Developmental Biology

Question 1

How do cleavage divisions set up assymetries in a embryo?

Answer 1

Cleavage divisions can set up asymmetries by segregating determinants –
- Polarised parental cell
- Assymetric localisation of cell-fate determinants (protein or mRNA)
- Unequal daughter cells

Question 2

How does development of an embryo involve emergence of pattern? What are the 2 sides the embryo is split into?

Answer 2

Development involves the emergence of pattern
Pattern formation is the process by which a spatial and temporal pattern of cellular activities is organised within the embryo so that a well ordered structure develops
Germ layers split into dorsal region (north) and ventral region (south)

Question 3

What do the different germ layers eventually develop into?

Answer 3

Endoderm – gut, liver, lungs
Mesoderm – skeleton, muscle, kidney, heart, blood
Ectoderm – epidermis of skin, nervous system

Question 4

What are the 5 key features of morphogenesis?

Answer 4

Development involves morphogenesis – creation of structure and form
- Differential proliferation
- Change in cell shape and size
- Cell movement
- Cell fusion
- Cell death

Question 5

What does grastulation do?

Answer 5

Gastrulation moves the germ layers relative to one another

Question 6

What is growth like in early development?

Answer 6

Development involves growth
- There is little growth during early development, while the basic body plan is being established
- Differential growth rates can result in a change in body proportions (huge freaky head to start off with)

Question 7

What class of stem cell is a zygote? What does this mean?

Answer 7

The zygote is totipotent – it generates all the cell types of the body and extra-embryonic tissues (eg bundle of nerve cells, RBCs, smooth muscle cells, adipose cells)

Question 8

How do embryos progressively commit to cell types? What is differentiation? What are the names of the first 3 forms of the embryo?

Answer 8

Development involves progressive cell commitment
- Early embryonic cells are pluripotent: over time their potential is gradually restricted
- Differentiation is the process of cells becoming structurally and functionally specialised, reflecting activation and maintenance of a particular pattern of gene expression
- Zygote – blastula – gastrula (ecto,meso,endoderm)

Question 9

How does cleavage produce a cluster of blastomeres?

Answer 9

Cleavage produces a cluster of blastomeres
- Zygote (16-20hrs), 2-cell (24hr), 4-cell (45hr), 8-cell (72hr)
- Divisions occur in the absence of growth
- Spherical blastomeres form a loose clump

Question 10

What forms after 3 days after fertilisation and what does it do? What does increased cell-cell adhesion maximise?

Answer 10

The morula forms 3 days after fertilisation and undergoes compaction
- E-Cadherin becomes restricted to regions of intercellular contact
- Increased cell-cell adhesion maximises contact between blastomeres, forming a compact ball of cells held together by tight junctions

Question 11

What does subsequent tangential cleavage produce?

Answer 11

Subsequent tangential cleavages produce one polarised and one non-polarised daughter cell

Question 12

What do the outer cells have? What do the inner cells do?

Answer 12

• The outer cells have distinct apical and basal surfaces. The nonpolarised cells form the inner cell mass. The inner cells communicate extensively through gap junctions.
• Apical (the exterior surface; microvilli)
• Basal or basolateral (the internal surfaces E-cadherin)

Question 13

What do the outer and inner cells give rise to? What is this the first step of?

Answer 13

The outer and inner cells give rise to 3 distinct lineages – trophectoderm and inner cell mass (ICM)
- This segregation is the first step towards differentiation

Question 14

How does a fluid filled cavity develop in a blastocyst?

Answer 14

A fluid filled cavity develops and the embryo is now called a blastocyst
1. The inner cell-mass will give rise to the embryo proper (and some extra-embryonic structures)
2. The zona pellucida prevents implantation in the oviducts
3. Flattened epithelial cells of the trophectoderm will form extra-embryonic tissues
4. Fluid filled blastocyst cavity (or blastocoel)

Question 15

How do the embryo hatch? Describe the early structures of the embryo.

Answer 15

Enzymes digest through the zona pellucida and the embryo hatches
Formation -
1. Zona pellucida surrounding outside
2. Trophoblast on inner side (derived from trophectoderm)
3. Hypoblast (surrounding of the inner cell mass)
4. Blastocyst cavity (inner cell mass)
5. Epiblast (inner cell mass)

Question 16

What does implantation require? What is the inner cell mass called?

Answer 16

Human development from fertilisation to day 6
- Implantation requires interactions between trophoblast integrins and laminin, and fibronectin extracellular matrix proteins expressed by the epithelial cells of the uterine mucosa
- The inner cell mass is now called the embryoblast

Question 17

What are the 2 layers formed by the embryoblast?

Answer 17

The embryoblast forms a flat disc of 2 layers, epiblast and hypoblast, with different fates

Question 18

What is the epiblast?

Answer 18

Epiblast (central)
- Columnar cells adjacent to newly formed amniotic cavity
- Will form the embryo proper

Question 19

What is the hypoblast?

Answer 19

Hypoblast (underneath Epiblast)
- Small cuboidal cells adjacent to the blastocyst cavity
- Will form extra-embryonic structures that will connect to the mothers circulation

Question 20

What is the primitive streak? What will it become? What happens during gastrulation?

