1
Q
What are the essential elements of a Morphogen?
A
- The signal (ligand) is soluble.
- Cells respond directly to factor via specfic receptors.
- The same receptor cell displays at least 2 different responses (besides nil) at different ligand concentrations.
- The signal causes a concentration gradient along a field of cells.
2
Q
Embryo Specification
A
The cell has the capability to take on its specific fate when placed in a neutral environment. This is reversible, however–if exposed to alternate cues, the cell can take on an alternate fate.
3
Q
Embryo Determination
A
The cell not only takes on its specific fate in a neutral environment, it takes on this fate even if exposed to alternate cues (irreversible commitment to the fate).
4
Q
Autonomous Fate Aquisition
A
Self-contained info; no extrinsic cues.
5
Q
Conditional Fate Aquisition
A
Non-autonomous; needs outside cues.
- Lateral fate specification
- Induction
- Syncytial
6
Q
What are the three germ layers of a developing embryo?
A
- Ectoderm (outer layer)
- Mesoderm (middle layer)
- Endoderm (internal layer)
7
Q
Roux Experiment
A
A defect experiment
- Let a fertilized egg cleave in half into the 2-cell stage
- Stick a hot needle into one of the cells to cause death
- See if whole or partial embryo develops
- Purpose was to see if determinants localized to different sides of the egg following fertilization, resulting in two blastomeres with different fates.
8
Q
Driesch’s Experiment
A
An Isolation Experiment
- Remove fertilization envelope from a 4-cell emrbyo
- Separate into 4 cells
- Each cell developed into a separate embryo
9
Q
Conklin’s Styela Blastomeres
A
An Isolation Experiment
- Begin with an 8-celled embryo
- Separate into 4 groups of 2 cells
- Different cell types observed depending on where the cell pairs were originally located
10
Q
Recombination Experiment
A
Take one set of cells and transfer it (within the same embryo) to another area where those specific cells are not normally found.
- Observe whether the transfered cells remain of the same cell type or switch cell type
11
Q
Transplantation Experiment
A
Transplant a group of cells from one embryo to another of the same type, and place the cells in an area where they are not normally found.
- Observe whether transfered cells remain of the same cell type or switch cell type
12
Q
What 2 steps are involved in cell commitment to a particular fate?
A
- Specification: the cell has the capability to take on its specific fate when placed in a neutral environment, however, the cell can take on an different fate if exposed to alternate cues
- Determination: the cell not only takes on its specific fate in a neutral environment, it takes on this fate even if exposed to alternate cues (irreversible commitment to the fate).
13
Q
What are the two major types of cell fate acquisition?
A
- Autonomous fate specification: self contained information; no external cues
- Conditional: non-autonomous; requires outside cue
- Lateral fate specification
- Induction
- Syncytial
14
Q
What is the Preformationist Theory?
A
All of the components of a miniature orgaism exist within the gamete.
- Ovists believed this was the case in the ovaries.
- Spermists believed this was the case in the sperm.
15
Q
Oocyte
A
The female gamete; the egg cell.
16
Q
What are the 7 basic stages of development starting with a sexually mature adult?
A
- Gametogenesis
- Fertilization
- Cleavage
- Gastrulation
- Organogensis → hatching (or birth)
- Larval Stages
- Metamorphosis (in some species)
17
Q
_____ polymerase is stabilized on the _____ site of DNA by _____ factors recruited by _____.
A
RNA** polymerase is stabilized on the **promoter site of DNA by transcription factors recruited by enhancers.
18
Q
What are the four general processes of fertilization?
A
- Contact and recognition between gametes
- Initiate blocks to polyspermy
- Fusion of genetic material (pronuclei) → syngamy
- Activation of metabolism and cell cycle re entry
19
Q
What are the 5 main steps leading to the fusion of egg and sperm cell membranes in the sea urchin?
A
- Sperm contacts jelly layer
- Acrosome reaction
- Digestion of jelly layer
- Binding to vitelline envelope
- Fusion of acrosomal process membrane and egg membrane
20
Q
What are the 5 main steps leading to the fusion of egg and sperm cell membranes in mice?
