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Embryology Lecture 1: from fertilization to the first week
Today
Embryological terminology Fertilization
Blastocyst formation
The embryonic disc ImplantationTerminology
Embryo: To grow within: within the egg for a reptile and within the uterus for a mammal
Embryology: humansfirst 8 weeks.
Embryonic period: is the first 8 weeks, crucial time when the 4 tissue types are being formedforming
organs.
Fetal period: from 9 weeks to birth: getting maturation of the structuresand they develop further. Most
differentiation has also occurred.
Teratogen: as the embryonic period is the most critical timemost susceptible to agents from the
environment that disrupts development. Agents from the environment that disrupt normal embryological
development
DNA is arranged in chromosomes in humans
Since all cells have the same genetic makeup, it impliesthat all cells are derived from a common cell
One cellis actually made from the fusion of twocells: a spermatozoa and am egg cell: to form a zygote
(a fertilised egg)
Is a special cell as this cell has the capacity to formevery other cell in the body: totipotent
It does so by a series of mitotic divisions BUT: mitosis is not the only methodas this would
double the number of chromosomes in each cell
Therefore meiosis occurs (reduction division) Occurs in the gonads to form the gonads
gametogenesis
A fertilised egg is very large (0.2 mm)can see it witha naked eye. WHY? as it will go through many cell
divisions rapidly: needs a lot of stored organelles and
materials
Are accumulated in the ooecyte through the process of oogenesis and gametogenesis making theooecyte a large cell
Spermatozoa are tiny? WHY? Because the sperm moves!need to deliver the genetic material tothe site of fertilisation: and need to travel large directions. There are also many of them
Results of fertilisation
Can tell it is a fertilized zygote as it contains pronuclei (one from the spermand from an ooecyte
DNa replication takes place inside the pronucleiand the pronuclei fusetogether to reform the actual zygote and restore dipliody (46 chrom)
Sex is determined at this time: the ooecyte has an x chromosome, and thesperm may carry an x or a y chromosome
Within 24 hours. The ooecyte undergoes mitosis: called cleavagefrst threecell dvivisions
Each cell is known as a blastomere: a precursor cell.
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CLEAVAGE TO IMPLANTATION: Embryogenesis 1
What are the mechanisms involved in embryogenisis:
As discussed before, mitosis is one The processes have inputs from the environment: induction mechanisms: cells that send signals Have cavity formation: to form tubes that is done by the pumping of fluid to form cavities Cell migration: the movement of cells from one part to another Folding: rearrange flat layers into tubes within tubes.
1. CLEAVAGE DIVISIONS produce a ONE LAYERED embryo
The first few mitotic divisions are called cleavage divisions, Takes place in the uterine tube, and within 24 hours the embryo has started to undergo a cleavage
division2- 48. These have taken place without increase in size of the embryo.
Each cleavage cell has occurred to reduce the daughter cell as the original zygote was VERY large This can occur because the daughter cells can initially use the mRNAs, proteins and excess
organelles stored in the oocyte.
Till the 8 cell stage, the cells are loosely arranged, and still encompassed in the zona pellucidiia,which is a gelly layer which surrounds the oocyteand forms with the ooecyte in the ovary
2. CLEAVAGE divisions produce a solid ball of cells called the MORULA
By three days, have a solid ball of cells called a morula that have become compacted and aredeveloped a polarized phenotype
This occurs through cleavage divisions, and enters the uterine cavity during this stage3. a CAVITY forms within the MORULA, transforming it into a BLASTOCYST
NOTE; that two regions are now identifiableupon the formation of the blastocyst The BLASTULA cavity begins to form in the interior at 5 days Contains fluid that is absorbed from the uterine cavity environment that displaces the cells to the
outside The fluid filled cavity is called the blastocoele After the third division, the cells become tightly adhered, and have a polarised phenotype where
the nucleus is basal and the organelles are apicalis important as the polarisation allows the
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development of the next stage: some cells become the embryo blast (body of the individuall) and
others are the tropoblast (which forms placenta)
An outer tropoblast (forms the embryonic part of the placenta Forms an inner cell mass (ICM) which forms the body of the embryo, (cells from this area) Still has the zona pellucida, and during this time, the mitotic divisions continue until the daughter
cell is about the same size as normal cells - > and from now the embryo starts to grow in size
4. IMPLANTATION
The next stagethe blastocyst implants into the uterus Before it does this, it needs to hatch from the zona pellucida (that surrounds the blastocyst,).
