embryoloygy 1st year

8/6/2019 Embryoloygy 1st Year

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Embryology notes Aditya Eranki

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

embryoloygy 1st year

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