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Year 2 Semester 2 Vertebrate Development > Organogenesis (Prior) > Flashcards

Organogenesis (Prior) Flashcards

1
Q

`What is organogenesis?

A

Organogenesis is the process by which organs arise from one of three germ layers during the later stages of embryonic development

This requires input from genetic programs, cell-cell interactions, and mechanical forces

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2
Q

What are the timings of blastocyst and gastrula formation in mouse and humans?

A

Mouse:
Blastocyst - E3.5
Gastrula - E6.5-7.0

Human:
Blastocyst - Day 5
Gastrula - Week 2-3

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3
Q

What do blastocysts and gastrula contain?

A

Blastocysts contain the ICM which gives rise to the 3 germ layers and the embryo proper:

  • Endoderm
  • Mesoderm
  • Ectroderm

They also contain trophectoderm, which are the outer cells that contribute to extra embryonic tissue

Gastrula contains the embryonic disc which encompasses the 3 germ layers

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4
Q

What organs do the three germ layers give rise to?

A

Endoderm:
lungs / intestines / thyroid / pancreas / bladder

Mesoderm:
heart / blood / bones / muscles / kidneys

Ectroderm:
epidermis / nervous system / eyes / inner ears

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5
Q

What is the route the ICM takes to produces its derivatives?

A
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6
Q

What is the process of heart development?

A

The vascular system-the heart, blood vessels, and blood cells- is the first organ system to develop in vertebrate embryos.

Early development - oxygen and nutrients need to be delivered to the rapidly developing tissues and subsequently to developing organs.

The heart begins development from the first mesodermal cells which migrate from the site of gastrulation toward the anteriolateral border of the trilaminar embryonic disc.

While migrating, these mesodermal cells will be rendered competent for differentiation toward the cardiac lineage

Day 21/22 the tubular heart starts to beat

At the beginning of the fourth week of development (CS10) the straight heart tube undergoes looping

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7
Q

When does the eye begin to develop?

A

Embryo at the end of 4 weeks of development showing the otic and optic vesicles

The vertebrate eye is essentially an extension of the forebrain

It extends from the ectoderm

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8
Q

What is the master gene for eye development?

A

Pax6

E.g.
The ectopic expression of mouse Pax6 in a Drosophila antennal disc results in compound eye structures developing on the antenna

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9
Q

Mouse timings and process of endodermal organogensis

A

E7: Endoderm specification:
- Endodermal tube is formed

E8: Hepatic specification
- Hepatoblasts
- Liver progenitors in posterior foregut

E9: Liver bud growth

Depending on where along the anterior posterior axis the progenitors sit, different organs will arise

Anterior foregut
- Lungs
Posterior foregut
- Liver
- Pancreas
Midgut
- Small intestine
Hindgut
- Colon

The liver and pancreas originate from adjacent regions of the definitive endoderm that gives rise to three diverticula (buds)

Formation of the organ buds and their outgrowth from the foregut endoderm are controlled by mesodermal signals.

These specify endoderm cells to hepatic and pancreatic fates and promote proliferation of the budding cells.

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10
Q

Pancreas development

A

The pancreas arises from ventral and dorsal endodermal buds located caudally to the liver

Cell proliferation rapidly generates a population of multipotent progenitors that differentiate to the mature epithelial cell types: endocrine, acinar, and ductal cells.

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11
Q

Do organs develop in isolation

A

No, organs do not develop in isolation

The gallbladder, hepatic, cystic, and common bile ducts, develop from the same endodermal region as the ventral pancreas.

As the ventral pancreas starts rotating around the gut to merge with the dorsal pancreas, the extrahepatic biliary tract develops separately from the pancreas while maintaining a connection with the common pancreatic duct.

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12
Q

What are the two main functional cells in the liver?

A

Hepatoblasts give rise to:
- Hepatocytes
- Ductal cells

Hepatocytes are responsible for detoxification and bile production

Ductal cells are bile ducts that collect the bile ducts produced by hepatocytes and take it away for digestion

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13
Q

How was the liver progenitor identified?