WHY? because the tropoblast must interact with the endometriumit exposes the tropoblast to the
endometrial lining of the uterine cavity
The embryo secretes a product that bores a hole in the zona pellucida and the embryo squeezes out(5 days)
Implants in the posterior uterine wall (or in the fundus); Starts implanting at about day 6
The chorion: originates from the combination of the tropoblast and ICM Amnion : from the inner cell mass Placenta: from the tropoblast. Day 6the blastocyst is making attachment of the endometrium, the tropoblast starts to differentiate
into two layers. The outer layer looses the cell membranes and becomes a mss of cytoplasm with
nuclei, called the syncytiotropoblast. Is derived from the underlying layer of the tropoblast now called
the cytotropoblast: as the cells here remain as cells and dont melt
The syncytiotropoblast secrete enzymes that erode way the endometrium and glandsthe glandscontain uterine milknourishment
Also signals the mother that maintain structure of the endometrium and stop menstrual bleed The endometrial layer increases in thickness in preparation for implantation each month The image has been rotated 90 degrees
clockwise =>
At the same time that the tropoblast hasdifferentiated, the ICM is also starting to
differentiate (discuss later)
By day 9spaces begin to form in thesyn..tropoblast, called tropoblastic lacuna and
they are filled with uterine milk- which
diffuses to nourish the embryo
By day 10, implantation is complete,endometrium grows over the site
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Topic 2: Embryonic and Extra embryonic development Epiblast formation the embryonic disc
Gastrulation
Notochord development
Embryonic development: formation of the TWO layered embryo from the completion of implant
FROM DAY 8: at the same time, there is differentiation of the ICM. Differentiation (two layers) termed a bilaminar embryo Lower layer: hypoblast lines the blastocoele cavity Upper layer called the epiblast Fluid filled space fills the epiblast called the amniotic cavitymore and more fluida layer of
epiblast cells is displaced and form the amniotic membrane. (NOTE: the fetus is actually enclosed in
the amniotic membrane) and the AM grows
The embryonic disk is bilaminarepiblast is continuous with the amniotic membrane Hypoblast migrates around the inside of the blastocoele cavity and forms the primary yolk sack. Cells derived from the yolk sac form the primordial germ cellsand also form the blood cells.END result? The dorsal central axis is established
Extra embryonic development
Note: that implantation is actually completed during this period: the endometrium totallycovers the embryo at the end of the second week
Simultaneously another layer also forms at 12 days that is CRUCIAL for subsequent development Called the extra embryonic mesoderm (whose origin is unknown). It is outside the embryo. Migrates around and fills the space between the cytotropoblast and the yolk sac, and also eventually
covers the amnion also
Spaces form in the extra embryonic mesodermforming a large cavity (extra embryonic caelom).This is also known as the chorionic cavity. IT GROWS VERY LARGE- as will be seen later
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SUMMARY: By the end of the second week, we have the bilaminar embryo with the yolk sac below and
amnion below suspended in the chorionic cavity with a thick stalk of embryonic mesoderm called the
connecting stalk
Topic 3: Gastrulation
Change from a Bilaminar to a trilaminar embryo
Change the point of view (from above embryo)
Are looking at the dorsal view of an embryo By the second week, a depression /groove formsalong the longitudinal midline of the embryo
called primitive pit
A mound forms at one endcalled the primitivenode
Together, these are known as the primitive streak Gives the embryo bilateral symmetry: Why is
this important? Tissue on the right side of the PS
gives rise to the right side of the body and vice
versa.