A
- Sperm activated by female reproductive tract
- Sperm binds to zona pellucida
- Acrosome reaction
- Sperm lyses hole in zona
- Sperm and egg membranes fuse
21
Q
The immunoglobulin superfamily protein _____ is required for sperm to fuse with an egg.
A
Izumo
22
Q
What are the two types of blocks to polyspermy?
A
- Fast Block: Electrical and transient; provides time for permanent block to occur
- Slow Block: Mechanical and permanent
23
Q
The membrane potential of sea urchin eggs before and after fertilization is about __ and __, respectively.
A
-65; 10
24
Q
_____ _____ exocytosis leads to a permanent block to polyspermy.
A
cortical graunule; this is triggered by sperm/egg binding.
25
What is the molecular mechanism of egg activation following gamete membrane fusion?
1. Soluble factors from sperm activate PLC
2. Both sperm protein and protein from the egg's PLC → PIP2 → IP3 pathway activate calcium channels linked to the ER
3. Calcium is released
4. Calcium gradient triggers corticle granule exocytosis
5. Permanent block to polyspermy complete
26
Actinomycin inhibits \_\_\_\_\_.
transcription.
27
What are the two main categories of cleavage and what determines which one the egg employs?
1. Holoblastic (full cleavage)
2. Meroblastic (partial cleavage)
The yolk thickness and density determines which form of cleavage the egg utilizes
28
What are the main categories of cleavage?
1. Holoblastic Cleavage (Complete)
1. **_Isolecithal_**--sparce, evenly distributed yoke
2. **_Mesolecithal_**--moderate vegetal yoke disposition
2. Meroblastic Cleavage (Incomplete)
1. **_Telolecithal_**--dense yoke throughout most of cell
2. **_Centrolecithal_**--yoke in center of egg; superficial cleavage (most insects)
29
What are the 5 main forms of cell movement during gastrulation?
1. **_Invagination_**: infolding of cell sheet into embryo
2. **_Involution_**: inturning of cell sheet over the basal surface of an outer layer
3. **_Ingression_**: migration of individual cells into the embryo
4. **_Delamination_**: splitting or migration of one sheet into two sheets
5. **_Epiboly_**: the expansion of one cell sheet over other cells
30
What are the two major states of cells in an embryo?
1. **_Epithelium_**: a sheet of cells, sitting on a basement membrane. Each cell is joined to its neighbors by specialized junctions and exhibits a distinct apical basal polarity; cells move as a group.
2. **_Mesenchyme_**: descriptive term for scattered cells embedded loosely in the extra cellular matrix; cells move as individuals.
31
The _____ protein is instrumental in mediating cell-cell adhesion.
cadherin
32
\_\_\_\_\_ have the ability to induce presumptive ectodermal cells to acquire other fates, as well as to induce a secondary axis in sea urchin embryos.
Micromeres
33
What are 5 common methods of studying embryos?
1. Fate Mapping (Lineage Tracing)
2. Defect Experiments (Including Ablations)
3. Isolation Experiments
4. Transplantations
5. Recombinations
34
Northern Blot
Used to determine the **when** and **how much** of a given mRNA is present.
35
Reporter Construct
This is a piece of cDNA that encodes some (easily) detectable protein that is under the control of a promoter/enhancer of the experimenter's choice. Green florescent proteins (GFPs) are often utilized to determine **when** and **where** a given **mRNA** is present. Two types of regulatory regions are used to make reporters:
1. _Consitutively Active (Basal)_: this is used when trying to force expression of a gene product in a cell type that normally does not express that gene (ectopic expression) or when trying to overexpress a given gene product ectopically.
2. _Cell Type Specific_: this is useful when the gene of interest has been identified and the experimenter wants to determine when and where it is expressed. Also useful for driving expression of gene products fused to the reporter when functional assays are called for.
36
Western (Immune) Blot
Determines **when** and **how much** of a given **protein** is present.
37
Immunoflorescence
Specific **antibodies** are used on whole embryos to determine **when** and **where** a **protein** is present.
38
What are 4 types of Loss of Function tests?
1. Gene Knockout
2. Knockdown
3. Function Blocking Antibody
4. Inactivate target protein via a dominant form of the protein (mutant)
39
Vitelline Evelope
A thin extracellular matrix directly surrounding the egg cell membrane in sea urchins and most animals.
