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Study Questions 2 Flashcards
cleavage
formation of blastula, rapid cell division, site of cleavage furrow
blastomere
Cells resulting from cleavage, have M and S cycles
blastula
when factors for cleavage are used up, goes into mid-blastula transition, ends in 128 cells with a cavity in the center
blastulation
the process of creating the cavity (blastocoele) in the blastula
mesolecithal
moderate amount of yolk (holoblastic)
centrolecithal
yolk in the center (Meroblastic)
isolecithal
uniform yolk distribution (holoblastic)
telolecithal
yolk at one end (meroblastic)
holoblastic cleavage
formation of complete and separate cells (not much yolk)
meroblastic cleavage
incomplete separation of cells (massive yolk prevents full cleavage)
superficial cleavage
cleavage occurs on the outside of the cell
discoidal cleavage
the top of the embryo is cleaved only
radial cleavage
equal cleavage
spiral cleavage
cleaved so that the top have bigger cells and bottom smaller
bilateral cleavage
bigger cells in the middle and smaller surrounding it
rotational cleavage
one bigger cell with smaller polar bodies surrounding it
determinate cleavage
irreversibly separates portions of the zygote with specific potencies for further development
indeterminate cleavage
cleavage in which all the early divisions produce blastomeres with the potencies of the entire zygote
blastocoele
the cavity in a blastula
direct development
type of development where embryo develops into a mature individual without a larval stage
indirect development
first there is a sexually immature larval stage which has different nutrient requirements from the adults
protostomes
develop mouth first from the blastopore, spiral cleavage and determinate development
deuterostomes
blastopore becomes anus, radial cleavage and indeterminate development
epiboly
movement of epithelial sheets that spread to enclose deeper layers of the embryo, by division, change in shape, or by several layers intercalating
involution
inward movement of expanding outer layer over internal side of remaining external cells
invagination
in-folding region of cells
Ingression
migration of individual cells from surface into interior of embryo
ectoderm
outer layer, body covering and nervous system
mesoderm
middle, skeleton and muscles, reproductive
endoderm
inner, digestive organs and intestines, respiratory
delamination
splitting of one cellular sheet into two
cell migration
happens during gastrulation
archenteron
, the invagination in a blastocoel, can extend through convergent or intercalate
primitive gut
initial invagination in the blastocoel
Is protein synthesis essential during early cleavage? Is mRNA synthesis essential during early cleavage?
mRNA synthesis is not necessary during early cleavage because of the already available and mRNA inthe oocyte that controls the initial rate of cell division and the placement of the blastomeres. The maternal mRNA is used until the mid-blastula transition when the mitosis promoting factor has beenall used up and then the zygotic genome is used.Protein Synthesis IS required, at minimum to produce cyclin B for Mitosis-Promoting Factor (MPF)
Actinomycin D does not arrest early sea urchin cleavage but blocks gastrulation. What conclusions can you draw from this observation?
- This shows that the maternal mRNA is used for translation. Actinomycin is an inhibitor
- Gastrulation is blocked hence the epithelial cells cant be differentiated, since maternal mrna is used up
What would you consider to be two most important things that an embryo accomplishes during the process of cleavage and blastulation?
The cell fate and the major embryo’s axis would be the most important things
How does the cell cycle which occurs during cleavage differ from the cell cycle of normal adult body cells?
a. the cell divisions don’t create a growth in cell, just increase in number
b. blastomeres have cell cycles with two steps M and S
c. when the cyclin and mitosis promoting factors are used up the cell reaches the maternal to zygotic transition
- >here the embryonic genome starts to get utilized
- Given that there is no transcriptional activity in the early stages after fertilization, how can a rise in egg protein levels be explained?
a. By the MBT as it uses the maternal mRNA
i. Valine becomes newly synthesized proteins
- When does the embryonic genome get utilized? Briefly describe the events that lead to this.
