extraembryonic development Flashcards
what are the four extraembryonic membrane in placental mammals and birds
- Chorion
- Amnion
- Allantois
- Yolk sac
what are the roles of the extraembryonic membranes
• Transfer of nutrient in and breakdown of products out
• Physically protect the embryo
• Protect the embryo from toxins, hormonal changes (from mother),
• Protects from temperature changes - Embryo is v metabolically active and exothermic. EEMs act a a sink and take the heat away from the embryo
• Protect from maternal immune system
• Allow growth (not pressing on the embryo)
• Cellular differentiation - gonadal stem cells, haematopoietic stem cells
• Drive hormonal changes
Signal to mother pregnancy is occurring (hCG)
Maintain uterine relaxations and secretions (progesterone) (later on)
End pregnancy and Induce parturition (oestrogen- CRH)
what are the two phases of uterine growth
proliferative phase: 17β estradiol, stimulates stromal and epithelial growth
and secretory phase: 17β estradiol and progesterone, nurtures embryo (rich in glycoproteins sugars and amino acids)
what is the first EEM to appear
the chorion
what forms the chorionic membrane
the highly mobile trophoblast cells which migrate into the uterine wall (endometrium)
how does the embryonic side of the chorio-allantoic placenta form in mammals
The allantois fuses with the chorion and mesoderm migrates in from the embryo itself (forms blood vessels)
where does the mammalian foetal placenta form
from trophoblast cells in one region of the chorion
where/when does the amniotic cavity form
between the hypoblast and epiblast in week 2
what cell layers does the trophoblast differentiate into when in enters the maternal tissue
inner cytotrophoblast and outer syncytiotrophoblast
when do the lacunae form in the expanding syncytiotrophoblast and when do they fuse to form lacunae networks
in the second week - day 12 they stop growth and fuse to form the lacunae networks
on day 12 describe how the chorionic cavity first forms
extraembryonic mesoderm cells appear between the cytotrophoblasts and the primitive yolk sac causing cavities to form and fuse
where does the maternal placenta form
the upper surface of the uterus
what are the four lineages of the trophoblast stem cell
The fusion lineage – syncytiotrophoblast
The invasive lineage – interstitial cytotrophoblast
Then later
Through endoreduplication – trophoblast Giant cells
Structural spongiotrophoblasts
(possibilities as to )why are the fetal and placental tissue are not immunologically rejected by the mother
reduced antigenicity of the trophoblastic tissues, paralysis of the mother’s immune system during pregnancy, and local immunological barriers between the fetus and mother.
what is a major difference between the fetal and maternal placental surface
The fetal surface of the placenta is smooth and shiny because of the apposed amniotic membrane. The maternal surface is dull and lobulated, with cotyledons of numerous placental villi and their branches.
what is the function of chorionic villi
This membrane allows the exchange of nutrients and oxygen and waste and carbon dioxide between the mother and the fetus.
what is the main function of the chorion
protect and nurture the embryo
what is the main function of the amnion
Protecting the fetus: The fluid cushions the baby, acting as a shock absorber.
Temperature control: The fluid insulates the baby, keeping it warm and maintaining a regular temperature.
Infection control: The amniotic fluid contains antibodies
what is the main function of the allantois
serves as an embryonic respiratory organ. It receives the excretions of the embryonic kidneys. It absorbs albumen, which serves as a nutrient (protein) for the embryo.
what is the main function of the yolk sac
provides an embryo with nourishment (food). It helps circulate gasses between you and the embryo. The yolk sac also produces cells that turn into important structures, such as the umbilical cord, blood cells and reproductive organs
(in the 9th&10th days) what tissues form the human body
primitive ectoderm (epiblast) and visceral endoderm (hypoblast)
what is endoreduplication
the process by which a cell undergoes multiple S-phases without an intervening mitosis and its accompanying cytokinesis. - This results in terminally differentiated cells that never re-enter the mitotic cell cycle. (typically when cells differentiate into specialised function)
what is the primary function of interstitial cytotrophoblasts
to anchor the growing fetus to the maternal uterine tissue
what is the primary function of endovascular cytotrophoblasts
penetrate maternal spiral arteries and route the blood flow through the placenta for the growing embryo to use.
what are the functions of syncytiotrophoblasts
- Control both maternal adaptive and innate immunity
- Physical separation
- Express cell surface non classical MHCI molecules (rather C-E-G) – inhibit NK cells, suppress CTL activity and upregulate the local CD4’ Treg cell differentiation
- Blocks innate immunity via the modification of neutrophil function and NET formation
- Produce hCG- signal to mother pregnancy has started
what is hCG
Human chorionic gonadotropin (hCG) is a hormone produced by the placenta during pregnancy. It helps thicken a person’s uterine lining to support a growing embryo and tells the body to stop menstruation. Stops degeneration of the corpus luteum and maintains progesterone formation
what maintains the corpus luteum in a normal cycle
luteinising hormone from the pituitary
what two hormones does the corpus luteum produce
progesterone and 17beta estradiol
what prevents the corpus luteum from degenerating in pregnancy
syncytiotrophoblasts in the embryo produce hCG (80% identical to LH) (negative feedback loop doesnt work on syncytiotrophoblasts)
what is the negative feedback loop involved in corpus luteum degeneration and how does pregnancy prevent it
hypothalamus+pituitary release LH (maintains corpus luteum)
corpus luteum releases progesterone and estradiol
progesterone switches of LH
hCG produced by the embryo syncytiotrophoblast is a survival factor for CL and prevents its degeneration
Progesterone has no effect on hCG production
what are the functions of progesterone
- Blocks further follicle development and ovulation (blocks FSH and LH)- so single embryo (or one timed wave of ovulation).