40
Zona Pellucida
A thick extracellular matrix that directly surrounds the egg cell membrane of mice and most mammals.
41
cDNA
Complemetary DNA is DNA synthesized from a mRNA template in a reaction catalyzed by the enzyme reverse transcriptase and the enzyme DNA polymerase. **When scientists want to express a specific protein in a cell that does not normally express that protein, they will transfer the cDNA that codes for the protein to the recipient cell.**
42
Gene Knockdown
Gene knockdown refers to techniques by which the expression of one or more of an organism's genes is reduced, either through genetic modification (a change in the DNA of one of the organism's chromosomes) or by treatment with a reagent such as a short DNA or RNA oligonucleotide with a sequence complementary to either an mRNA transcript or a gene. If genetic modification of DNA is done, the result is a "knockdown organism". If the change in gene expression is caused by an oligonucleotide binding to an mRNA or temporarily binding to a gene, this results in a temporary change in gene expression without modification of the chromosomal DNA and is referred to as a "transient knockdown".
43
Gene Knockout
A gene knockout is a genetic technique in which one of an organism's genes is made inoperative ("knocked out" of the organism). Knockout is accomplished through a combination of techniques, beginning in the test tube with a plasmid, a bacterial artificial chromosome or other DNA construct, and proceeding to cell culture. Individual cells are genetically transfected with the DNA construct. Often the goal is to create a transgenic animal that has the altered gene.
44
Describe the generalized Wnt signal transduction pathway.
Wnt → Frizzled → Disheveled --I GSK3 --I ß-catenin → Transcription
## Footnote
End result: vegetal cells specified within the sea urchin embryo.
45
Double Negative Gated Circuit
Pathway in which there are inhibitors of inhibitors.
46
Feed Forward Circuit
Pathway in which genes activate or enhance production of other gene products.
47
What are the four basic mechanisms by which embryos maintain terminally differentiated cell states?
1. Transcriptional positive feedback
* The initial stimulus activates expression of a transcription factor, which further promotes gene transcription
2. Chromatin remodeling
* The initial stimulus activates expression of the Trithorax protein, which inhibits nucleosome formation
3. Autocrine stimulation
* The initial stimulus activates transcription of autocrine ligands whose function is to promote further transcription of the same genes in the same cell
4. Paracrine loop between cells
* The initial stimulus activates transcription of a paracrine ligand that promotes further transcription of the same genes in neigboring cells
48
Collective Migration
When a group of cells move in the same direction (i.e., lengthen).
49
Cell Intercalation
When a group of cells compact together and cause lengthening.
50
Why are C. Elegan embryos commonly studied for developmental research?
1. Transparent
2. Major types of bodily systems
3. Hermaphroditic
4. Rapid embryogenesis
5. Easy to grow
6. Molecular biology friendly
51
How many somatic cells does an adult C. elegans have?
959
52
Inductive Signaling
A one-way signaling event whereby one cell sends a signal to another "competent" cell (i.e., one that has a receptor for the signal).
53
Lateral Specification
A signaling process whereby:
* Transmission of the same signals occur between two cells
* Both cells express the signal and receptor
* Both cells have equivalent fate potentials
54
Thomas Hunt Morgan
Developed fruit flies as model genetic organism and pushed embryologists to consider genetic information as a key aspect of development.
* Won the Nobel Prize in Physiology/Medicine for his discovery "that genes are carried on chromosomes."
55
Section differentiation in *Drosophila* is caused by what phenomenon?
Asymmetric distribution of maternal determinants in in the unfertilized egg.
56
What three genetic pathways work together to form the A-P axis during oogenesis in Drosophila?
1. Anterior: Bicoid gradient
2. Posterior: Nanos gradient
3. Terminal: Torso protein
57
What are bicoid's primary known functions?
1. Activates anterior GAP genes (e.g., *hunchback*, *orthodenticle* and *buttonhead*) that promote proper anterior formation.
2. Represses maternal *caudal* mRNA by binding to its 3' UTR site, thereby preventing translation
Since *caudal* mRNA is present evenly throughout the embryo, repression of it in the anterior portion leads to it being translated and functional exclusively at the posterior end.
58
Nanos
A maternal mRNA that is sequestered at the posterior end of the oocyte.