a. Cleavage is driven by Mitosis-Promoting Factor (MPF)
b. MPF has two subunits, CyclinB and cdc2
c. CyclinB up-regulates cdc2, which is protein kinase
d. cdc2 phosphorylates several factors (histones etc.), inducing mitosis chromatin condensation, organization of mitotic spindle etc.
e. CyclinB is regulated by factors in the cytoplasm (which cytoplasm?)
f. When the above factors are used up, the embryo has reached the Mid-Blastula Transition (MBT), also called maternal to zygotic transition (MZT) – embryonic genome starts to get utilized
- What are the major factors that influence the pattern of cleavage?
a. The amount and distribution of yolk affects where cleavage occurs
b. Factors in the egg cytoplasm influence angle of mitotic spindle
- Distinguish between complete (holoblastic) cleavage and incomplete (meroblastic) cleavage.
holoblastic - formation of complete and separate cells (not much yolk) uniform (isolecithal) or moderate yolk (mesolecithal)
b. meroblastic - incomplete separation of cells (massive yolk prevents full cleavage) yolk at one end (Telolecithal) or center (centrolecithal)
- Distinguish between determinate and indeterminate cleavage.
a. determinate cleavage, irreversibly separates portions of the zygote with specific potencies for further development
b. indeterminate cleavage, cleavage in which all the early divisions produce blastomeres with the potencies of the entire zygote
- Why does the terrestrial environment present more difficulties for embryonic
development than the fresh water environment, and why does the fresh water environment in turn present more difficulties for development than the marine environment?
- Terrestrial has more predators than freshwater
* Marine has more solubility and nutrients available than fresh water
- What major embryonic and reproductive adaptations are seen in terrestrial and fresh water animals for dealing with these environments and helping to ensure survival of the species?
a. Their eggs have egg protection
i. Fresh water: retained by parent or gets attached to substrate
ii. Terrestrial: in a shell, moist environment or carried internally
- What are the major developmental features which allow the metazoans to be classified into two groups, the protostomes and the deuterostomes?
a. Whether the mouth or anus was developed first from the blastopore
b. Protostomes: majority of coelomate invertebrates
i. Blastopore develops into mouth
ii. Spiral cleavage
iii. Determinate development: the fate each cell will have in the adult organism has already been determined the early blastula (8-cell stage)
c. Deuterostomes: Echinodermata and the ancestors of the Chordata
i. Blastopore becomes the anus
ii. Radial cleavage
iii. Indeterminate development: each of the cells of the early blastula if separated remains capable of developing as complete organisms (cells are totipotent)
- Which germ layer(s) form pseudocoelom/coelom in pseudocoelomates vs. coelomates?
a. Pseudocoelom: between mesoderm and endoderm
b. Coelom: entirely within the mesoderm
- What determines cell fate?
a. Asymmetric cell divisions
b. Cell-to-cell interactions
c. Cell-cell communication(s)
d. Position of the cell in an embryo
- Explain the role of asymmetric cell divisions (or cell-cell communication(s) or position of the cell in an embryo) in determination of the cell fate
a. Asymmetric: create cells with different cytoplasms as they are more concentrated in one area and are divided where one cell has an abundance and other doesn’t
i. could be bound to egg cytoskeletal elements only at one side of the mother cell two different daughter cells’ shapes
ii. could be transported along the cytoskeleton and end up only in one daughter cell different metabolic activity and/or motility between daughter cells
iii. could be associated with the centrosome they will influence mitotic spindle formation
b. Symmetric (cell-cell interaction and communication)
i. Interactions (between neighbouring cells)
1. tight junctions – create sheets of cells
2. anchoring junctions – connect the cytoskeletons of adjacent cells, or connect cells to matrix /basal lamina
3. communicating junctions – permit small molecules to pass between cells
a. gap junctions – in animal cells
b. plasmodesmata – in plant cells
ii. Communications
1. Cells reciving signals from eachother
2. Induction – interaction (mostly at close range but not always) between two or more cells or tissues with different histories and properties.
a. Inducer – cell that produces/sends a signal (morphogen) that will change cellular behavior of the
b. Responder – cell being induced; the target cell
3. Target cell must be competent i.e. capable of responding (has to have a receptor for a morphogen produced by inducer)
- Competence – the ability of a cell or tissue to respond to a specific inductive signal. It can change with time.