A

Using a process called lineage tracing

Lineage tracing is where in certain cells, you can turn on a florescent reporter

This was done in the liver at E9.5 in mice, at a gene called Lgr5+ using the reporter “TdTomato” (red)

The embryo is then left until E11.5

At E11.5 red cells were found in the liver

This tells us there is a progenitor in the liver at E9.5 that is positive for the gene Lgr5+

The embryo was then left to be born, and after a month red cells were located in the liver including hepatocytes and ductal cells

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14
Q

What happens if progenitors are destroyed in the liver and the pancreas?

A

Using a specific transgenic mouse, embryonic liver and pancreas progenitors can be targeted for destruction

After such treatment, the embryonic liver grows back to a normal size, indicating that it does not arise from a fixed number of cells

In contrast, if some of the cells of the pancreas in a mouse embryo are destroyed (by the technique described above) after the pancreatic ‘bud’ has formed, a smaller than normal pancreas develops.

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15
Q

What are planarians?

A

Planarians are tiny flatworms with the ability to regrow the entire organisms

Planarians contains lots of multipotent progenitors that reside within the adult called neoblasts

These neoblasts are able to migrate to the site of injury to differentiate into the required materials

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16
Q

What are hydra?

A

Hydra have similar regeneration to planarians but use a different technique

Although Hydra is devoid of pluripotent stem cells, it has three stem cell types (ectodermal and endodermal epithelial stem cells and interstitial stem cells) throughout the body

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17
Q

What is regeneration?

A

Regeneration is the reactivation of developmental mechanisms in postembryonic life to restore missing or damaged tissues

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18
Q

Do mammals exhibit regeneration?

A
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19
Q

Do mammals exhibit regeneration?

A

Mammals have a very limited regenerative capacity

Severe damage to tissues or organs (e.g., hearts, limbs, or spinal cords) does not induce regenerative responses but rather induces healing concomitant with fibrotic scarring

However, they can regenerate certain structures

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20
Q

How does liver regeneration occur?

A

In the mammalian liver, no regenerating blastema is formed

The liver regenerates the same volume as it lost. Acute damage -each cell appears to generate its own cell type

A reserve population of multipotent progenitor cells divides when these tissues cannot regenerate the missing portion

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21
Q

How do somatic stem cells know when its time to regenerate?

A

When cells are damaged, they release soluble chemoattractants, such as chemokines, which recruit somatic stem cells to the site of injury

These cells may then differentiate into the target tissue cell type

Somatic stem cells can induce local changes, such as the generation of new blood vessels that promote wound healing

The regenerative capacity of endogenous somatic stem cells may not always be sufficient to repair diseased or damaged tissue

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22
Q

What things are required to grow tissues in a lab?

A
  • A 3D cellular cluster that is derived from stem/progenitor cells that are able to self-renew and self-organise
  • A similar cellular architecture to primary tissue that is genetically stable
  • Developmental signals to direct the developmental pathW
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23
Q

What is an organoid?

A

Organoid is an artificial, self-organising tissue that resembles an organ

Being like an organ, it must satisfy criteria that also define an organ

2 main types:
- Pluripotent stem cell derived
- Adult derived

They generally contain multiple cell types but can contain just 1

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24
Q

How can an organoid be grown?

A

Organoids can be generated from adult stem cells.
They can also be generated from PSCs by directed differentiation

In both cases, stem cells or progenitors are grown in an environment that mimics their in vivo counterpart, allowing for self-organisation into a tissue with a remarkably similar structure to that in vivo

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25
Q

What are the easiest organoids to make?

A

Ectrodermal tissues are the easiest to make

If you place the 3D cell aggregate and don’t do anything with it, cerebral organoids will be generated

The use of growth factors and small molecules such as dual-SMAD will drive brain-region specific orgnaoid generation

The use of Wnt inhibition + dissolved MG will result in a retinal organoid

The use of Wnt inhibition + dissolved MG + low FGF and BMP inhibiton results in inner ear organoids

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26
Q

Kidney organoid generation

A

Early activation of Wnt and FGF9 drives kidney organoid generation

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27
Q

What are some adult tissue derived mesodermal organoids that can be generated?

A

Bone
- -serum
- +TGFß1

Fallopian tube
- Wnt
- RSP01
- EGF
- FGF10
- noggin

Endometrial
- RSPO1
- EGF
- FGF10
- noggin
- TGFß inhibitor

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28
Q

What are some PCS derived endodermal tissue organoids?