* Is translated immediately after fertilization and sets up a gradient from P→ A
* *Nanos* proteins enter nuclei and activate genes that direct posterior development
* *Nanos* proteins also repress translation of another maternal mRNA called *Hunchback* (by binding to the 3' UTR site)
* Phenotype of an embryo lacking *Nanos* from the mother: "anteriorized"
59
When does an embryo become "dorsalized?"
When the embryo does not make ventral structures, such as when *dorsal* is absent.
60
Dorsal Protein
A repressor of genes that direct dorsal fate and an activator of genes that direct ventral fate.
* mRNA is present throughout the embryo
* Translated at 90 minutes post fertilization
* Enters nuclei only on the ventral side
61
What are three examples of ways in which nuclear entry of a transcription factor can be regulated?
1. Binding partners
2. Phosphorylation
3. Proteolysis
62
What are the two primary tasks of vegetal cells?
1. Differentiate into endoderm
2. Induce overlying cells to become mesoderm
63
VegT
Activates genes for endoderm fate (autonomous).
* Also activates genes encoding the signals secreted by dorsal Veg cells (TGFßs)
64
The body axes are triggered at _____ and realized during \_\_\_\_\_.
fertilzation; gastrulation
65
Both blastomeres of an amphibian embryo require _____ \_\_\_\_\_ material in order to give rise to a complete embryo.
grey crescent
66
What scientist won the Nobel Prize for his work on induction?
Spemann
67
Maternal Vg1
Encodes TGFß (initiates the process, then VegT takes over and drives zygotic expression of TGFß; eventually establishes a positive feedback loop)
* KD of Vg1 → no organizer
68
What are the 4 major functions of the Organizer?
1. **Initiate the movements of gastrulation**
2. **Self-differentiate into dorsal mesoderm** (prechordal plate, chordamesoderm → notochord, etc...); this is an "induced" fate that has become determined by the time of gastrulation
3. **"Dorsalize" the surrounding mesoderm** into paraxial (somite) mesoderm (otherwise it forms "ventral" mesoderm)
4. **Dorsalize the endoderm** and induce formation of the neural tubes
69
What effect does Noggin have on BMP?
Noggin is a BMP inhibitor
70
What are the 3 primary functions of BMP?
BMP triggers signaling pathways that result in:
1. Expression of "ventral" fate genes in mesoderm
2. Expression of "epidermal" fate genes in ectoderm
3. Repression of "neural" fate genes in ectoderm
71
When _____ cells secrete Wnt inhibitors, what happens?
organizer;
1. Alleviate repression of various anterior neural fates (in ectoderm)
2. Promotion of dorsal fates in the mesoderm (coupled with "lack of activation" of ventral fates)
72
What are the two basic varieties of Wnt inhibitors?
1. _Wnt Mimics_: bind to the Wnt receptor, but do not activate it; they are receptor antagonists; works like a competitive inhibitor.
2. _Wnt Receptor Mimics_: bind to Wnts directly and compete with the endogenous Wnt receptor (they mimic the receptor ligand binding domains); basically titrate the Wnts (an example is Frzb)
73
What tissues are the placenta composed of?
Maternal tissue (decidual cells) and fetal cells that form in the chorion.
74
What is a streak and how does it affect cell behavior?
A streak is a "stable structural entity" that continually proliferates and generates an ordered series of cell types through an epithelial-mesenchymal transition (EMT).
* The first cells to exit give rise to extraembryonic, cardiac and cranial mesoderm as well as definitive endoderm.
* Subsequent cells give rise to lateral plate and then paraxial mesoderm tissue.
* Node cells secrete proteins that specify these fates.
75
What are 5 common themes in vertebrate gastrulation?
1. Gastrulation and axis formation are tightly linked
2. Internalization of endoderm and mesoderm
3. Epiboly of ectoderm around entire embryo
4. Convergence of internal cells to the midline
5. Extension of the body along the A-P axis
76
What are the two signaling centers in mammals that regulate cell fates along the A → P axis?