- Even if competent the responding cell wont develop in the way needed if its not induced
- Juxtacrine – cell membrane proteins on the surface of one cell are recognized by receptors on the adjacent cell – direct contact
- Paracrine - morphogen released from a cell has an effect on neighboring cells
- Endocrine - hormones (= morphogen) released from a cell affect other cells throughout the body
- (Autocrine - cell secretes paracrine factor = morphogen and responds to it)
iii. Position of cell in embryo - Different fate depending on how much morphogenes they get
- Define induction and competence.
- Competence – the ability of a cell or tissue to respond to a specific inductive signal. It can change with time.
- Induction – interaction (mostly at close range but not always) between two or more cells or tissues with different histories and properties.
- Define instructive and permissive effects.
- Instructive
- A signal from the inducing cell is necessary for initiating new gene expression in the responding cell:
- Permissive
- The responding cell has already been specified; needs only to be in right environment that allows the expression of those traits (so either cell is moving, or environment is changing, or both)
- What is a morphogen? What are the types of cell-cell communications in respect to morphogen production and distribution?
a. Morphogen (“form-giver”) is a diffusible signaling molecule (= ligand; ion, hormone, protein: transcription/translation factor or paracrine factor), produced by the inducer cell.
b. Gives the positional information and the pattern formation
i. Through quantitative (amount of morphogens whiech regulated cell proliferation and differentiation) and directional (gradient of morphogens establishes cell polarity)
- How do morphogen gradients pattern developing tissues? (You can use diagram to aid your explanation.)
a. Gives the positional information and the pattern formation
i. Through quantitative (amount of morphogens whiech regulated cell proliferation and differentiation) and directional (gradient of morphogens establishes cell polarity)
- Answer briefly the following three connected questions: a) What is signal transduction? b) What is the most common outcome of signal transduction? c) What is the relationship between signal transduction and cell specification?
a. Signal transduction: the events within the cell that occur in response to a signal, in/activate gene expression
b. Almost always lead to chang in cells gene expression
c. Different cell types can respond differently to the same signal
- List the (3) modes of cell type specification and give the two most important characteristics we discussed in class. (Can you give examples?)
a. Autonomous
i. Found in majority of invertebrates
ii. Cell specification happens before major cell migrations (which are happening during gastrulation)
iii. Cell specification is due to the asymmetric cell division; cell acquired certain unique maternal cytoplasmic morphogenetic determinants
iv. Invariant cell divisions
v. If a blastomere is removed a part of the body (which would originate from that blastomere) will not develop as other blastomeres cannot replace the lost one
b. Conditional
i. -Cell specification happens during or after major cell migrations
ii. -Cell specification is due to interactions between cells, so cell-cell communications and the actual position of a cell are important
iii. Cleavages are variable
iv. All vertebrates and some invertibrates
c. Syncytial
i. Interactions are between one cell itself (insects)
ii. Many nuclei in one cell
iii. During the first few cleavages karyokinesis happens without cytokinesis, forming one huge multinucleate cell - syncytium
iv. Cytoplasmic regions can easily interact (there are no membranes to stall or prevent flow of cytoplasmic determinants); interactions lead to the specification of body regions (NOT to cells’ specifications at this early stage).
v. Cell membranes form in later blastula stages (= cellularization of the blastoderm)
vi. After cellularization some cells will have autonomous and some conditional specification (depending on a position of the nucleus at the cellularization time, when cell membranes form around it)
vii.
d. Two characateristics:
i. Not influenced by neighbouring cells, fate is determined (Sea Urchin)
ii. Not irreversibly committed, can change its fate depending on the postion of the cell in the embryo
- Define the conditional specification and give an example from the nematode [worm] C. elegans. (or a sea urchin)
• Conditional specification:
o Cell’s fate depends on its position in the embryo as the neighboring cells produce signals which will determine this fate
o Animal cells themselves (even though they are competent) will not be able to produce larva
o When large micromeres are grafted to animal cap, they induce animal cap cells to generate endoderm (produce paracrine and juxtacrine factors which will specify neighboring cells)
- Define autonomous specification of cells and give an example from the sea urchin (or a nematode).
