Lecture 8 + 9 - Embryology Flashcards
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)
outline what happens in the Pre Embryonic period
fertilisation cleavage the formulation of morula (clump of cells) compaction - forming the blastocyst Implantation begins
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)
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
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
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
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
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
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.
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
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
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
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
what happens on day 9 - after implantation
rapid development of the synctiotrophioblast
primitive yolk sac forms
yolk sac in contact with the cytotrophoblast
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
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
