact (or locally disrupted)
(eg ungulates)
Invasive vs non-invasive embryo implantation
- invasive = blastocyst breaches the endometrium epithelium into the endometrial wall
- non-invasive = attaches and stays on surface of uterus (local disruption but not invasive)
Non-invasive implantation
- blastocyst grows rapidly (extraembryonic growth
- maximizes surface area to absorb uterine nutrients
- eg pig blastocyst elongates x50 (2 to 100 mm) on day 6 to day 12 pc
- embryo-maternal interfacea cross epithelium (caruncles-cotyledons)
- embryos not through the epithelium but sit on the surface
- cotyledon on embryo
- caruncle on endometrium
- sit together but never breach each other
- blastocyst has 2 cell populations
- trophoblast around grows quicly to give maximum surface area
- free-floating must absorb nutrients across the surface
Invasive implantation
- blatocyst implants while small (extraembryonic growth)
- trophoblast invades between epithelium into underlying stroma, but embryo part stays really small
- depth of invasion limited by speed of stromal/decidual response - poor decidual response triggers deeper invasion
- the blastocyst can’t grow until it hooks up to a blood supply (ass opposed to above feeding off uterine secretions)
- stroma undergoes decidualization (quality of response decides how well it invades
- good decidual response = doesn’t need to invade very far
- poor decidual response = embryo keeps burrowing down through the endometrium until it hooks up to a blood vessel
Invasive implantation
blastocyst
- blastocyst with external trophoblast, internal cavity, pluriblast/inner cell mass to go on to give the embryo proper
- the blastocyst with trophoblast around the outside with villa with zona in the way to stop embryo to binding before it gets to the uterus
- without you get ectopic pregnancy - anywhere other than uterus
- trophoblast then interdigitates
Invasive implantation appears to be
an inflammatory cascade
- up-regulation of MMPs and, simultaneously, of TIMPs
- MMPs break down extracellular matrix
- TIMPs = tissue inhibitors
- upregulate MMPs and inhibitors to control
- increased PGs (via PTGS/COX-2)
- positive effect on human implantation of endometrial scratching
Endometrial scratching
(invasive implantation)
- cause inflammation in women who are having trouble conceiving
- not all women
- this technique of scratching the endometrium before putting the embryos back doesn’t work for all women
- if look just at women who have miscarriage or repeated pregnancy failure – scratch their endometrium and replace embryos the chances of conceiving go up massively
- problem with the inflammation so have to mechanically cause inflammation before put embryo in
- so inflammatory cascade is required for embryo attachment and implantation, even if you have to cause it
Decidualization
- mother provides deciduon - endometrium which responds to the embryo being there
- in decidualization whent he conceptus attaches we see in the underlying stroma (myometriuim doesn’t change):
- rapid changes in endometiral stroma underlying the conceptus, including
- increased vascular permeability
- oedema
- losening of the ECM between cells
- stromal cell differentiation
Decidualization
inflammatory cascade mediated by:
- increased prostaglandins
- pro-inflammatory cytokines
- eg IL-11 (doesn’t normally appear on list)
- VEGFs
- vascular endothelial growth factor - want more blood vessels
- angiopoietins
- about angiogenesis
- the tissue will become highly vascularized to form a placenta
- prolactin-related peptides
- eg proliferin
- for tissue growth, but wouldn’t have expected to be there (expect inflammatory mediators)
Decidualization
up-regulation of
11βHSD1 enzyme
- makes cortisol from cortisone
- generates active cortisol from intert cortisone
- hydroxyl group gives cortisol its name (the alcohol cortisol needs to be active)
- increases cortisone around
- cortisol active, cortisone inert
- cortisol to cortisone switches the enzyme off completely
- in decidualization as the endometrial cells decidualize upregulate the enzyme that does the opposite
- turns cortisone into cortisol to increase the amount of cortisol
- upregulate MMPs as well as TIMPs to keep MMPs under control
- here want prostaglandins and inflammatory cascade but don’t want to go too far (only localized) so upregulate machiner to make anti-inflammatory steroid
Decidaulization
rodents
- induced by presence of embryo
- primary (local) followed by secondary (widespread) decidualization reactions
- rodents only decidualize if there’s an embryo there
- normal rat/mouse think anything in its uterus must be an embryo
- respond to something in uterus not necessarily embryo
- get primary then secondary just because “embryo” there
Decidualization
primates
- occurs spontaneously in response to progesterone
- where from?