- Barrier at cervix, mucus plug stops microorganism and sperm entry. Cervical plug.
- Induces uterine endometrium to make a nutrient-rich food source for embryo (secretory phase- needs estrogen too). (important for early stages)
- Keeps uterine myometrium ‘quiescent’. (non contractile – remove it and it will contract highly and try to expel embryo)
- Progesterone initially made by CL , later in pregnancy by placental syncytiotrophoblast (4-7 weeks) luteoplacental shift.
- Induces breast tissue growth ready for lactation. (later in pregnancy)
what is the luteoplacental shift
The shift from the corpus luteum to the placenta as the site of production of estrogen and progesterone in amounts that are sufficient to maintain pregnancy in humans
what is the cytotrophoblast shell
the external layer of cytotrophoblasts from the fetus that is found on the maternal surface of the placenta - it firmly secures the placenta to the mother endometrium
what is the decidua basalis
The area of endometrium between the implanted chorionic vesicle and the myometrium, which becomes the maternal part of the placenta.
what are maternal spiral arteries
small arteries between villi on the embryonic side lay a vital role in supplying nutrients to the placenta and fetus
remodelled into highly dilated vessels by the action of invading trophoblast
why are maternal spiral arteries spiralised and what is responsible
reducing pressure on the blood (bad blood would rupture)
spiral formation driven by trophoblastic giant cell
describe the formation of the placenta from human E14-E20
E14 Cytotrophoblasts grow out into the endometrium increasing the contact surface at the primary chorionic villus
E16 Extra-embryonic mesoderm grows out into the primary villi increasing the surface contact further – the secondary chorionic villus
E20, blood vessels form in the secondary villi mesoderm- this forms the tertiary chorionic villus, and cytotrophoblast cells cover the syncytial cells- the cytotrophoblast shell
what are trophoblast giant cells
first cell type to terminally differentiate during embryogenesis and are of vital importance for implantation and modulation of post-implantation placentation.
what are the functions and features of trophoblast giant cells
- Highly polyploid (switched from a mitotic to endoreduplicative cell)
- Like metastatic tumour cells, they breach basement membranes and invade deeply into the maternal decidualized uterine stroma.
- Highly angiogenic and vasodilatory properties, remodel arterial walls, enable them to redirect maternal blood flow towards the implantation site.
what are trophoblast giant cells important for
• Important for angiogenic events, remodelling (spiralisation), hormones for vasodilation, phagocytic, immune evasive
where are spongiotrophoblasts found
between the villi and the cytotrophoblast cells - in the intervillous spaces which is flooded with maternal blood
what is a discoid placenta
A placenta in which part of the chorion remains smooth, while the other part interacts with the endometrium to form the placenta. - in humans and rodents
what are the average dimensions of the human placenta
- The placenta average 22-24cm diameter and 2cm thick – typically weights 500grams
what are the main substances transferred from the mother to the foetus
water, oxygen, nutrients and IgG antibodies
what are the main substances trasnferred from foetus to mother
metabolic wastes including urea, uric acid and creatine (via developing foetal kidney and the allantois) and carbon dioxide and heat
do the mother and foetus exchange cells through the placenta
– under normal conditions, maternal cells do not cross the placenta during gestation.
however fotal cells are highly invasive - (have been found to enter the mothers tissue)
what endocrine functions does the placenta have
- Human chorionic gonadotropic hormone: prevents shedding of uterine wall (menses) when the corpus luteum atrophies- similarities to LH and induces progesterone formation. Signals that pregnancy has started.
- Progesterone increases uterine secretion (foetal nutrition). Also prevents contractions of the uterus
- Oestrogen drives growth of the uterus and increase uterine blood supply. More important later ingestation. Significant for the end of pregnancy (prep embryo, change muscle and ligaments to allow expulsion)
- Human placental lactogen increase foetal metabolism and general growth and development. Act s with growth hormone to stimulate IGF-1 production and regulating intermediary metabolism
what cloaks the foetus from the maternal immune system
the placenta;
Secretes neurokinin-B, physical masks cells of foetal placenta.
