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Development Exam 2 Flashcards

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1
Q

What are HOX genes?

A

Homeodomain transcription factors. They are HIGHLY conserved throughout eveloution. Crucial for patterning the long axis and limbs of developing animals. They are found in 4 (or more) complexes on 4 (or more) different chromosomes

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2
Q

Describe the relative time and location of expression of different Hox genes.

A

The anterior Hox genes (a1, b1, d1) are expressed anterior and earlier. The posterior Hox genes (a10,11,12…) are expressed more posteriorly and later in time.

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3
Q

Which group of Hox genes (anterior or posterior) play a major role in limb morphogenesis?

A

Posterior Hox genes are important for limb morphogenesis.

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4
Q

How does HOX gene expression relate to the Head-Tail patterning of an embryo?

A

The 3’ to 5’ position of HOX genes is recapitulated in the Head-Tail (Rostral-Caudal) pattern of expression in developing embryos. For example, Hoxb1 is expressed more anterior than Hoxb4, which is expressed more anterior than Hoxb9

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5
Q

A sample of tissue from the anterior most regon of the neck of the embryo is removed or tested for Hox gene expression. Will it express one single Hox gene? Or a combination or several Hox genes?

A

The same tissue can express different Hox genes at the same time, the combination of which determines the unique longitunal identity of the developing tissue.

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6
Q

If the pattern of HOX gene expression is altered, what happens to the affected cell population?

A

The positional identity is changed

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7
Q

Switches between Hox gene expression between adjacent genes (c5 to c6, for example) cause what feature in vertebral development?

A

Changes in expression from c5 to c6 defines the cervical-thoracic boundary. Simlar changes define the boundaries between other vertebral segments as well.

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8
Q

What are key targets for evolutionary selection in terms of Hox genes?

A

The relative timing and patterning of gene expression

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9
Q

Where is the anteriorormost Hox gene expressed?

A

In the posterior brain

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10
Q

Describe the regulation between Hox genes.

A

They impact each other’s regulation in positive and negative manners. For example, the loss of posterior HOXes tends to allow more anterior HOXes to expand posteriorly. The region that develops depends on a complex combination between genes turning on and off

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11
Q

What determines the local pattern identity as the node is retreating?

A

A combnination of the retinoic acid gradient and the expression of Hox genes.

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12
Q

What is reverse genetics?

A

An experimental technique that starts with a gene believed to have a function of interest. Mutations are then engineered into the genes to produce measureable LOF or GOF mutations

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13
Q

What is a homeotic transformation?

A

When a loss of function mutation is accompanied by a replacement with an alternative fate specification. For example, when a cervical vertebrae is posteriorized and expresses a rib bone.

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14
Q

Describe the process of genetic knockouts in mice used to test Hox function.

A

A drug resistance gene is inserted into the middle of an exon for a Hox gene via homologous recombination. This causes a disrupted reading frame of the Hox gene. The only cells that will survive in culture are the ones that took up the drug resistance, and therefore have disrupted Hox gene expression.

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15
Q

Describe the anatomical changes seen in hoxc8 knockouts

A

Homeotic transformation. The first lumbar vertebrae is anteriorized into a 14th thoracic vertebrae, expressing the rib formation usually seen only in thoracic vertebrae.

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16
Q

True or False: Hox genes only regulate the patterning of the spine segments.

A

FALSE. Hox genes pattern all three germ layers to signal development of the right organs and structures in the right locations

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17
Q

Describe the Hox gene expression in migrating neural crest cells.

A

Migrating crest cells (“going mesenchymal”) retain their hox-gene expression/identity even after they leave the neural crest. They are specified and determined.

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18
Q

What structures form from the dorsal neural crest cells?

A

Most of the bones of the face, inner ear, hyoid bone and pharyngeal structures. This is all possible because migrating neural crest cells retain their Hox gene expression and positional identity as they migrate and form these bone structures.

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19
Q

Is the notochord an organizing center?

A

Yes. It is the key organizing center for the dorsal-ventral axis development.

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20
Q

What are the two cell types that make up the somites?

A

Dermomyotome (Dorsal and lateral 3/4) and Scleromyotome (Ventral and medial 1/4)

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21
Q

What do the dermomyotome and sclerotome each develop into?