A

Gut
- EGF
- noggin
- RSPO

Thyroid
- EGF
- FGF
- LGF
- insulin
- TSH

Stomach
- EGF
- noggin
- RA
- (Wnt)

Liver bud
- HGF
- dexamethasone
- oncostatin

Lung
- noggin
- FGF
- Wnt
- TGFB1

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29
Q

What are some adult-tissue derived endodermal tissue organoids?

A

Gut
- RSPO
- EGF
- noggin
- TGFB1

Stomach
- RSPO
- EGF
- noggin
- FGF
- TGFB1

Pancreas
- RSPO
- EGF
- noggin
- FGF
- TGFB1

Liver
- RSPO
- EGF
- HGF
- FGF
- TGFB1
- Nic
- FSK

Lung
- RSPO
- noggin
- FGF
- TGFB1
- Nic
- p38i
- RhoKi

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30
Q

How are intestinal organoids formed?

A

By isolating Lgr5+ stem cells from the crypts of the intestinal epithelium and growing in matrigel with the correct growth factors, an organoid with all of the required cell types for the intestines will be generated

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31
Q

How are pancreas organoids formed?

A

Duct within pancreas is isolated and grown within matrigel with correct growth factors

Organoid generated is a single layered structure, the same as would be in an in vivo pancreatic duct

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32
Q

What are some reasons to form pancreatic organoids?

A

Investigate biology of healthy tissue

Model diseases

Investigate cell therapy to cure pancreatic diseases

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33
Q

Liver hepatocyte organoid generation

A

Liver ducts can be isolated and grown in 3D environment to form a ductal liver organoid

This can then be exposed to growth factors to differentiate into hepatocytes

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34
Q

Liver organoid generation overview

A
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35
Q

What is a Body Plan?

A

A body plan is the general structure, arrangement, and organization of the different parts of an organism.

Includes aspects such as symmetry (bilateral, radial, etc.), the presence and type of body cavity (coelom, pseudocoelom, acoelomate), segmentation, and limb disposition.

Determines how an organism’s body is organized and functions, influencing its development, evolution, and adaptation to the environment.

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36
Q

Where does a chick embryo develop?

A

In an egg, consisting of an eggshell, yolk, and an embryonic disk on top of the yolk.

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37
Q

What is the area of cytoplasm on top of the yolk called, and what are its two main regions?

A

It is the area of the small disk of cytoplasm, with the area pellucida (gives rise to the embryo) and the area opaca (does not give rise to the embryo).

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38
Q

What is the significance of the posterior marginal zone?

A

It is where the Koller’s sickle forms, initiating the primitive streak, which determines the anterior-posterior axis.

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39
Q

What is the primitive streak, and why is it important?

A

The primitive streak is a structure that extends from the posterior marginal zone towards the midline of the area pellucida, forming Hanson’s node, which acts as the organizer and establishes the anterior-posterior axis.

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40
Q

Describe Koller’s sickle and its role in development.

A

Koller’s sickle is a structure at the posterior margin of the chick embryo that initiates the formation of the primitive streak, crucial for axis development.

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41
Q

What happens as the primitive streak regresses in the chick embryo?

A

The neural tube is laid down, and the mesoderm on either side begins to segment into somites, forming the segmented body plan.

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42
Q

What is Hanson’s node, and why is it important?

A

Hanson’s node is the condensation of cells at the anterior end of the primitive streak, serving as the primary organizer in the embryo.

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43
Q

What did graft experiments using quail and chick embryos demonstrate?

A

Early quail organizer transplanted into chick embryos induces the formation of a body axis, showing the organizer’s potency to induce embryonic cells.

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44
Q

What do the results of graft experiments reveal about early vs. late organizers?

A

Early organizers can induce extensive body structures (head, trunk, tail), while later organizers primarily induce tail structures due to decreased potency.

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45
Q

How does gravity influence the formation of the posterior marginal zone in chick embryos?

A

The rotating egg during development positions the lighter components (embryonic disk) at the top, specifying the posterior marginal zone.