Answer 20

The primitive streak forms after 2 weeks –
- The primitive streak forms on the surface of the epiblast in the region that will become the posterior of the embryo
- This is the first sign of the anteroposterior axis
- During gastrulation, Epiblast cells migrate towards the primitive streak and invaginate (move inwards), displacing the hypoblast

Question 21

What are the 3 germ layers created during gastrulation of the epiblast cells?

Answer 21

During gastrulation, Epiblast cells migrate through the primitive streak to create 3 germ layers –
- The first cells to invaginate form the endoderm
- The next cells to invaginate form mesoderm
- The remaining Epiblast cells form ectoderm
The germ layers are morphologically distinct and have different fates, after gastrulation, the flat embryo rapidly folds into a 3 dimensional body

Question 22

What happens during neurulation of the ectoderm?

Answer 22

During neurulation the ectoderm folds along its central axis to form the neural tube
- The neural tube folds elevate and fuse
- Neural crest cells form near the site of fusion and migrate away
- The epidermis fuses above the neural tube

Question 23

What does the paraxial mesoderm segment into? What does this generate?

Answer 23

The paraxial mesoderm segments into somites (day 23 – weird looking alien corn structure, has anterior and posterior neuropore)
- Somites generates trunk and limb muscles, dermis and vertebrae

Question 24

What does the endoderm give rise to? What body features does it provide in later development?

Answer 24

The endoderm is internalised and gives rise to the epithelial linin of the gastrointestional tract
- Endoderm also provides the stomach, liver and pancreas and the epithelial lining of the respiratory tract

Question 25

What process happens during the embryonic period? What could this cause?

Answer 25

During the embryonic period (weeks 3-8) the germ layers give rise to tissues and organ systems (organogenesis) - Stem cell populations establish each of the organ primordia - Organogenesis is sensitive to genetic and environmental influences - This is the period when most structural birth defects are induced - Looks like an alien still, just with a spinal curvature and formation of a head

Question 26

What are the common stages of early animal development?

Answer 26

The development of most animals proceeds through common stages- - Fertilisation - Cleavage to form the bastula - Gastrulation to reorganise the structure of the embryo and generate germ layers - Neurulation - Organogenesis

Question 27

What is meant by the term epigenesis?

Answer 27

Development is progressive: a simple embryo with a few cell types in a basic pattern is gradually refined to generate a complex organism with many cell types showing highly detailed organisation. This process is known as epigenesis.

Question 28

How are different cell types determined?

Answer 28

Differentiated cells are characterised by the proteins they contain

Question 29

What is genomic equivalence? What is it proved by?

Answer 29

Genomic equivalence: somatic cell nuclear transfer (SCNT) shows that’s almost all somatic cells have a compete copy of the genome - A differentiated cells nucleus can direct development of a new individual (if removed and then placed in new cell) proving it had not lost any of its genetic potential as it became specialised

Question 30

How was genomic equivalence tested using frogs?

Answer 30

Wild type female donor of enucleated host eggs + albino parents of tadpole that supplied donor nucleus = results in clone of albino frogs

Question 31

How does cleavage result in differentiation? How does this affect cells developmental fates?

Answer 31

Differential gene expression underlies differentiation - During cleavage, cells become different from one another - Achieved by partitioning cytoplasmic determinants eg transcription factors - Determinants specify body axes and influence gene expression, affecting cells developmental fates - Once initial asymmetries have been established, subsequent interactions between cells influence their fate and bring about differentiation of specialised cell types - Inductions are mediated by diffusible signals or by cell surface interactions

Question 32

What are the model organisms that are suitable for the testing theories about development of embryos?

Answer 32

Model organisms – suitable for experiments that address our questions about embryo development - Mouse - Zebra fish - Fruit fly - Frog - Nematode worm - Chick

Question 33

What is useful about Xenopus eggs?

Answer 33

Xenopus eggs have visible and molecular polarity - Pigmented animal hemisphere, non-pigmented vegetal hemisphere - VegT (T-box transcription factor) mRNA is localised at the vegetal pole

Question 34

What does vegetally localised VegT required for?

Answer 34

Vegetally-localised VegT is required for development of endoderm and mesoderm

Question 35

When VegT binds DNA, what does it activate the expression of?

Answer 35

Vegetally-localised VegT is required for development of endoderm and mesoderm VegT binds DNA and activates the expression of: - transcription factors that mediate endoderm differentiation - Nodal TGF b signalling factors that are secreted and induce responding cells to form mesoderm (induction)

Question 36

What is the evidence for VegTs function?

Answer 36

Proof that a molecule performs a particular function in normal development requires it to be: - Expressed in the right cells at the right time - Necessary for that function to take place - Sufficient for that function to take place

Question 37

Where is the VegT gene usually expressed?

Answer 37

VegT mRNA and protein can be detected in the cells that will give rise to endoderm

Question 38

How is development affected by blocking gene function (loss of function experiment)?