A

Dermomyotome: dermis and muscle. Sleromyotome: Vertebra and ribs

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22
Q

What happens when a notochord is transplanted from one developing embryo into an ectopic location in another developing embryo?

A

The transplanted notochord acts as a second inductive center. Two floor plates form, and the somite on the side of the transplant is ventralized, only expressing sclerotome.

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23
Q

What is the ventralizing signal produced by the notochord?

A

Sonic hedgehog (Shh)

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24
Q

What are the sources of the two ventralizing signals from the notochord?

A

The primary signal Shh signal comes from the notochord. The floor plate of the neural tube, induced by the notochord, also produces Shh signaling to create the second signal.

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25
What happens if Hensens node from a Chick embryo is removed and sandwiched between Xenopus animal cap tissue? What does this say about evolution?
The chick organizing signal is able to induce neural fates in Xenopus cells. This shows that the Hox genes and their related signaling pathways are highly conserved across nearly 270 million years of evolutionary divergence.
26
Describe how the Grrafian follicle develops into the blastocyst in human embryological development.
The Graafian follicle, in the ovaries, bursts out into the body cavity. The fibria of the fallopian tube sweep the egg into the lumen of the fallopian tubes. Fertilization (normally) takes place in the fallopian tubes. The egg travels through the tubes, undergoing mitotic divisions to form the blastocyst as the egg travels to the uterus.
27
Describe what happens after a human egg implants in the uterine wall.
The trophectoderm lacunae fuse with maternal blood vessels, surrouding the embryo with pools of maternal blood.
28
What is the function of the corpus luteum?
HcG signals the corpus luteum to become a progesterone signaling center. This causes retention of the endometrium, giving a "Do not menstruate" signal to the body.
29
Why are forelimbs formed earlier than hindlimbs?
Because they are closer to the head, where patterning by the organizing node begins.
30
How can identical twins form?
The inner cell mass can hyperproliferate and then separate within a single blastocyst leading to the development of two embryos developing in the same placenta (monochorionic twins). Blastomeres may detach from each other between the 2 and 8 cell stage, thus separating the placenta (dichorionic twins).
31
How can conjoined twins form?
If a hyperproliferated inner cell mass fails to properly separate, a large epiblast containing two developmental axes can form. This can lead to conjoined twins with some distinct organs and some shared organs. They share endoderm.
32
What happens if Hensen's node does not retreat fast enough in the developing embryo?
Neural hypertrophy can occur. Neurectoderm forms at the expense of ectoderm, leading to an oversized head. This can lead to internal organs being exposed to amnion because not enough skin is formed.
33
What happens if Hensen's node does not retreat far enough posteriorly?
The tissue posterior to the location where the node stops will not be pattened. This is the cause of sacral teratomas
34
What can failure of gastrulation cause?
Inadequate mesoderm available for formation of normal structures. Ex: sirenomelia, a condition where newborns have a single, incomplete limb bud containing leg bones from two limbs. Failed development of posterior structures (anus, intestines...) are often fatal.
35
What happens if the neural tube fails to close?
The amnion will have direct access to the central nervous system, leading to neurodegeneration. Ex: spina bifida, anencephaly.
36
What supplement is taken by mothers to help prevent neural tube defects?
Folic Acid
37
What do the six branchial arches and the maxillary prominences develop into?
The face: arches, palate ridges and philtrim have to seal together to properly form sinuses, nose, palate (hard and soft), and primative pharynx.
38
What is the most common branchial arch developmental defect?
Cleft palate
39
Why can a cervical rib (from a homeotic transformation) be dangerous in humans?
The cervical rib could shift and block the carotid artery which can cause sudden death
40
Describe the similarities between human Waardenburg syndrome and mice splotch mutants.
They have similar phenotypes due to defects in the PAX3 gene. Epidermal pattern errors are seen with non-pigmented hair, and many neural tube defects are also seen.
41
Describe the organism C. Elegans
C. Elegans is a free living, soil dwelling nematode. Most are functional hemaphrodites.
42
What are the genotypes for the hermaphrodite and male C.Elegans, respectively?
Hermaphrodite: XX Males: XO
43
Why is C. Elegans a good model organism to study human development?