1. The Node ("organizer")
* Specifies the entire body; similar to the amphibian organizer to some degree
* Expresses Chordin and Noggin genes
2. Anterior Visceral Endoderm (AVE) Cells
* Work together with the node to specify anterior structures, especially in the nervous system
* Express Otx-2, Cerberus, Lim-1 genes
Due to redundancy, KOs and KDs of these organizer-like genes are mostly ineffective unless multiple genes are silenced
77
Briefly described the steps involved in dorsal/ventral specification in mammal development.
1. After the 5th cell division, the inner cell mass (ICM) separates into hypoblast (primitive endoderm) and epiblast (primitive ectoderm & mesoderm)
2. The hypoblast forms on the side that is exposed to the blastocoel fluid → defines future "ventral"
3. The ICM cells in contact with trophoblast (upper layer) become the epiblast layer → defines future "dorsal"
78
What are the two types of L-R axis regulation?
1. Global level
* Inversion of embryonic turning
* All asymmetrical organs are on the wrong side in mice
* Homozygous for lof inv; not a real problem
2. Organ-specific level
* Situs inversus viscerum
* Homozygous lof mutation randomizes the l-r orientation of each asymmetric organ independently
* This can have dire consequences
79
Nodal and PitX2 expression occur exclusively on the _____ side of an embryo.
left
80
What process allows embryonic fluid to move within the yolk sac to promote proper development?
Coordinated cilia on Node cells.
Kartangener's Syndrome is a disease whereby the nodal cilia are unable to beat, resulting in asymmetric organ placement.
Cilia beat reversal → reversal of the L-R axis!
81
Nodal Vesicular Parcel (NVP)
An enclosure secreted by node cells that contains sonic hedgehog and retinoic acid and delivers these contents to the left side to trigger correct L-R development.
82
What is the term used to describe both of the hometotic regions on chromosome 3 of *Drosophila*?
The homeotic complex
83
A general rule is that the expression of a given homeotic gene is _____ regulated by the homeotic gene products that are posterior to it.
negatively
84
What are the structural and functional definitions of a homeotic gene?
1. **_Structural Definition_**: A transcription factor that has as part of its protein structure a consensus sequence that binds target DNA called the **homeodomain**.
2. **_Functional Definition_**: Regulates batteries of other genes that specify patterning in the embryo (specification of body parts, including organs).
85
Homeotic Selectors
Also known as **master control genes**, these work alone at the top of GRNs.
* Often directly target realisator genes as well as regulators
* Positive feed forward and feedback loops are common
86
The expression of a homeotic gene in a given area should be both necessary and sufficient to drive the formation of a given structure (body part) if that homeotic gene is also a \_\_\_\_\_.
master controller
87
What biochemical matter is responsible for the differential temporal expression of PHA-4 targets?
The early targets have a higher affinity for PHA-4 than the late targets.
88
Homeotic genes are called _____ genes in mammals.
Hox
89
What are the 3 major derivatives of the ectoderm germ layer?
1. Surface ectoderm (epidermis)
2. Neural crests
3. Neural tube
90
What are the 4 stages involved in ectoderm to neural cell differentiation?
1. **_Competence_**: ectodermal cells are exposed to proper combination of signals, resulting in initiation of "neuronal gene expression"
2. **_Specification_**: cells have recieved instructions to become "neuronal" (and can do so without continued instruction), but can still be redirected down alternative fate pathways--now called **neuroblasts**
3. **_Determination (Commitment)_**: cells enter neural differentiation pathway; irreversible fate even when exposed to other signals
4. **_Terminal Differentiation_**: cells leave mitotic cell cycle and express neuronal specific genes
91
Epidermis
The outer layer of skin.
92
Epithelium
A sheet of cells held together tightly (as opposed to mesenchyme); can be derived from any germ layer; the epidermis and neural tube are ectodermal epithelia while the lining of the gut is an endodermal epithelium.
93
What are the two ways to form a tube from the neural plate?
1. Primary Neurulation--shaping, elevation, convergence, closure
2. Secondary Neurulation--formation through simple congregation
Many vertebrate embryos utilize the 1º type in the anterior and the 2º type in the posterior
94
The process of neurulation begins at the _____ of an embryo and progresses towards the _____ end.
anterior; posterior
95
How often do neural tube defects occur among humans, and what are the two main varieties?