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46
Q

What were the results of the space shuttle experiment on chick embryo development?

A

In low gravity conditions, most embryos did not develop normally due to the lack of proper posterior marginal zone formation; only one out of eight embryos developed normally in space.

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47
Q

Describe the process of left-right asymmetry establishment in chick embryos.

A

The left side is specified by Sonic Hedgehog, which activates Caronte, BMP, and then Pitx2, while the right side is inhibited by activin, preventing Sonic Hedgehog signaling.

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48
Q

What does in situ hybridization show about left-right asymmetry in chick embryos?

A

In situ hybridization shows that Caronte is found on the left side when viewed ventrally, indicating the left side specification by Sonic Hedgehog pathway.

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49
Q

How is left-right asymmetry established in other vertebrates like fish and amphibians?

A

It’s influenced by the flow of embryonic fluids driven by cilia, creating a vortex that determines the left side.

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50
Q

What role do human embryonic stem cells play in similar experiments?

A

When cultured to form an organizer-like structure and transplanted into chick embryos, they can induce axis formation, demonstrating cross-species induction potential.

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51
Q

Explain the significance of gravity in chick embryo development.

A

Gravity ensures the proper formation of the posterior marginal zone by positioning the lighter embryonic disk at the top during the egg’s rotation through the oviduct.

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52
Q

What happens when chick embryos develop in low gravity or no gravity environments?

A

The absence of gravity leads to improper formation of the posterior marginal zone, resulting in abnormal development of most embryos.

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53
Q

How does the primitive streak establish the body plan in chicks?

A

It sets up the anterior-posterior axis, and as it regresses, it helps lay down the neural tube and segment the mesoderm into somites.

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54
Q

What is the significance of the organizer’s potency?

A

Early organizers can induce extensive body structures (head, trunk, tail), while later organizers primarily induce tail structures due to decreased potency.

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55
Q

Explain the concept of the organizer in human and chick embryo experiments.

A

The organizer from human embryonic stem cells, when transplanted into chick embryos, can induce axis formation, showcasing evolutionary conservation of developmental mechanisms.

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56
Q

Describe the role of Koller’s sickle in primitive streak formation.

A

Koller’s sickle marks the posterior end of the chick embryo and initiates the formation of the primitive streak, which is essential for setting up the body axes.

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57
Q

How do cilia contribute to left-right asymmetry in vertebrates?

A

Cilia in vertebrate embryos create a fluid flow that establishes the left-right axis by moving embryonic fluids and determining the left side.

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58
Q

How is the anterior-posterior axis formed in chick embryos?

A

The axis is established by the primitive streak, which originates from the posterior marginal zone, with Hanson’s node forming at the anterior end and regressing to lay down the neural tube and somites.

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59
Q

What experimental evidence supports the role of the organizer in axis formation?

A

Graft experiments, such as transplanting quail organizer cells into chick embryos, demonstrate the ability of the organizer to induce axis formation and embryonic development.

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60
Q

What are the implications of the space shuttle experiment for understanding embryo development?

A

The experiment shows that gravity is crucial for proper posterior marginal zone formation and subsequent embryo development, highlighting the role of physical forces in developmental biology.

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61
Q

How does Sonic Hedgehog influence left-right asymmetry in chick embryos?

A

Sonic Hedgehog activates Caronte on the left side, which then activates BMP and Pitx2, leading to left-side specification. On the right side, activin inhibits Sonic Hedgehog, preventing this pathway.

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62
Q

What was observed in embryos with left-right asymmetry using in situ hybridization?

A

In situ hybridization showed that Caronte expression is localized to the left side, confirming the role of Sonic Hedgehog in left-right asymmetry.

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63
Q

What is the outcome of transplanting early and late organizers in quail and chick graft experiments?

A

Early organizers induce the formation of head, trunk, and tail structures, while late organizers primarily induce tail structures. This demonstrates the higher potency of early organizers.

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64
Q

What is the importance of the left-right asymmetry in vertebrates?

A

Left-right asymmetry ensures the correct positioning of organs such as the heart and liver, which are essential for proper physiological functioning.

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65
Q

How did the space shuttle experiment test the effects of low gravity on embryo development?

