Lecture 8 + 9 - Embryology Flashcards by alex finney

what are the three embryonic periods ? and the time they take.

Pre - embryonic - first 2 weeks
embryonic - weeks 3 - 8 ( Building of the complex body systems)
fetal - weeks 9 - 38 (growth)

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outline what happens in the Pre Embryonic period

fertilisation
cleavage
the formulation of morula (clump of cells)
compaction - forming the blastocyst
Implantation begins

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what happens in fertilisation ? first 24 hours

1) oocyte is released from ovary
2) travels along Fallopian tube
3) sperm (3 day viable) fertilises oocyte (1 day viable) in the ampulla
4) the zygote will now travel to ideal implantation site at the posterior uterine wall (cell is dividing as it travels)

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explain cleavage - 30 hours

happens 30 hours after fertilisation
the result is 2 blastomeres of equal size forming
surrounded by a zona pellucida which is a glycoprotein shell

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after cleavage a morula forms. what is this? - 3 - 4 days

the Morula is a collection of totiponent cells (can become ANY CELL)
the cells are getting smaller due to constriction by the zona pellicular

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compaction is the next step.

explain the compaction step - days 4-6

formation of the first cavity - the blastocyst cavity

1st differentation happens - 2 parts form
forms embryoblast (later becomes fetus)
forms trophoblast (supporting cells - placenta ect)

still surrounded by zona pelluicda

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after compaction the hatching happens, what is this ? days 4-6

the blastocyst hatches from the zona pellucida
cells now free to enlarge
allows for interaction with uterine surface for implantation

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after hatching implantation can happen. explain this step day 6

now approx 100 cells

it now begins to penetrate through uterine epithelium into uterine stroma

only 8 of these make the embryo
rest are used in fetal membrane development

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briefly explain assisted reproductive techniques

ooctes fertilised in vitro - allowed to divide to 4 or 8 cell stage
morula then transfered to uterus

we can do a pre- implantation genetic diagnosis (PGD)
remove a cell from morula and tested for serioud inheritable diseases, prior to transfer to mother.

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define totipotent and pluripotent

totipotent - TOTAL POTENT - can be any cell type - only after the first cell divisions
this is before compaction

after compaction cells are
Pluripotent - can become many cells
multilineage potential

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next is week 2, the week of twos

outline the steps of week 2

two cell layers emerge from trophoblast
the synctiotrophioblast - multi neucleonic sheet of tissue for nutrient transport from mother - it allows acess to glands and sinusoids withing the endometrium
cytotrophoblast - stem cells for the synctiotrophioblast

the embryoblast forms the bi laminar disc
made of the
epiblast
hypoblast

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implantation happens over days 9 - 10

explain implantation

the conceptus (embryo and support cells) penetrates through uterine epithelium into uterine stroma - implantation is interstitial

this establishes a maternal blood flow within placneta
the embryo now has support from mothers blood
it has access to glands and sinusoids withing the endometrium

establishes the chronic villus - the basic structural units for materno - fetal exchange

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what are the key conditions linked to implantation defects?

where it implants at an inappropriate site

ectopic pregnancy - implantation at the uterine body (commonly the filopian tube)
can lead to life threatening haemorrhage - need an operation

placenta previa - implants too low in uterine wall
can cause haemorrhage in pregnancy
requires a C section

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what happens on day 9 - after implantation

rapid development of the synctiotrophioblast
primitive yolk sac forms
yolk sac in contact with the cytotrophoblast

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what happens by day 11 ?

summary - extraembryonic mesoderm forms

the primitve yolk sac membrane pushed away from cytotrophoblast by acellular extraembryonic reticulum

the recticulum layer converted to extraembryonic mesoderm by cell migration

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day 12

uteroplacental circulation begins

this happens by
the maternal sinusoids being invaded by syncytiotrophoblast - lacunae (lakes) become continuous with sinusoids (blood flows in lakes)

uterine stroma prepares for support of the embryo

day 13

formation of secondary yolk sac,

defintive yolk sac pinches off from primitve yolk sac

day 14

spaces within extraembryonic mesoderm form the embryonic cavity

the embryo and its cavities suspended by a connecting stalk that is a column on mesoderm that will go on to become the umbilical cord

bleeding at this time can be confused with menstrual bleeding

day 14 - aka where are we at , at the end of the second week ?

conceptus (embryo and its support membranes) has implanted

the embryo, yolk sac (bottom) and aminotic cavity (top) are suspended in the supporting sac

connected by a connecting chord, held within the chorionic cavity

embryo now has access to nurtients

50% of all zygotes are lost in first 2-3 weeks

Challenge - give a full explanation of the first two weeks, with days

dont worry if you cant do this fully, still helpful to try

the embryonic period starts at week 3 -8
marked by start of gastrulation
all systems are built in this time
sensitive time with the greatest risk of birth defects forming
what first forms in the ?

the primitive streak,

with a pit and node

explain the primitive streak

primitive streak with a pit and node
it marks the onset of gastrulation

this is the latest we can currently experiment on human embryos - ethics/law - animals after this point

what happens during gastrulation?

