Gastrulation

Question 1

During week 2, how do the cells of the blastocyst further differentiate?

Answer 1

Trophoblast:

• cytotrophoblast - an inner layer of mononucleated cells
• syncitiotrophoblast - an outer layer of multinucleated cells that lack distinct cell boundaries

Embryoblast:

• Hypoblast - small cuboidal cells adjacent to the blastocyst cavity
• Epiblast - high columnar cells adjacent to the amniotic cavity

Question 2

After further differentiation of the trophoblast and embryoblast, what structure is formed?

What cavity appears?

Answer 2

the flat bilaminar disc is formed from the hypoblast and epiblast

• at the same time that this disc is formed, the amniotic cavity appears within the epiblast

Question 3

What are the 3 key roles of the syncytiotrophoblast?

Answer 3

• immune protection
• production of hCG
• formation of the placenta

Question 4

What is meant by trilaminar disc formation?

Which cell type is responsible for this?

Answer 4

gastrulation involves the formation of a trilaminar disc which contains the 3 primary germ cell layers

• the epiblast will give rise to:
  
  • ectoderm
  • mesoderm
  • endoderm

Question 5

What is meant by gastrulation and when does it occur?

What other systems start to develop around the same time?

Answer 5

• gastrulation occurs during week 3
• it involves establishing 3 primary germ cell layers from which all body systems will develop
• the vascular system and neural plate / neural tube also start to develop during week 3

Question 6

What else does gastrulation allow the establishment of?

Answer 6

• gastrulation allows the establishment of the 3 body axes:
• cranial-caudal axis
• right-left axis
• dorsal-ventral axis

Question 7

What can cause disruption to gastrulation and what can this lead to?

Answer 7

• aberrant gastrulation can be caused by genetic mutations / abnormalities or maternal toxins
• at the start of gastrulation, epiblast cells are “fate-mapped”
  
  • the cells have a set destination as to what type of cell they will differentiate into
• gastrulation is highly sensitive to insults, which can cause catastrophic coongenital abnormalities as body axes are being established during this process

Question 8

What is the major change that takes place in the embryo between the 2nd and 3rd weeks?

Answer 8

During week 2:

• the ICM is separated into 2 layers - the epiblast and hypoblast
• these 2 layers together make up the bilaminar disc

During week 3:

• the bilaminar disc becomes a 3-layered trilaminar disc through the process of gastrulation

Question 9

What are the 3 layers of the trilaminar disc?

Answer 9

• ectoderm
• mesoderm
• endoderm
• these 3 layers will give rise to all the tissues of the embryo

Question 10

What structure marks the beginning of gastrulation?

Why is this important in laterality?

Answer 10

• gastrulation begins with the formation of the primitive streak on the surface of the epiblast
• this consists of the primitive pit, node and groove
• the primitive node is important in laterality as it contains cilia cells that sweep cells in certain directions

Question 11

How does formation of the primitive streak establish body axes?

What happens following its formation?

Answer 11

• formation of the streak divides the embryo into right and left sides and establishes this body axis
• the epiblast cells detach and migrate through the primitive streak
• the first layer of epiblast cells to invaginate into the streak settle to form the endoderm
• the second wave of migrating cells form the mesoderm
• the remaining epiblast cells do not migrate and form the ectoderm

Question 12

Which signalling factors are important in formation of the trilaminar disc?

Answer 12

• cell migration and specification is controlled by FGF8, synthesised by cells of the primitive streak
• FGF8 controls cell movement by downregulating E-cadherin, which usually binds the epiblast cells together
• FGF8 controls cell specification in the mesoderm by regulating BRACHYURY expression

Question 13

When does the primitive streak regress?

What does it do during this process?

Answer 13

• it is a transient structure that begins to regress after the epiblast cells have migrated through
• it lays down cells beneath it as it regresses
• the cells located most dorsally will form the notochord

Question 14

What are the 3 body axes and when do they start to be established?