A

A: Embryos were fertilized and developed in low gravity conditions aboard the Endeavour Space Shuttle, and the results showed that most embryos did not develop normally without proper gravity.

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66
Q

How is the neural tube formed in chick embryos?

A

The neural tube forms as the primitive streak regresses, laying down the neural tissue that will develop into the central nervous system.

67
Q

What role does the posterior marginal zone play in establishing body axes in chick embryos?

A

The posterior marginal zone is crucial for the formation of the primitive streak, which sets up the anterior-posterior axis and initiates body axis development.

68
Q

What experimental techniques are used to study left-right asymmetry in chick embryos?

A

In situ hybridization is used to visualize the expression of genes like Caronte, which are involved in specifying left-right asymmetry.

69
Q

What is the role of Spemann’s organizer in Xenopus embryos?

A

Spemann’s organizer is located at the dorsal lip of the blastopore and induces the anterior-posterior axis. It can induce a new body axis when transplanted to another location or a different host embryo.

70
Q

How is the anterior-posterior axis determined in Xenopus embryos?

A

The axis is determined by the site of sperm entry, which induces cortical rotations and sets up the organizer on the opposite side of the embryo, becoming the dorsal side.

71
Q

What induces cortical rotations in Xenopus embryos?

A

The fusion of the sperm with the egg induces cortical rotations, which help determine the location of the organizer.

72
Q

How does the potency of the organizer change over time in Xenopus embryos?

A

The organizer’s potency changes spatially and temporally. Early organizer grafts induce anterior structures, while later grafts induce more posterior structures like the trunk and tail.

73
Q

What structures are formed when different regions of the organizer are grafted at early gastrulation?

A

The anterior-most part forms head structures with balancers; slightly posterior parts form head structures with eyes; further posterior parts form trunk structures, and the most posterior parts form only trunk structures.

74
Q

What is the significance of segmentation in vertebrate embryos?

A

Segmentation creates specific sections of cells that can be specified with different regional identities, forming structures like vertebrae.

75
Q

What are the characteristics of segments in simpler animals like earthworms?

A

In simpler animals, segments are repeating units along the body, each containing similar types of cells and having the same possibilities as each other.

76
Q

How does segmentation in vertebrates differ from that in simpler animals?

A

Vertebrates have a segmented body plan where segments can specialize. This is most evident in the skeleton, allowing vertebrates to adapt and modify segments for different functions.

77
Q

What are somites, and what is their role in segmentation?

A

Somites are transient structures that form from an anterior to posterior sequence in the embryo, giving rise to vertebrae and other segmented structures.

78
Q

What does it mean when somites are described as transient structures?

A

Somites exist temporarily during embryonic development and are used to form permanent structures like vertebrae, after which they are no longer present.

79
Q

What is the role of Hox genes in segment identity?

A

Hox genes are transcription factors that specify the identity of segments in the embryo. They are arranged in clusters and expressed in a collinear manner, reflecting their order on the chromosome.

80
Q

How are Hox genes organized and expressed in Drosophila and Tetrapod vertebrates?

A

In Drosophila, there is one Hox cluster with eight genes expressed collinearly. In Tetrapod vertebrates, there are four Hox clusters (A, B, C, D) due to duplication events, and they are also expressed collinearly.

81
Q

How does anterior repression affect Hox gene expression?

A

Later Hox genes repress the expression of earlier Hox genes, creating clear boundaries of expression. For example, HoxD4 represses HoxD3, resulting in a defined anterior boundary.

82
Q

What does in situ hybridization reveal about Hox gene expression in mouse embryos?

A

In situ hybridization shows the spatial expression patterns of Hox genes. For example, HoxB1 is expressed anteriorly, HoxB4 in the trunk, and HoxB9 posteriorly, each with distinct anterior boundaries.

83
Q

How does Hox gene expression differ between species like chicks and mice?

A

Differences in Hox gene expression patterns result in anatomical differences. For example, the cervical region is longer in chicks than in mice due to broader expression of certain Hox genes in the chick.

84
Q

What is a homeotic transformation, and how is it demonstrated in Drosophila?