Lymphocytic suppressor cells in the foetal placenta block IL2 and limit maternal cytotoxic T cells
what part of the mebryo is not surrounded by the amnion
its junstion with the yolk sac/ allantois and body stalk
what blood vessels does the umbilical cord have
two umbilical arteries and one umbilical vein
how does the placenta connect to the foetus
by an umbilical cord (approx 55-60cm long)
how does the amnion form around the embryo
Amnion precursors start to appear in man just after implantation
The epiblast forms an epithelial plate with a lumen in it
The lower surface is the epiblast and the upper are amnioblasts
As the embryo and the amnion grows – it slowly rotates
Then the embryo folds – head and tail, and to the sides (forming a cavity in the embryo)
pulling the amnion around
so the amnion surrounds almost all the embryo
when amniotic fluid is produced the amnion expands to ultimately adhere to the inner surface of the chorion
when in amniotic fluid produced and what is its purpose
about 5 weeks in humans
it allows free movements of the foetus during the later stages of pregnancy. portects it from trauma and maintains temperature
what is amniocentesis
a procedure in which amniotic fluid is removed from the uterus for testing or treatment: genetic and chromosomal testing
describe the formation of the yolk sac
cells from the hypoblast migrate out the mebryonic disk and cover the inner cytotrophoblasts: the spaces are (outer) extraembryonic coelom and (inner space) primary yok sac) - E9
a second sheath forms (visceral endoderm)
the extraembryonic mesoderm grows out between the cytotrophoblasts and the paretal endoderm then opens to forma wider extacoelomic cavity
pinching of the yolk cavity occurs to form two sacs the smaller pinched one dies and the secondary (definitive) yolk sac is formed
what is different about mouse yolk sacs
instead of forming a flat disk, the parietal endoderm grows down then up then forward then rotates to the left meaning that it gets pulled around the embryo and the yolk sac surrounf the embryo
what are the roles of the yolk sac
early nutrient transfer (in man E15 to 3 months) (because placenta doesnt do this for a few weeks) vascular components (first site of blood vessel endothelial development and hematopoeisis in mammals - yolk sac hematopoietic stem cells) primordial germ cell ancestors (migration from primary ectoderm into the yolk sac about E20 near exit of allantois)
when do the PGCs move back into the embryo fom the yolk sac
4-6 week, moving along the yolk sac/ allantois wall into the wall of the gut to colonise the gonadal ridge
how do marsupial EEMs differ from mammals
born in underdeveloped state.
Doesn’t produce chorioplacental membrane,
make choriovitili placenta not allantoic.
no proper vascular supply instead diffusion of secreted material from the glandual uterus
how are fish EEMs different to mammals
large yolk sac, no chorion, no alltois.
why does the mammalian allantois remain small
befcause materal in it can diffuse away as it fuses with the chorion (it doesnt have to retain waste)
what is the allantois
This is an outgrowth (diverticulum) initially from the yolk sac (then hindgut) and pushes into the stalk joining to the chorion (part of the umbilicus), in man ~E16
fusion with the chorion forms the chorioallantoic membrane
what is the viteleine duct
an embryonic structure providing communication from the yolk sac to the midgut during fetal development. Normally, it obliterates spontaneously and separates from the intestine between approximately the 5th and 9th weeks of gestation
expansion of the _________ leads to the elongation of the umbilical cord
amniotic cavity
what problems can occur when getting rid of the allantois
patent urachus - an opening between the bladder and belly button
umbilical polyp - a lesion in belly button (didnt close all the way to the end)
bladder diverticulum - a puch in the bladder wall
urachal cyst - a sac-like pocket of tissue that develops in the urachus and remains
what structures specialise to form the embryonic placenta and the maternal placenta
the chorion and allantois form embryonic placenta
the endometrium forms the maternal placenta
in birds what are the functions of each of the EEMs
The amnion protects the embryo in a sac filled with amniotic fluid.(shock absorber)
The yolk sac contains yolk — the sole source of food until hatching. Yolk is a mixture of proteins and lipoproteins.- it is very large initially reducing during development
The chorion lines the inner surface of the shell (which is permeable to gases) and participates in the exchange of O2 and CO2 between the embryo and the outside air.
The allantois stores metabolic wastes (chiefly uric acid) of the embryo and, as it grows larger through development, also participates in gas exchange.
what are monochorionic twins
two foetuses share a single placenta (70% of identical twins)
if separation occurs in the first 1-3 days how do twins form
dichorionic diamniotic - two blastocysts form and the babies develop in two separate placentas and amniotic sacs
if separation occurs between 4-8 days how do twins form
Monochorionic diamniotic twins - identical twins who share a placenta (monochorionic) but each have their own inner sac (diamniotic)
if separation occurs between 8-13 days how do twins form
monoamniotic monochorionic twins - share a common amniotic cavity
if separation occurs between days 13-15 how do twins form
conjoined twins
what is tetragmetic chimerism
fusion of two early embryos
Most same sex chimera (XX/XX or XY/XY) are healthy and often detected by chance.
what is microchimerism
Transfer of stem cells if blood vessel anastomosis occurs – as foetal immune system is not competent to remove foreign antigens any invading haemopoetic stem cells survive
seen is ~8% of twins and ~21% of triplets