They are transparent, making it easy to see mutations in development without expensive techniques. The genes are highly conserved between C. Elegans and Humans. Structures from all three germ layers form in C. Elegans.
44
What is the Dauer stage?
A shunt pathway in the C.Elegans developmental life cycle that can be induced by starvation. The C. Elegans are able to survive 3-4 years in this stage without any food due to a completely suspended metabolism.
45
What was the C.Elegans research won the 2002 Nobel Prize?
The description of normal development, mutant identification, and discovery of programmed cell death (PCD)
46
What is unique about the cell liniage and position of cells in C. Elegans?
They are invariant, so each cell can be given a name based on its position and progrenitor cell. Cells will be in same locations at same time in any developing nematode.
47
Describe the lineage of the ABa cell.
The ABa cell is the anterior daughter of the AB cell.
48
How many cells are in the adult C. Elegans? How many undergo PCD during development?
959 cells make up the adult C. Elegans. 113 undergo PCD.
49
What cells are formed from the first division of the Zygote?
AB and P
50
What are the fates of the AB cell?
Mostly hypodermis and neurons. Some muscle.
51
What are the fates of the P cell?
Muscle, intestine, neurons, germ line.
52
How do mosaicism and regulative development differ?
Mosaic development depends upon localized cytoplasmic factors while regulative development depends upon cell-cell interactions.
53
What significant discovery came from Strome's work with P granules?
It confirmed the highly mosaic nature of the early C. Elegans embryo. The P granules, when labeled with antibodies are found only in the posterior lineage. This is crucial for the proper development of the germ line.
54
What are the names of the germ line cells in C. Elegans?
Z2 and Z3
55
What is the effect of the MES1 mutation?
The Maternal Effect Sterile 1 mutation causes a visible loss of P-granule segregation. With this mutation, P-granules are found outside of the P-lineage. Homozygous mothers have LOF mutation leading to abberant germ line development, mostly sterile.
56
In what tissues are P-granules found in MES mutants?
Some are properly located in Z2 and Z3 (germ line) but they are also scattered throughout the epidermis and the muscle.
57
How is the level of fertility related to the number of P-granules in Z2 and Z3?
The more P-granules in the germ cells, the higher the fertility.
58
What does the PGL-1 gene encode?
an RNA binding protein that localizes with P-granules. This protein is a molecular component of P-granules, that when mutated causes temperature dependent sterility. This is strong evidence for cytoplasmic determinants.
59
What example shows how crucial cell-cell interactions are for determining fates in the developing C. Elegans?
ABp mus contact the P2 cell or it will become an ABa cell. This shows that P2 is the source of ABp signaling.
60
What does the glp-1 gene encode?
A transmembrane receptor. The mRNA translation is repressed in the P cell and restricted to the AB cell. This receptor binds to the apx-1 protein produced by the P2 cell.
61
From what cell is the anterior pharynx derived in C. Elegans?
The ABa
62
What is morphogenesis?
Change in form over developmental time driven by patterning, cell division and differentiation.
63
What are the causes of morphogenesis?
(1) Regulation of cell adhesion (2) Change in cell movement (3) Orientation of cell division (4) Change in cell shape
64
What is an anlage?
a group of co-determined cells whose interaction leads to the establishment of an organ or tissue (ex: lens primordia, branchial arch primordia) "The final patterning before differentiation commences"
65
What is a primordium?
A group of cells most often associated with the production of a specific organ.
66
Can a primordium contain cells from more than one anlage?
Yes. Primordia can be composed of cells arising from distinct anlagen joining together. (ex: the neural retina and the lens are from different anlagen)
67
What is a compartment?
A group of cells that are physically or informationally isolated from adjacent cells by distinct adhesion and signaling. Experiments trying to combine separate compartments end with rejection.
68
What is often a molecular change seen following specification and determination?
Gene expression is altered, often leading to changes in adhesion molecules found on the cell surface.
69
What is the function of cell adhesion molecules in terms of development?
They facilitate local communication between cells. The presence of one CAM vs. another can lead to different fate specification events.
70
What are the three cell-cell adhesion junction types?
(1) Adherens junctions: between cadherins, catenins and actin (2) Desmosomes: between cadherins and intermediate filaments (3) Calcium-independent adhesion via IgG
71
What is the function of cell-matrix adhesion molecules?