A

A homeotic transformation is when one body part develops into another due to changes in Hox gene expression. In Drosophila, this can result in legs forming where antennae should be if antenna-specific Hox genes are replaced by leg-specific ones.`

85
Q

How can homeotic transformations be induced in vertebrates like mice?

A

By knocking out specific Hox genes, researchers can induce transformations. For example, knocking out Hox10 results in extra ribs, indicating a transformation of lumbar to thoracic identity.

86
Q

What happens when Hox10 is knocked out in mice?

A

Knocking out Hox10 results in a homeotic transformation where lumbar vertebrae acquire thoracic identity, leading to extra ribs.

87
Q

What happens when Hox11 is knocked out in mice?

A

Knocking out Hox11 results in an extension of lumbar vertebrae, indicating its role in specifying sacral identity.

88
Q

Why is it important to specify species when discussing vertebrate development?

A

Different species have unique mechanisms for establishing body axes and asymmetry, so specifying species ensures accurate understanding and comparison.

89
Q

How does the sperm entry point determine the dorsal side in Xenopus embryos?

A

The site of sperm entry induces cortical rotations, and the organizer forms on the opposite side, establishing the dorsal side.

90
Q

What is the role of Hox genes in Drosophila compared to Tetrapod vertebrates?

A

In Drosophila, a single Hox cluster of eight genes determines body segment identity. In Tetrapod vertebrates, four Hox clusters (A, B, C, D) due to gene duplications control segment identity in a similar collinear fashion.

91
Q

What is anterior repression, and why is it important?

A

Anterior repression is when later-expressed Hox genes inhibit earlier Hox genes, ensuring clear boundaries and specific segment identities.

92
Q

How do Hox gene expression patterns affect segment identity in vertebrates?

A

Hox genes specify segment identity along the anterior-posterior axis. Changes in their expression can lead to transformations, altering segment identities and resulting in anatomical differences.

93
Q

How do Hox genes control vertebral identity in mice?

A

Hox genes determine the identity of vertebrae. For example, Hox10 specifies lumbar vertebrae, and its knockout results in extra thoracic vertebrae with ribs.

94
Q

What is the significance of homeotic transformations in understanding Hox gene function?

A

Homeotic transformations, where one body part is replaced by another, demonstrate the role of Hox genes in specifying segment identity and provide insights into developmental processes.

95
Q

How do Hox genes exhibit collinearity in expression?

A

The order of Hox genes on the chromosome corresponds to their expression pattern along the anterior-posterior axis, a phenomenon known as collinearity.

96
Q

What does the expression pattern of Hox genes reveal about their function?

A

The spatial and temporal expression of Hox genes determines segment identity, with distinct anterior boundaries and overlapping expression domains contributing to diverse structures.

97
Q

How does the expression of HoxB1, HoxB4, and HoxB9 differ in mouse embryos?

A

HoxB1 is expressed anteriorly, HoxB4 in the trunk region with a clear anterior boundary, and HoxB9 more posteriorly, showing the collinear expression pattern.

98
Q

What is the role of Hox clusters in tetrapod vertebrates?

A

Tetrapod vertebrates have four Hox clusters (A, B, C, D) due to gene duplication events, which control the development of different body segments in a collinear manner.

99
Q

How does anterior repression create clear expression boundaries for Hox genes?

A

Later-expressed Hox genes inhibit the expression of earlier ones, creating distinct boundaries and ensuring specific segment identities.

100
Q

How do Hox genes contribute to the formation of cervical and thoracic vertebrae in chicks and mice?

A

Hox genes like Hox5 and Hox6 have broader expression in chicks, leading to longer necks. Differences in Hox gene expression result in anatomical variations between species.

101
Q

What happens if Hox gene expression is altered during vertebrate development?

A

Altering Hox gene expression can change segment identities, leading to transformations such as extra ribs or altered vertebral identities.

102
Q

What is the significance of studying Hox gene expression in different species?

A

Comparing Hox gene expression patterns across species helps understand the evolutionary and developmental mechanisms that lead to anatomical diversity.

103
Q

What is the significance of segmentation in animals?

A

Segmentation allows for the division of the body into repeating units, enabling specialization and adaptation of different segments. This is seen in both invertebrates (e.g., earthworms and millipedes) and vertebrates.