About one in 500 are affected
* Posterior pore closure defects: Spina Bifida
* Anterior closure defects: Anencephaly (1/1000 pregnancies)
96
A _____ binding protein has been identified that plays a crucial role in neural tube closure.
folate; this protein localizes to the edges of the neural tube as it closes, although the mechanism by which this happens and the precise function of the folate protein are still unknown.
97
What are the 3 levels of differentiation of the neural tube?
1. _Gross anatomical_: bluges and constricts to form chambers of brain and the spinal chord (obvious along A-P axis)
2. _Tissue level_: organization of cells into functional units
3. Differentiation of neuroblasts into nerve cells and glia
98
The neural tube is _____ along the D-V axis.
polarized
99
What are the two main paracrine signaling molecules that dictate the D-V patterning of the neural tube?
1. _Sonic Hedgehog_: secreted initially from the notochord
2. _BMPs_: secreted from the dorsal ectoderm
These two molecules form opposing gradients, and cells are specified based on the proportion of each in their immediate vicinities
100
Describe the Cre-Lox P recombination method for creating conditional KOs.
* Takes advantage of a bacteriophage enzyme called Cre recombinase (Cre)
* Cre recognizes DNA sequences called "Lox P" (very specific)
* Cre cuts each LoxP sequence in half and recombines them (a "splicer")
Basic idea:
* Flank the gene you want to KO with the LoxP sequences
* Design Cre cDNA under control of a promoter that you control (use a tissue specific, drug sensitive, or heat sensitive promoter)
* When Cre is made, your Lox'ed target gene will be spliced out
101
What distinguishes the human brain from brains of other primates?
1. Retention of fetal neuronal growth after birth
2. Migration of cells from proencephalon to diencephalons
3. High transcriptional activity
4. Specific form of FoxP2 gene (speech, language)
5. Continuation of brain maturation into adulthood
102
Snail
A transcription factor that **downregulates E-cadherin** expression (also regulates expression of other genes involved in specification).
103
Rho GTPase
Regulates actin-based cytoskeleton (shape changes and cell movements)
104
What are the 4 main neural crest domains listed from the anterior to posterior ends?
1. **_Cranial (cephalic) NC_**: cells migrate dorso-laterally to produce craniofacial mesenchyme → bone, cartilage and connective tissues of face, cranial nerves
2. **_Cardiac NC (b/n cranial and trunk NC; somites 1-3)_**: musculo-connective tissue of arteries, lots of other cell types as well (melanocytes, neurons, etc.)
3. **_Trunk NC (somites 6-tail)_**: dorsal root ganglia containing sensory neurons; sympathetic ganglia; adrenal medulla, nerve clusters around aorta
4. _**Vagal and Sacral NC (somites 1-7; posterior to somite #28**)_: generation of parasympathetic ganglia of the gut
105
Are neural crest cells multipotent?
Yes!!
106
Are NCCs from one region "restricted" to giving rise to specific cell types? What experiment was done to determine this?
**Partially**; An experiment was conducted in which a trunk NCC was isolated and labeled immediately after development. The trunk NCC was then transplanted into the cranial region.
Results:
* Trunk NCC migrates to proper "cranial" locations in the head region
* However, it cannot make cartilage or corneal cells (cranial NCC fates)--probably related to earlier Hox expression
107
What are limb fields and what specifies their formation?
Limb fields are regions of the lateral plate mesoderm that are capable of giving rise to limbs, and they are specified by a gradient of Hox genes along the A-P axis early in development.
* Limb fields can induce myoblasts to migrate out from somites and enter the limb bud to form the musculature
* No other regions of the lateral plate mesoderm can do this
108
How were limb fields initially identified?
* Removed groups of cells to see if limbs could still form
* Transplanted groups of cells to see if new limbs form at new site
* Marked cells to see if their decendents ended up in limbs
109
How does the limb bud form?
1. A proliferation of mesenchymal cells from the somatic region of the lateral plate causes a physical bulge → these cells go on to become the skeletal limb elements
2. Lateral myotome releases myoblasts that eventually move into the bud to become the musculature
3. The bud grows and lengthens along the proximal-distal axis due to the proliferation of the cells
110
What is the AER?
Apical Ectodermal Ridge → cells overlying the limb bud mesenchyme that take on unique properties.