Also have cranial and caudal differentiation: theends
The primitive node points towards the cranialend and below is caudal.
Also has dorsal and ventral (dorsal is the image) Epiblast cells are adjacent to the primitive streak start to proliferate (towards and then through the
primitive groove) and fill up the space between the epiblast and the hypoblast. (migrates ventrally
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.
The epiblast cells are migratingthrought he primitive streak, and
initially they replace the hypoblast
which is displaced laterally and lining the yolk sack. Forms then endotherm
Then, epiblast cells now fill in the space between the newly formed endoderm and the epiblast and iscalled the mesoderm
The bilaminar embryo becomes trilaminar (during the third week) The non migrating epiblast becomes known as the ectoderm SUMMARY: first waveendoderm second wavemesoderm and non migrating called ectoderm
form the four primary tissue types that make up the body.
Notochord formation:
Same view as before (looking at the dorsal view) from theamnion
Looking at the wave of migration of mesoderm As the epiblast cells move down, the mesoderm migrates
cranially and laterallyforming different structures.
The epiblast that migrates down the primitive pit and forms themesoderm migrates cranially along the central axis of the
embryo. To form the notochord!
Now looking at a sagital section of the embryoThe migrating epiblast cells that form the notochord join up with
the underlying mesoderm and eventually detach and form a rod
a transverse section (cranial end) the notochord has alreadyformed
At the caudal end has not already forms Therefore is growing in length by adding from the cranial
caudal sequence.
NOTOCORD? Induces changes in the other tissues Induces the ectoderm over it to form nervous tissue. It
also induces the mesoderm adjacent to it form somites
which then form the vertebral column, skeletal muscles
and the CT of the skin
FAILURE to do so results in spina bifidathe vertebralcoloum does not fuse in the lumbar region
BUCCOPHARYNGEAL AND CLACAL MEMBRANES
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the migration of the mesoderm fills up the space between the ectoderm and mesoderm EXCEPT thecranial end (depression where the ectoderm endoderm are adhered). This is called the BCmembrane
(forms the mouth)
At the caudal end, the endoderm and the ectoderm are also adhered, and form the cloacal membrane(anal/genital/urinary opening)
Embryology Topic 4: germ layer and folding Neural tube formation and its derivatives Development of the mesoderm including
Somites formation and body cavity
Formation
Embryonic folding and formation of gut
Tube
Process of folding causes some of the endoderm to be incorporated into the body of the embryo to form
some of the gut
Formation of the neural tube: fate of ectoderm
This is a transverse section, looking at the dorsalview
The notochord sends signals to the overlyingectoderm to thicken and form neural plate
particularly in the cranial region
AS the ectoderm thickens, it goes into wavesfolds and between the two folds is a depression
called the neural groove
At the top of the crest are neural crest cells As the folds form, they eventually start to fuse in
the middle in the middle forming a TUBE called the
neural tube which is the bases of the CNS
The tube closes up cranially and then caudally lastto form a complete tube
Along the tips of the folds are neural crest cells:which separate once the tube is formed and form aflattened mass (as you can see =>) over the top of
the tube to form the PNS, parts of the face and jaw
The neural tube forms the CNS => the brain and the spinal cord. The neural crest cell separates leaving the surface ectoderm fusing and forming the epidermis layer of
the cell
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Neural tube- different view
We are looking at a dorsal view (directly). Occurs late week 3 Have neural plate formationgrow into wavesneural folds fuse together proceed cranially andcaudallyand two opening remaincranial and caudal neuroporespina bifida and exencephaly
A saggital cross
Note: the mesoderm is starting to form block like structuressomites !!! (later) While the ectoderm is forming the neural tubethis occurs at the same time NOTE: discussed separately. End of week 4 / beginning of week 5 Notice: that the cranial regions is growing faster than
he caudal region (the neuralectoderm) causes the
embryo to do a headtail fold. The neuroectocderm
(neural tube in the cranial region is a lot more thick as
you can notice).