104
Q

What are the characteristics of segments in simpler animals like earthworms?

A

In simpler animals, segments are repeating units along the body, each containing similar types of cells and having the same possibilities as each other.

105
Q

How does segmentation in vertebrates differ from that in simpler animals?

A

Vertebrates have a segmented body plan where segments can specialize. This is most evident in the skeleton, allowing vertebrates to adapt and modify segments for different functions.

106
Q

What are somites and what process forms them?

A

Somites are blocks of mesoderm that segment along the body of an embryo. The process of their formation is called somitogenesis.

107
Q

Describe the direction and process of somitogenesis.

A

Somitogenesis proceeds in an anterior to posterior direction, with periodic budding off of somites from the presomitic mesoderm flanking the neural tube.

108
Q

What structures do somites give rise to?

A

Somites develop into vertebrae, ribs, the dermis of dorsal skin, skeletal muscles of the back, and skeletal muscles of the body and limbs.

109
Q

What is the role of the paraxial mesoderm?

A

The paraxial mesoderm, adjacent to the notochord, gives rise to the head structures and somites.

110
Q

What are the four distinct zones of the mesoderm?

A

The mesoderm is divided into four zones: notochord, paraxial mesoderm, intermediate mesoderm (forming gonads, kidneys, adrenals), and lateral plate mesoderm (forming lymph nodes, heart, etc.).

111
Q

What is the presomitic mesoderm?

A

The presomitic mesoderm is the unsegmented mesoderm located posteriorly, which will eventually form somites.

112
Q

How is the formation of somites regulated?

A

The formation of somites from the paraxial mesoderm is regulated by signalling factors such as sonic hedgehog from the notochord and Wnt signalling from the ectoderm.

113
Q

What is the “clock” gene expression in somitogenesis?

A

The “clock” gene expression involves a wave of cyclic gene expression moving from the posterior to the anterior, with cells turning genes on and off in a synchronised, phase-shifted manner.

114
Q

What is the role of the determination front in somitogenesis?

A

The determination front is the level at which a somite will form, determined by gradients of retinoic acid and FGF/Wnt.

115
Q

What is the Notch signalling pathway’s role in somitogenesis?

A

Notch signalling is crucial for synchronous activity of adjacent cells and formation of somite boundaries through juxtacrine signalling.

116
Q

How does the presomitic mesoderm’s positional identity affect somite formation?

A

The presomitic mesoderm retains its positional identity, forming corresponding structures even when transplanted to a different stage embryo.

117
Q

What determines the identity of somite segments?

A

Hox genes determine the identity of somite segments, specifying different regions along the anterior-posterior axis.

118
Q

What is Spondylocostal Dysostosis (SCD)?

A

SCD is a condition resulting from defects in somitogenesis, leading to skeletal and muscular deformities due to asynchronous formation of somites.

119
Q

How does cyclic gene expression timing differ among vertebrates?

A

The timing varies: 90 minutes in chicks, 2.5 hours in mice, 30 minutes in zebrafish, and 5-6 hours in humans, using similar molecular factors.

120
Q

How does somitogenesis in corn snakes differ from that in mice?

A

Corn snakes have multiple waves of cyclic gene expression travelling along the PSM, unlike the single wave seen in mice.

121
Q

What structures in the vertebrate head exhibit segmentation?

A

Segmentation in the head includes pharyngeal arches, and segmented structures in the hindbrain, controlled by Hox genes.

122
Q

How is somitogenesis studied in vitro?

A

Through culturing mouse embryonic stem cells under specific conditions, forming gastruloids that recapitulate somite formation.

123
Q

What are the key pathways involved in somitogenesis?

A

Notch, Wnt, and FGF signalling pathways, along with gradients of retinoic acid and FGF/Wnt, are key in somitogenesis.

124
Q

What is the importance of the clock and wavefront model in somitogenesis?

A

It is crucial for understanding the periodic and synchronized formation of somites and the role of gene oscillations.

125
Q

How do somites partition into different areas during development?

A

Somites partition into sclerotome (vertebrae, ribs), myotome (skeletal muscles), and dermatome (dermis)

126
Q

How does somitogenesis proceed in different species?