They act as an anchor via friction and electrostatic interactions in order to slow cellular progress and allow for greater chance for cell-cell interactions.
72
What are re-aggregation experiments, and what do they tell us?
Experiments forcing two cell types that don't normally interact to interact. When ectoderm and endoderm are transplanted to connect to one another in culture, they will eventually separate to minimize contact points. This shows that these cells "know" who they should be interacting with. Distinct anlagen have distinct adhesion properties.
73
If presumptive epidermal cells and presumptive neural plate tissues are degraded by proteases and left in culture as a mixture, what will happen?
They will spontaneously reaggregate, sorting themselves so that the epidermal cells are on the outside covering the future CNS
74
Why is cell adhesion important for neural tube closure?
The CAMs allow for the cells to be pulled in together to form the neural tube.
75
What will most likely happen to a group of cells that begin to express fewer and fewer CAMs?
That tissue will most likely migrate away from its current location to develop into something else.
76
How do cells and tissues change their shape?
Movement proteins (actin/myosin) can expand or contract regions of cells during different cell stages.
77
What are the three major examples of changes in cell shape?
(1) Cleaving cell during mitosis: contractile ring contraction causes cytokinesis (2) Apical constriction: constrict one end of the cell, but leave the other the same (3) Migrating cells: myosin pulls in direction of actin allowing cells to go mesenchymal.
78
Describe the changes in shape seen in presumptive mesoderm during gastrulation.
Convergence and extension allow for the cells to stretch toward future anterior, allowing for formation of structures like the notochord.
79
What is the function of bottle cells?
They allow for involution of the marginal zone because of the apical constriction of these cells. As the apical regions are squeezed, the contractile ring bound to the adjacent cells contracts to from the blastopore lip, thus initiating gastrulation. These cells "push" migrating mesoderm inward.
80
How are epiblastic animals different than xenopus in terms of CAMs?
Cells in epiblastic animals migrate as mesenchyme, so the CAM must not connect cells as strongly. They still must possess a loose association of cells in order for proper gastrulation.
81
Describe "the circle of limb life" and the signaling molecules involved.
Hox and Tbox --\> Retinoic Acid --\> FGFs and Shh; Shh--\> FGF and more Hox
82
How are the molecules involved in limb development different from those involved in axial patterning?
They are the same molecules, but temporally distinct.
83
What is the first feature formed during limb development?
Limb buds form first, indicating that limb development has begun.
84
What is the apical ectodermal ridge?
The AER is a condensation of ectoderm found at the distal tip of limb buds and forming limbs. Its presence indicates that limb primordia are present.
85
What is the early limb bud composed of?
A core mesenchymal mesoderm and an epithelial ectodermal layer
86
Do all parts of the limb develop from the mesenchymal core?
No. Most do, but the muscle cells migrate from the adjacent somites.
87
What is the progress zone?
A portion of cells just proximal to the AER containing rapidly dividing and proliferating undifferentiated cells.
88
When do cells begin to differentiate?
When they leave the progress zone and cartilage is formed.
89
Describe the order in which differentiation occurs in terms of autopod, stylopod, and zeugopod.
Proximal is formed first. Stylopod develops first, followed by zeudopod, and then lastly autopod.
90
If the zeugopod does not differentiate properly, what structures will be absent?
The radius and ulna will be missing if the zeugopod does not develop properly.
91
If the autopod does not differentiate properly, what structures will be absent?
The digits will be absent if the autopod does not develop properly.
92
If the stylopod does not differentiate properly, what structures will be absent?
The humerus will be absent if the stylopod does not develop properly.
93
What is the most anterior digit on a chick wing?
The 2nd digit (No 1st digit exists in chickens)
94
As the limb size increases, what changes occur within the limb?
Bone replacing cartilage and innervation
95
Wher is the polarizing region found?
Immediately posterior and a little proximal to the progress zone. AKA: zone of polarizing activity (ZPA)
96
True or False: Cell lineage determines fate in limb development.
False. Cell position, NOT lineage, determines the cells fate.
97
What is the effect of the AER?
AER is crucial for progress of cell proliferation.
98
What is the effect of the ZPA?
The ZPA determines the relative anterior-posterior and dorsal-ventral orientations within the developing limb
99
What is the Model of Pattern Formation?