Answer 14

1. anteroposterior (craniocaudal)
2. left-right
3. dorsoventral

• establishment of the body axes occurs prior to gastrulation
• the A-P and D-V axes are established before the L-R and probably during the morula stage

Question 15

Which axis is determined during the blastocyst stage?

Which region of the embryo is important in formation of this axis?

Answer 15

anteroposterior (A-P) axis

• cells destined to form the anterior visceral endoderm (AVE) at the cranial end of the endoderm layer of the bilaminar disc migrate towards the head region

Question 16

What genes are expressed by cells of the anterior visceral endoderm (AVE)?

Answer 16

• the cells of the AVE express genes that are essential for head formation, including transcription factors:

1. OTX2
2. LIM1
3. HESX1

Question 17

What secreted factors are produced by cells of the anterior visceral endoderm (AVE) and what is their role?

Answer 17

1. cerberus
2. lefty1

• these factors (both part of the TGF-B family) inhibit nodal activity (TGF-B) in the cranial region of the embryo

Question 18

How is the dorsal end of the embryo established during the blastocyst stage?

Answer 18

• the absence of cerberus and lefty1 at the caudal end allows nodal expression to continue
• nodal establishes and maintains the primitive streak
• once the streak has formed, nodal upregulates a number of genes responsible for formation of dorsal and ventral mesoderm and head / tail structures

Question 19

What is the role of HNF-3B?

Answer 19

• it maintains the primitive node
• later it induces regional specificity in the forebrain and midbrain regions
• without HNF-3B, embryos fail to gastrulate properly and lack forebrain and midbrain structures

Question 20

Where is BRACHYURY expressed?

What are its 2 main roles?

Answer 20

• expressed in the primitive node and notochord
• it controls formation of dorsal mesoderm in the middle and caudal regions of the embryo
• it is responsible for cell migration through the primitive streak

Question 21

What can the absence of BRACHYURY result in?

Answer 21

caudal dysgenesis

• as BRACHYURY is important for mesoderm formation in the middle and caudal regions of the embryo, its absence results in shortening of the embryo

Question 22

What other 3 factors rely on BRACHYURY expression?

Answer 22

• nodal
• lefty1
• lefty2

Question 23

What is secreted by the primitive streak that is involved in establishing laterality (left-right sidedness)?

Answer 23

• when the streak appears, cells in the streak and node secrete FGF8
• FGF8 induces expression of nodal
• nodal expression is restricted to the left side by the accumulation of serotonin (5-HT) on the left side

Question 24

What is the result of high concentrations of serotonin (5-HT) on the left?

Answer 24

• high concentrations of 5-HT activate the transcription factor MAD3
• MAD3 restricts nodal expression to the left side of the primitive streak

Question 25

Why is serotonin concentrated on the left side of the primitive streak? What happens if this balance is disturbed?

Answer 25

* it is concentrated on the left side as it is **broken down by _monoamine oxidase (MAO)_** on the **right side** * abnormal / disturbed serotonin signalling can lead to laterality defects

Question 26

What 3 midline genes are responsible for establishing laterality?

Answer 26

* *SONIC HEDGEHOG (SHH)* * *LEFTY1* * *ZIC3* * these are all involved in **establishing the midline** as well as _**preventing *NODAL* expression from crossing over**_ to the right side

Question 27

What is the ultimate result of nodal protein being present in the left lateral plate mesoderm?

Answer 27

* nodal protein initiates a **signalling cascade** that includes activation of ***_LEFTY2_*** * LEFTY2 then **upregulates / induces *_PITX2_*** * PITX2 is a homeobox-containing transcription factor that acts as a **"master gene"** responsible for **_establishing left-sidedness_** * PITX2 is expressed on the **left side of the heart, stomach and gut primordia** as they assume their normal asymmetric body positions

Question 28

What can happen if there is ectopic expression of *PITX2*?

Answer 28

**_laterality defects_** * if it is expressed on the right side, this results in conditions such as situs inversus and dextrocardia

Question 29

Whilst genes regulating right-sided development are less well-defined, which one is known to be involved?