111
After mesenchymal cells condense within the growing limbs, they first differentiatie into _____ and then into \_\_\_\_\_.
cartilage; bone
112
What are the T-box genes and what role do they play in limb development?
T-box genes are a family of genes that encode a specific kind of transcription factor (DNA binding domain is called a "T box" domain.
* Tbx5 is expressed in the anterior limb field and Tbx4 is expressed posteriorly
* A autosomal dominant Tbox5 allele in humans results in Holt-Oram syndrome (abnormalities of heart and upper limbs)
113
Progress Zone
The PZ is a group of proliferating mesenchymal cells that lie just underneath the AER (200 um deep)
114
Zone of Polarizing Activity
The ZPA is a region of the lateral plate mesoderm that specifies A-P patterning of limbs.
* ZPA releases a signal (morphogen) that forms a gradient across the embryonic limb bud. The cell fate at different distances from the ZPA is determined by the local concentration of the morphogen
* High conc. = posterior fate
* Low conc. = anterior fate
115
Wnt7A
Expressed in dorsal limb ectoderm and required for the formation of dorsal features within the limbs.
* Signaling results in expression of Lim1 (a transcription factor) in dorsal mesenchyme cells
* Lim1 activates genes that then dictate "dorsal" limb fates in the mesenchymal cells
* If Lim1 is expressed on the ventral side, then those cells develop a dorsal phenotype
116
Nail-Patella (Turner Kiser) Syndrome
A human genetic disorder (autosomal dominant) that results in missing or small, poorly developed nails and malformed keencaps and elbows.
117
Sculpting of the autopod requires cell \_\_\_\_\_.
apoptosis
118
What are the 4 primary functions of PCD in development?
1. Adjust cell numbers
2. Eliminate dangerous or injured cells
3. Delete unneeded structures
4. Sculpting
119
What is August Weismann's Hypothesis regarding cell differentiation?
"As a cell becomes more differentiated or specialized, the genetic information in that cell is diminished or changed (irreversibly)."
120
Stem Cell
A cell that is capable of dividing many times to give rise to at least two different cell types (as well as maintain itself).
* Found in specialized regions of tissues called "niches"
* Progenator cells differentiate into new cell types but stem cells also make exact duplicates to continue the stem cell line
* The differentiating cells leave the niche and travel to locations where they are needed
121
Pluripotent
A type of stem cell that can give rise to most other cell types in the body.
* Embryonic stem cells
122
Multipotent
Stem cells that can give rise to a few cell types.
* Adult (and cord) blood stem cells
123
Totipotent
A stem cell capable of giving rise to any cell type or a complete embryo.
124
Where do embryonic stem cells come from?
The inner cell mass
125
What are the 3 main steps to utilizing stem cells to repair or replace damaged organs or body parts?
1. Isolate stem cells
2. Instruct them
3. Deliver to patient
126
How is the grey crescent in frog embryos formed?
Through reorganization of the cytoplasm and **cortical rotation**.
127
In frogs, the point opposite sperm entry is the future _____ region of the embryo and where _____ will initiate.
dorsal; gastrulation
128
In the late blastula stage, what tissues do the animal cap, marginal (equatorial) and vegetal cells end up becoming?
Ectoderm, mesoderm and endoderm, respectively.
129
BMP ligands operate through SMAD \_\_\_
1/5
130
Activin, Vg1, Nodal and TGFß ligands operate through SMAD \_\_\_
2/3
131
The _____ protein _____ dorsalizes the amphibian embryo.
soluble; Noggin
* Noggin physically binds to BMP2,4 and prevents these proteins from binding to their receptors
132
SMADs
Intracellular proteins that transduce extracellular signals from transforming growth factor beta ligands to the nucleus where they activate downstream TGFß transcription.
133
Describe the function of a dominant-negative allele.
A dominant-negative allele is a mutant form of a protein that has a **dominant** effect on the wild type (endogenous) protein and this effect is **negative** in terms of **function**.
* The idea is to disrupt the interaction between protein dimers, trimers, and so on to essentially "poison" a functional complex
134
Placenta
A tissue derived from both maternal tissue (decidual cells) and from fetal cells that form the chorion.
135
How do early cleavage patterns differ for echinoderms and amphibians & mammals?