As a result, some of the endoderm that is lining the yolksac is incorporated into the body of the embryo (more
later) and forms the gut tube
Mesoderm development
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Taking place at the same time as the neural tube As soon as the mesoderm forms, it starts to differentiate into block and sheet line structure Differentiates into Block like structures: somitesform in a cranial and caudal sequence (cranially
and caudally). \
Can date an embryo by number of somites present Next to (paraxial) the notochordhave the paraxial mesodermforms somtes The intermediate mesodermnext to somitesforms urinary and gentital system Lateral plate mesoderm give srise to most of smooth muscle, cist CT and the cardiovascular system
(the cranial to the oral membrane). The heart starts of in front of your mouthfolding moves heart
SOMITES
AS the somites start to form , they differentiate into three regions, the sclerotome, myotome,dermatome.
Sclerotome (hard): forms the vertebrae and the ribs It migrates and surrounds the neural tube, thusforming the vertebrae. The vertebrae are segmented - note that the somites are forming as blocks.CONSERVED through evolutionribs also segmented
Myotome (muscle) forms the skeletal muscles of the bodythe mass of cells migrate around ...forming the body wall. Againthese are segmented
Dermatome: forming the dermis (CT) of the skin. The cells of the dermatome migrate aroundinnervating the various parts of the skinforming a segmented pattern downwards in stripes.
NOTE: that the nerves from the neural tube follow the segmented pattern as well
Lateral plate mesoderm: The mesoderm that is lateral to the somites
- Towards the end of the week 3start to see fluid filled spaces within the lateral plate mesodermwhich eventually fuse to form a horseshoe shaped cavity that runs around the embryoits called
the intra embryonic coelom and this gives rise to three cavitiesperitoneal, pleural and pericardial
cavities.
- The IEC (intra embseolom) separates the mesoderm now into to two layersthe visceral layer andthe parietal layer
- The visceral layer with the underlying endoderm form the organs that make up the gut and the resptract
- The parietal layer and the overlying ectoderm form the body wall
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- When it folds, it folds towards the ventral surface so the visceral layer is going to be on theoutside, and the parietal layer will be on outside of the inside cavity
- CONTEXT => in the adult: the visceral peritoneum lines the organs- parietal lines the walls.
- EMBRYONIC FOLDINGThe rapid development of the mesoderm and the ectoderm forming the neural rube causes theembryo to foldfolds towards the ventral surface
- There is also a lateral foldby the end of the fourth week.- Both take place at the same side
HEAD TAIL FOLDING (with the pictures)
- Is best viewed in a saggital section =>- Note that the heart bulge is in front of the
buccopharyngeal membrane (mouth)
- The rapid growth of the ectoderm growingespecial rapidly at the ectoderm to form theneural tube causes a head and tail fold. FOLDS
VENTRALLY
- AS it folds, part of the endoderm that is on theventral surface (lines the yolk sac) is
incorporated to the inside of the embryo
forming the gut tube
- Has a foregut (bounded by the BC membrane),and the hindgut bounded by cloacal membrane
and a midgut still continuous with the yolk sac.
The aural and cloacal membrane are still fused
but in week 5 they open and there is a
continuous tube from the BC membrane/
amnion to the cloacal membraneneural canal
- With the head tail folding, the heart is caudal tothe mouth
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Lateral folding
- This is best viewed from a transverse cross section- The edges of all three germ layers grow rapidly in a ventral direction causing the embryo to do a
lateral fold towards the ventral surface.
- AS a result, part of the endoderm lining the yolk sac is incorporated inside the embryo to form thegut tube and runs the length of the embryo
- The endoderm forms the LINING of the digestive system (epithelium). AND the epitheliumcomponents of the accessory organs. Remember, that the organs grow off the Gut. The CT is
formed from the mesoderm.
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- The IM celom forms a cavity- is the peritoneal cavity in the diagram. The body wall is the parietalperitoneum and the visceria. Peritoneum is on the side of the