Structures appear to develop based on their A/P and D/V position.
100
What is the proximal-distal position determined by?
A temporal mechanism determines the proximal distal position as the AER and progress zone move from proximal to distal.
101
What happens when the AER is removed?
Removal of the AER results in loss of distal structures, the severity of which is determined by the time of removal.
102
What happens when a graft of an AER is transplanted onto the dorsal surface of a developing limb bud?
Ectopic outgrowth is induced as another growth axis is introduced by the new AER.
103
What is the major signal associated with the AER?
The fibroblast growth factors, specifically FGF-8 throughout the ridge and FGF-4 in the posterior region
104
How is it known that the AER uses FGF signaling?
A time-releasing bead of FGF-4 introduced to a developing limb can mimic the effect of the AER
105
In the absense of the progress zone and ZPA, what happens to the AER?
It loses its identity and function. This shows the interplay between the AER, progress zone and ZPA.
106
Which will have the more properly developed limb: AER removed after 1.5 days or AER removed after 4 days?
The AER removed after 4 days will have a more developed limb with a completely patterned stylopod and zeudopod. The time of the AER removal dramatically changes the structure of the resulting limb.
107
Can an FGF-8 signal compensate for the removal of the AER?
Yes. FGF-8 soaked beed applied to a limb without an AER results in almost normal limb development.
108
If a graft from the posterior ZPA to the anterior of another limb will cause what?
A mirror image duplication of the digits will occur. 4-3-2-2-3-4
109
After a graft of ZPA to the anterior side, are the cells of the new digits from the donor bud or the host bud?
The cells in the new digits are from the host bud, indicating that the transplanted ZPA actually reorganizes the recipient tissue.
110
What are the two major candidates for the diffusible signal associated with ZPA signaling?
1) Retinoic acid (RA) 2) Sonic hedgehog (Shh)
111
Why is Shh a good candidate for the posterior morphogen of the limb bud?
It is expressed in the posterior portion of the limb bud at the correct time.
112
What will an Shh coated bead implanted on flank mesoderm cause?
A limb bud will form showing that the Shh is responsible for the ZPA activity. Wherever an Shh bead is implanted becomes the ZPA.
113
How does a population of mesenchyme know that they are in the correct location along the long axis of the body?
The HOX code.
114
Which Somites correspond with wing? Which correspond with leg?
Somites 15-20 =wing. Somites 26-32 = leg.
115
What are Tbx genes and what do they do?
Tbx genes are the transcription factors activated by Hox gene expression within the limbs. Tbx 5 is associated with wings, and Tbx 4 is associated with legs. They both stimulate Fgf10 leading to Fgf8 and limb development.
116
Would a wing-like limb be expected to express more Tbx-4 or Tbx-5?
Tbx-5 is associated with wings, and would be expected to be present in a higher concentration.
117
Does the Hox code completely specify what limb will develop where?
No. The Tbx expression also plays a role. Tissues patterned by the Hox code are not completely specified and can be tricked into forming alternate limb structures by introduction of Tbx signaling.
118
Describe the Tbx4 and Tbx5 content of a leg-like mosaic limb.
The limb will express Tbx4 in greater concentration than Tbx5, but both will be present.
119
What will the transplantation of leg bud mesenchyme to wing bud cause?
As the bud emerges, proximal structures are already patterned, so the stylopod and zeudopod will be normal. The autopod would develop with leg like features.
120
Does the process of organ formation use the same mechanisms for developing as limbs do?
Yes. The same signaling molecules act to give cells positional identity and lead to proper development. Organs are just sacks of mesenchyme during development just like limbs.
121
What controls the D/V axis patterning?
Wnt signaling in the ectorderm specifies D/V limb patterning by local gene expression. Transplanting left limb to right limb flips D/V axis, but not A/P or P/D axes
122
What are the main signaling molecules associated with the dorsal and ventral axis in limb development?
Wingless-7a (Wnt7a) is dorsal signal. Engrailed is ventral signal.
123
What does the knockout of Wnt7a1 and Wnt7a2 cause?
A homeotic transformation in which all dorsal identity is lost and replaced by ventral identity. "The dorsal side has been ventralized"
124
How are digit and tendon location and identity encoded?
They are encoded by the Hox genes. The Hoxa cluster gives proximal-distal identity (Hoxa9=prox, Hoxa9-13=dist). The Hoxd cluster and [Shh] gives anterior-posterior identity (Hoxd9, low [Shh]=anterior Hoxd9-13, high [Shh]=posterior)

Development (2 decks)

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