Cleavage type:
* Echinoderms/Amphibians: holoblastic and radial
* Mammals: holoblastic and rotational
Temporal Features
* E/A: first cell division within minutes to hours post fertilization
* M: first cell division within 2-3 days post fertilization
136
Early zygotic transcription is a defining feature of \_\_\_\_\_\_.
mammals
137
The 3 primary tissue types develop _____ implantation.
before
138
Describe the differences betwen monozygotic and dizygotic twins.
* **Monozygotic**: one egg forms identical twins
* formed from a single embryo whose cells somehow became dissociated
* **Dizygotic**: two eggs form fraternal twins
* formed from two separate fertilization events
Identical twins are fairly rare (though increasing)--they occur in about 0.25% of human births
* can be produced by the separation of early blastomeres, or even separation of inner cell mass into two regions inside the same blastocyst
139
The _____ region of the spinal cord is where neurons recieve input from sensory neurons while the _____ region is where motor neurons reside.
dorsal; ventral
140
Give the basic elements of the Hedgehog signal transduction pathway.
Hedgehog --l Patched --l Smoothened --\> Ci (activator) --\> transcription
141
Describe the molecular model for initiation of the limb bud in chicks that occurs between 48-54 hours of gestation.
1. FGF10 is initially expressed thoughout the lateral plate mesoderm (broad, low levels)
2. **Wnt8c** signaling in the Hox-specified limb field (hind limb) *stabilizes FGF10 expression in that area*
3. FGF10 now accumulates at high levels in the (future) **hindlimb** area
4. As FGF10 levels are maintained in the hind limb areas, **Wnt2b** begins to *upregulate and stabilize FGF10* expression in the **forelimb fields**
5. FGF10, now at high levels in these areas, is detected by the overlying ectodermal cells. *This induces these cells to begin expressing* **FGF8** (and they are now the AER cells) through a loop involving **Wnt3a**
6. The FGF8 in turn is detected by the lateral plate mesoderm cells and causes them to upregulate FGF10 expression to lock in the paracrine loop
142
Which is dominant, Tbx4 or Tbx5, and why?
Tbx4; a direct sufficiency test was conducted that forced Tbx4 expression throughout the flank. The limbs that developed in the anterior region often became legs instead of wings.
143
The _____ dictates the type of limb formed (hind or fore) and also the patterns along the P-D axis.
mesenchyme
144
What is the relationship between the AER and underlying mesenchyme cells?
Mesenchymal cells induce and sustain the AER but the AER is also required for sustained growth and development of the limb. The AER prevents mesenchymal cells directly underneath it from forming cartilage and keeps them in a state of mitotic proliferation.
145
What are the 3 main limb bone elements from P→D?
1. Stylopod
2. Zeugopod
3. Autopod
146
\_\_\_\_ expression in ZPA is influenced by ____ expression in early limb bud and then in turn influences further ____ expression to pattern the growing limb the along A-P axis.
SHH; Hox; Hox
147
Where specifically does one obtain an embryonic stem cell?
From 5 day old blastocysts
148
What are 5 common problems with utilizing stem cells for theraputic purposes?
1. Isolating ES cells
2. Directing the lineage
3. Isolating and directing adult stem cells
4. Controlling cell division
5. Immunological incompatibility
149
What are the stages involved in somatic cell nuclear transfer procedures for therapeutic cloning?
1. Remove the nucleus from the oocyte
2. Insert the nuclues from the patient's skin cell
3. Induce development without sperm
4. Resulting stem cells are genetically identical to the patient
150
Hydatiform Moles
Tumors that form in humans that result from a sperm fertilizing an egg that lacks a pronucleus. The sperm genome duplicates and the early cells survive, but no embryo develops and instead a large "placental-like" mass of cells results.
* This naturally occuring phenomenon gave the first clue that the pronuclei were not equivalent
151
What experiments demonstrated that mammalian pronuclei were not equivalent?
Either male or female pronuclei can be removed (using a micropipette) from a newly fertilized mouse egg and then added to other recently fertilized mouse eggs that have had one or the other of their pronuclei removed (it is possible to visually distinguish female from male pronuclei).
Zygotes with two female pronuclei or two male pronuclei will undergo a few rounds of cell division, but will not develop through birth.