- progesterone from gland corpus luteum
- most species polytocus so have several corpa lutea which make progestrone to make the endometrium
- decidualization every month (assumes an embryo is there) and so prepares for pregnancy
- in some species the deciduon only forms in the event of pregnancy (rodents)
- in others deciduize every month assuming a pregnancy should be there
Placental classification
Classify mammalian placenta on the basis of
extent/shape of placenta
Placental classification
types
- discoid
- zonary
- cotyledonary
- diffuse
Discoid placentas
also bidiscoid
distinct disc on particular region of implanting blastocyst to become embryo
- primates
- lagomorphs
- rodents
- insectivores
- bats
Zonary placentas
placentas in a discrete zone
- carnivores
- seals
- elephants
Cotyledonary
- sheep
- cows
- giraffes
- deers
- goats
Diffuse placentas
- general contact between embryo and reproductive tract
- diffuse foci but lots of them between the embryo and the endometrium
- prosimians (discoid shape is late evolution in primates, primate ancestors still have diffuse)
- pigs
- horses
- camels
- moles
- cetaceans
- kangaroo
- discoid
- entire conceptus trophoblast everywhere
- special piece of trophoblast interacts with deciduon and forms the placental disc which is then delivered as afterbirth
- zonary
- carnivores
- coceptus still elliptical
- just a single zone like a belt to form the placenta
- cotyledonary
- gray is conceptus (embyo)
- purple is contact point with uterus
- cotyledon meets with coruncles to bind on endometrium
- diffuse
- lots of points of contact
- not even distinct cotyledons
Classify mammalina placenta on the basis of
maternal tissue in contact with the conceptus
Placental classification
classify mammalian placenta on the basis of maternal tissue in contact with the conceptus
- blood (maternal tissuue that contacts is blood)
- eg primates, lagomorphs, rodents
- capillary endothelium
- eg carnivores with zonary have capillary endothelium
- endothelial cells from mom’s capillaries between her blood flow and fetal blood flow
- endometrial epithelium
- eg sheep and cows
- non-invasive placentation
- fetal tissue not in touch with blood or vessels but endometrium and endothelium
- epithelial syncytium (eg pics and horses)
- fusion of cells between mother’s epithelium and binucleate cells (special trophoblast cells that come out from the fetus)
- syncytium between mom’s epithelium and fetus’s binucleate cells
a - haemachorial
d - endotheliochorial
e - epitheliochorial
f - synepitheliochorial
Haemochorial
(picture)
Haemochorial placenta
- chorion bathed directly with blood
- haemochorial placentation
- involves blood and chorion
Chorion
(fetal/embryonic membrane derived partly from the mesoderm - especially the trophoblast and cytotrophoblast and syncytiotrophoblast together)
endotheliochorial
Endotheliochorial placenta
- chorion separated from blood by mom’s endothelial cells
- capillary endothelium doesn’t break down
- seen in carnivores, seals, elephants
Epitheliochorial (picture)
Epitheliochorial placenta
- conceptus has choirion around it but maternal epithelium still intact because noninvasive placentation
Synepitheliochorial
(picture)
Synepitheliochorial placenta
- epithelium fuses with binucleate cells out from chorion (chorion tropholastic cells) to get synepithelio-chorial
Placental classification
classify mammalian placenta on the basis of
number of layers in chorionic trophoblast
- how many layers of chorion are there in the trophoblast between the embryo which needs the nutrients and the mother
- layers of chorion to be traversed by anything going into/out of the embryo
- layers of chorion between maternal and embryonic circulation
- 1 (eg primates, carnivores, pigs, horses)
- 2 (eg lagomorphs)
- 3 (eg rodents)
Three layers of placental classification
- extent/shape of placenta
- maternal tissue in contact with the conceptus
- number of layers in the chorionic trophoblast
Haemochorial placentae
primates =
haemomonochorial placenta
Haemochorial placentae
lagomorphs =
haemodichorial placenta
Hameochorial placentae
rodents =
haemotrichorial placenta
Haemochorial placentae
- primates = haemomonochorial placenta
- lagomorphs = haemodichorial placenta
- rodents = haemotrichorial placenta
Epitheliochorial placentae
pig/horse =
diffuse epitheliochorial placenta
Epitheliochorial placentae
sheep/cows =
cotyledonary synepitheliochorial placenta
(syncytium between endometrial epithelium and fetal binucleate cells)
Haemochorial placentae can be
mono-
di-
tri-
-chorial
Epithelio placentae
non-invasive
chorion fuses with epithelium from mother
diffuse placentae and cotyledonary placentae
Epitheliochorial placentae
pig/horse =
diffuse epitheliochorial placenta
Epitheliochorial placentae
sheep/cow =
cotyledonary synepitheliochorial
(syncytium between endometrial epithelium and fetan binucleate cells)
Placentome
placenta only contacts at a discrete point
Diffuse vs Cotyledonary
- cotyledonary - really distinct areas with defined structure with contact between placenta/embryo and mother
- diffuse - light contact points all the way over the conceptus