primitve streak appears
the bilaminar disc becomes a trilaminar disc
by migration and invagination of cells of the epiblast
the hypoblast is displaced and a 3rd layer forms

we now have the -
ectoderm - outer - skin, nervous system
mesoderm - middle - Viscera - vascular system - heart and vessels, bone, cartillage, muscle
endoderm - inner - epithelial lining of GI, UI, Respiratory tracts, parenchyma of glands

mouth and anus precursors form at this point

gastrulation occurs to ensure correct placement of precursor tissue.

what is situs invertus

a left right asymmetry - heart liver ect on wrong side of the body

normally leads to no problems, unless there is both normal and mirrored disposition
can result in immotile cilia

happens during gastrulation

ciliated cells at node sends signals that define leftness and rightness to the left and right , send them the other way

what happens involving the formation of the notochord | what is the process of neurulation ?

we form the notochord, within the mesoderm layer it is a solid rod of cells running through the middle has a important role in signalling differentation, but it is diffusion limited notochord directs conversion of overlying ectoderm to neurectoderm (local diifusion part), resulting in a thickened nerual plate there this neural plate, due to notochord signals, raise up from the disc and curl towards each other, to form the neural tube

explain the structure of the embryo around the notochord | draw if you can

central notochord in mesoderm, topped with ectoderm, bottomed with endoderm notochord surrounded by paraxial mesoderm with intermediate mesoderm further out in 4 blocks then forks into two mesoderm branches with a intraembryonic coelom the branches are called the somatic mesoderm (skin) and the splanchnic mesoderm (viscera)

what is the formation of somites

the organisation of paraxial mesoderm into segments always occurs next to neural tube before moving outward 1st PAIR appear at day 20 - 3 pairs appear a day (craniocaudial (brain to apex)sequence) until 42-44 pars appear by the end of week 5 some will dissapear until 31 total are left

explain the structure of somites

they appear as regular block of mesoderm cells arranged around a small cavity - next to neural tube this is followed by organised degeneration - the ventral wall of the somite breaks down leading to formation of sclerotome we have dorsal, scelrotome (skeletal), ventral portions of the somite now further orgainsation of the dorsal portion forms the combined dermomytome then the dermotome (dermis) disperses while the myotome proliferates and disperse we now have 3 layers

what are the three layers of a developed somite

dermatome - dermis / skin section myotome - muscle section/ mucles sclerotome - hard tissue section - bones

what are the implications of segmentation ( somites ) | 3 implicatons

1) orgainsing the mesoderm into somites gives rise to structures vertebrae, ribs, intercostal muscles, spinal chord segments. it guides innervation 2) the 31 segements, mean 31 pairs of spinal nerves, 31 sections of the body, each tied to a pair of spinal nerves 3) skin, muscle from somite is innervated with corresponding spinal nerve

on our 3 layered embryonic disc now we have a buccopharyngeal membrane (future mouth) , and a cardiogenic area ( future heart) resting above it.happ out heart is not above our mouth , what changes?

we undergo folding !!! driven by neural tube growth - head folds then tail - cephalocaudal folding then lateral folding ectoderm is now all the external areas of the body !

as this folding happens, important changes occurs, like heart re positioning. what exactly happens? illustrate if you can - saggital plane folding

saggital section folding - we have a pericardical sac (primitve heart) neural tube grows large - causes embryoninc sac to curl up, pushes pericardial sac into the correct place (thorasic region) simultaneously - ectoderm folds over, the above amniotic sac encloses the whole embryo some yolk is pinched off, the rest forms part of the embryonic body

explain and draw transverse section of trilaminar disc folding

somites are forming 2 mesoderm layers form - somatic - keeps connection with ecotderm and splanchnic - keeps connection with endoerm to give GI tract. somite growth - downward pressure on lateral margins of disc somatic and splanchlic layers are widening apart and forms a cavity - intraembryonic coelom yolk sac pinched off, to form a yolk sac tube inside the embryo - will become a gut tube amniotic sac, has engulfed embryo

what does folding achieve ?

draws toghether margins of the disc pulls amniotic membrane around the disc - embryo becomes suspended in amniotic sac pulling connecting stalk (becomes umbilical cord) vertically - to right place creates a ventral body wall creates the primordium of the gut ( yolk sac ) , puts the heart and primordium of the diaphram in the coorect places creates a new cavity within the embryo

summarise what has happend by the end of the forth week

nervous system starts to from (neural tube) embryo has folded to put it all in the right place segments have appeared giving specific tasks to specific cells

challenge - summarise the embryonic period | hint - gastrulaton - neurulation - segmentation - folding (saggital and transverse)

dont worry if you cant get this just try anyway

challenge - explain all of embryology so far encompassing the pre embryoninc period - fertilisation cleavage the formulation of morula (clump of cells) compaction - forming the blastocyst Implantation begins then the embryonic period gastrulaton - neurulation - segmentation - folding (saggital and transverse)

good luck fool !

challenge - explain all of embryology so far | encompassing - no hints this time!!

ahahahaha