Answer 29

***_SNAIL_*** * this is a transcription factor that is restricted to the **right lateral plate mesoderm** * it is thought to regulate effector genes responsible for establishing the right side

Question 30

What is caudal dygenesis and why does it occur?

Answer 30

* also known as **sirenomelia** or ***"mermaid syndrome"*** * **_insufficient mesoderm_** is formed in the **_caudalmost region_** of the embryo * mesoderm contributes to formation of the **lower limbs**, **urogenital system** and **lumbosacral vertebrae**, so there are abnormalities in these structures * a single fused lower limb is seen with an absent genitourinary system

Question 31

Why does caudal dysgenesis occur? What signalling factors could potentially be involved?

Answer 31

* abnormal cell migration occurs due to: 1. **premature regression** of the primitive streak 2. **insufficient migration** of epiblast cells the germ layers are formed in a **cranial-caudal fashion**, so the head end is formed but the tail end is not * ***_NODAL_*** and ***_BRACHYURY_*** are important in **maintaining the streak**, so it is likely to be a problem with one of these signalling factors

Question 32

What is the treatment for sirenomelia?

Answer 32

* it is difficult to treat as you are trying to create systems / limbs out of tissue that has failed to form correctly

Question 33

What is holoprosencephaly (HPE)? How does a child with this condition present?

Answer 33

* HPE results from disruption and **_death/loss of cells in the midline_** during gastrulation * the **forebrain is small** and the **2 lateral ventricles often fuse** to form a single ventricle * the **eyes are close together** or **fused** (hypotelorism) * it affects the **_cranial region_** - tissue has formed but there has been **excessive apoptosis** of cells in the **midline**

Question 34

What toxin is known to be associated with HPE? What signalling factors could potentially be involved?

Answer 34

* high doses of **_alcohol_** during gastrulation (start of 3rd week) can disrupt the process and kill cells in the **anterior midline of the trilaminar disc** * this produces a deficiency of the **midline in craniofacial structures** * **_SHH and HNF-3B_** may be involved, but signalling factors are not necessarily involved

Question 35

How can a teratoma result from abnormal gastrulation?

Answer 35

* **remnants of the primitive streak** **persist** in the sacrococcygeal region * there is **_continuous migration of epiblast cells_** through the streak * the epiblast cells are **pluripotent** so will proliferate and differentiate into structures formed from **_all 3 germ layers_** * teratomas also formed by PGCs migrating to extragonadal sites

Question 36

What is situs inversus? How might someone present and what are the associated risks?

Answer 36

a condition where the **positioning of _all_ organs** is **_reversed_** in a mirror image arrangement * individuals with this condition are not at high risk for other congenital abnormalities * their progeny is at higher risk of having a **laterality defect** and an even higher risk of a **cardiac malformation** * 20% of patients have **_Kartagener syndrome_** (bronchiectasis and chronic sinusitis due to **abnormal cilia**)

Question 37

What is situs ambiguous? Why does it occur?

Answer 37

a condition in which **one or more organs are _abnormally reversed_** in position * it is a laterality defect that results from **failure to establish the L-R axis**

Question 38

How can you tell if someone with situs ambiguous is predominantly right or left-sided?

Answer 38

* those with **left-sided bilaterality** have **_polysplenia_** * those with **right-sided bilaterality** have **_asplenia**_ or a _**hypoplastic spleen_**

Question 39

What risks are associated with having situs ambiguous?

Answer 39

* individuals are at a higher risk of having other birth defects, such as: * midline malformations * neural tube defects * cleft palate * anal atresia * 90% will have a congenital heart defect

Question 40

Why are SSRIs sometimes contraindicated in pregnancy?

Answer 40

* serotonin (5-HT) is important in **_establishing laterality_** * disrupting 5-HT signalling can result in **situs inversus, dextrocardia, situs ambiguous** and many **heart defects** * there is an increased risk of heart defects and other congenital malformations if SSRIs are taken during pregnancy