a. Large micromeres are formed– they are the only cells in Sea Urchin whose fate is determined autonomously (are not influenced by neighboring cells) – they have autonomous specification
b. Large micromeres can (and do) influence other cells
- Describe the mechanism by which the primary mesenchyme cells detach from the other vegetal pole cells and enter the blastocoel. What is the name of this cell movement?
a. After hatching from the fertilization envelope, vegetal side of blastula is thick and and flat
b. At the center of this “vegetal plate” a cluster of large micromeres extend filopodia from inner surfaces
c. These cells dissociate from the monolayer and migrate into the blastocoel (ingression; it leads to epithelial to mesenchymal cell transition, EMT)
d. These primary mesenchyme cells form the larval skeleton – mesoderm formation
- Describe the mechanism which accounts for the invagination of the vegetal pole cells during gastrulation in the sea urchin.
a. Mechanism of invagination: vegetal cells secrete chondroitin sulfate proteoglycan (CSPG) into hyaline inner lamina causing it to expand. This change in extracellular matrix changes the shape of cells. They become conical (narrow at basal side), causing invagination
b. First invaginate into the blastocoel forming primitive gut archenteron ⇒endoderm formation; the opening formed is called a blastopore
- Explain and give specific examples of how changes in cell adhesion and changes in the migratory properties of cells are important during early embryonic development. (This is a general question – keep it in mind. At this point you could talk about primary mesenchimal cells; there will be more examples as we progress though the course material.)
- Each movement changes the fate hence changing the cell
- Wont be able to form larva
- Inducle animal cap cells which generate endoderm
- Change in where the invagination occurs – primary mesenchymal cells
- The Archenteron is formed from the vegetal plate in sea urchin gastrulation. Describe the mechanisms of its formation with specific reference to functions and cellular movements of primary and secondary mesenchyme cells.
a. Mechanism of invagination: vegetal cells secrete chondroitin sulfate proteoglycan (CSPG) into hyaline inner lamina causing it to expand. This change in extracellular matrix changes the shape of cells. They become conical (narrow at basal side), causing invagination
b. First invaginate into the blastocoel forming primitive gut archenteron ⇒endoderm formation; the opening formed is called a blastopore
c. Second and third stages of archenteron invagination - archenteron extending dramatically
d. Up to three fold increase in length, short gut becomes long thin tube
e. Extension via convergent extension, cells migrate over one another, intercalate and flatten
f. Secondary mesenchyme cells (non-skeletogenic!) extend filopodia pulling up archenteron.
- Every organism on Earth has evolved to have the ability to differentiate germ cells during its early development to give rise to future progeny of that particular species. Describe how C. elegans manages this task during its early development. What cytoplasmic factor plays a role in specifying the germs cells and how is it localized in germ cells? (Keep in mind this question; we will have a lecture about germ cells at the end of the term.)
a. Through the morphogens as receptors that are bound will change what is going to be.
- What kind of molecules are P granules? What are their functions and in what organism were they discussed? Where are they found initially in the egg and where after fertilization and how does that occur?
a. In C. Elegans
b. RNA granules
P-granules are ribonucleoprotein complexes (protein part -mostly translation factors) that are localized randomly in the cytoplasm.
Shortly after fertilization the P-granules move by the PAR (partitioning) proteins responsible for specification of germ cell, so that they only enter P1 blastomere.
Uniform distribution of the P-granules immediately after fertilization
With the establishment of the posterior (through sperm entry) and the formation of mitotic spindle, posterior PAR proteins start aggregating granules in the posterior (condensation).
In the anterior, anterior PAR proteins dissolute P-granules. Dissoluted granules can be moved really fast towards posterior
Outcome: only posterior daughter cell inherits P-granules