Answer 38

- Removing VegT from the oocyte interferes with both endoderm development and mesoderm introduction - VegT-depleted: Epidermis remains consistent throughout structure

Question 39

What are the consequences of ectopic gene expression (gain of function experiment)?

Answer 39

- VegT expression in the ectoderm results in expression of endoderm-specific genes - Cells in normal animal pole express ectoderm genes only - In VegTmRNA there is ectopic expression of endoderm specific genes in animal pole

Question 40

What does Wnt signalling trigger?

Answer 40

Wnt signalling causes b-catenin to accumulate in the nucleus and activate target gene expression

Question 41

What is the role of GSK-3b in relation to b-catenin?

Answer 41

Wnt signalling causes b-catenin to accumulate in the nucleus and activate target gene expression - GSK-3b destroys b-catenin - If GSK-3b is inactivated, b-catenin can enter the nucleus

Question 42

What does sperm entry and cortical rotation trigger? What does rotation redistribute?

Answer 42

Sperm entry triggers cortical rotation, translocating dorsal determinants that initiate Wrt signalling and establish the dorsoventral axis - Cortical rotation redistributes determinants including Wnt11 and inhibitory GSK-3-binding protein (around outside) to the prospective dorsal side, where they activate the Wnt signalling pathway

Question 43

What does Wnt11 and CSK3-binding protein inhibit? What does this allow?

Answer 43

Wnt11 and CSK3-binding protein inhibits GSK3b allowing b-catenin to be stabilised in dorsal cells - B-catenin is initially present throughout the vegetal region, but targeted for destruction by GSK-3. After cortical rotation, the dorsal determinants activate Wnt signalling ad inhibit GSK-3 in the future dorsal side - B-catenin is stabilised and enters nuclei on the dorsal side. Ventral nuclei do not receive B-catenin

Question 44

What is b-catenin essential for? What does overexpresssion and depletion show?

Answer 44

B-catenin is essential for dorsal development - Over expression and depletion of B-catenin show that it is required for normal development of dorsal axial structures

Question 45

How does b-catenin activate the singalling pathway to express dorsal transcription factors such as goosecoid?

Answer 45

- Nuclear b-catenin in dorsal cells - B-catenin interacts with Tcf3 to activate expression of siasmois - Siasomois (acting synergistically with proteins induced by Nodal signalling) activates the expression of goosecoid

Question 46

What does VegT and b-catenin activate? What does this result in? High/low?

Answer 46

VegT and B-catenin activate Nodal expression, resulting in a nodal gradient across the endoderm. - High nodal plus b-catenin induces dorsal mesoderm (the organiser) - Low nodal induces ventral mesoderm

Question 47

What does the organiser (high nodal plus b-catenin) do?

Answer 47

The organiser secretes signals that pattern the mesoderm and neutralise the dorsal ectoderm

Question 48

What does grafting dorsal organiser cells to the ventral side of a host induce?

Answer 48

Grafting dorsal organiser cells to the ventral side of a host induces a secondary axis complete with head and nervous system - Host cells respond to signals produced by the grafted organiser, patterning the mesoderm and inducing neural tissue - Goosecoid transcription factor activates the expression of genes needed for organiser function

Question 49

What do cells differentiate in response to? How does they respond?

Answer 49

Cells differentiate in response to the signals they receive, patterning individuals tissues and organs - Signal tells cells where they are in time and space (positional information, regional specification) - Cells respond by activating transcription factors that turn target genes on/off - The combination of transcription factor found in cell determines which tissue specific genes its expresses, and thus how it differentiates

Question 50

What do signals surrounding the mesodermal somites do? What does they induce?

Answer 50

Signals from surrounding tissues pattern the mesodermal somites, instructing cells to differentiate into muscle - These signals induce expression of muscle-inducing transcription factors, eg MyoD - Cells differentiate into skeletal muscle

Question 51

How does the MyoD signalling work?

Answer 51

Nucleus (embryonic precursor cell) – Master regulatory gene MyoD (off), other muscle specific genes (off) Signals induce MyoD – myoblast (determined) – mRNA (MyoD protein, transcription factor) - Combination of transcription factors ensures cells express the appropriate tissue-specific genes

Question 52

What does overexpression of MyoD lead to? Why is it important the regulatory proteins are carefully managed?

Answer 52

Overexpression of MyoD can convert non-muscle cells into muscle cells - Fibroblast + MyoD = blastocyst = myotube A single protein can act as a master switch, initiating a complex programme of differentiation. It is therefore important that these regulatory proteins are themselves careful managed. In normal development, combinations of different signals and transciption factors ensure cells differentiate properly

Question 53

What is meant by the term cell lineage?

Answer 53

Cell lineage: the developmental history of a differentiated cell, traced back to the cell from which it arises

Question 54

How is terminal differentiation of a particular cell type induced? What is the fundamental control mechanism of differentiation?

Answer 54

- A series of decisions leads to terminal differentiation of a particular cell type - Cytoplasmic determinants and inductive signalling interactions establish and pattern the germ layers by activating expression of transcription factors - The combination of transcription factors determines how a particular cell differentiates - Transcription is the fundamental level of control of differential gene expression during development