A

Although the underlying mechanisms are similar, the timing of somite formation varies, being faster in some species (e.g., zebrafish) and slower in others (e.g., humans).

127
Q

What experiment demonstrates the early specification of somite identity?

A

Transplanting the PSM from a later-stage embryo to an earlier stage embryo, showing that somites retain their original identity (e.g., thoracic segments forming ribs in the cervical region).

128
Q

What is the role of Mesp2 in somite boundary formation?

A

Mesp2 is activated in the presumptive somite region and induces Ephrin-A4 expression, leading to epithelialisation and boundary formation.

129
Q

How do Hox genes influence the hindbrain segmentation?

A

Hox genes specify the identity of different segments (rhombomeres) in the hindbrain, similar to their role in trunk segmentation.

130
Q

How can a 2D system for studying somitogenesis be created from a 3D system?

A

By cutting posteriorly to include progenitors, placing them on a dish where they migrate laterally and maintain cyclic gene activity, forming a 2D culture.

131
Q

What is the significance of studying somitogenesis using in vitro models like gastruloids?

A

In vitro models allow for detailed observation and manipulation of developmental processes, providing insights into gene function and regulatory mechanisms.

132
Q

Summarize the key points of vertebrate segmentation and somitogenesis.

A

Segmentation in vertebrates involves the formation of somites from the PSM, regulated by a clock and wavefront model. Notch, Wnt, and FGF pathways, along with Hox genes, play critical roles in determining somite identity and segmentation patterns.

133
Q

What are rhombomeres?

A

Rhombomeres are segments of the hindbrain that are defined by specific Hox gene expression, similar to the segmentation in the vertebrate trunk.

134
Q

What are the different regions formed from mesodermal segmentation?

A

Notochord, axial mesoderm (somites), intermediate mesoderm (gonads, kidneys), and lateral plate mesoderm (lymph nodes, heart).

135
Q

How do the gradients of retinoic acid and FGF/Wnt contribute to somitogenesis?

A

These gradients determine the termination front, where somites form, by providing positional information through opposing concentration gradients.

136
Q

How do cyclic genes exhibit oscillatory expression patterns?

A

Cyclic genes turn on and off in a synchronised, phase-shifted manner, creating a wave of expression that moves anteriorly from the posterior.

137
Q

How does the process of epithelialisation contribute to somite formation?

A

Epithelialisation, induced by Ephrin-A4, leads to the formation of clear morphological boundaries between somites from the unsegmented mesoderm.

138
Q

Why is understanding the clock gene regulation important in developmental biology?

A

It helps elucidate the mechanisms of periodic and synchronized cell differentiation, essential for proper body segmentation and development.

139
Q

What are the implications of Notch pathway mutations in humans?

A

Mutations in the Notch pathway can lead to conditions like Spondylocostal Dysostosis (SCD), causing skeletal and muscular deformities due to disrupted somitogenesis.

140
Q

What is the relationship between the Notch, Wnt, and FGF pathways in somitogenesis?

A

These pathways show crosstalk and oscillatory behaviour, working in conjunction to regulate somite formation and segmentation.

141
Q

How does the Notch pathway’s juxtacrine signalling mechanism work?

A

Notch signalling requires physical contact between adjacent cells, with the Notch receptor on one cell interacting with its ligand on another cell, leading to cleavage and gene expression.

142
Q

What is the role of progenitors in the tailbud during somitogenesis?

A

Progenitors in the tailbud drive the elongation of the embryo, contributing to the anteriorly shifting positions of cells as somites form posteriorly.

143
Q

How do somites influence the development of skeletal muscles and dermis?

A

Somites differentiate into the myotome (skeletal muscles) and dermatome (dermis), contributing to the formation of these structures in the developing embryo.

144
Q

How do researchers visualise cyclic gene expression in somitogenesis studies?

A

Researchers use transgenic lines expressing fluorescent markers for specific clock genes (e.g., lunatic fringe) and observe the dynamic expression patterns under a microscope.

145
Q

What is the significance of multiple waves of cyclic gene expression in some vertebrates?

A

Multiple waves allow for more rapid segmentation, seen in highly segmented creatures like corn snakes, providing insights into the diversity of segmentation mechanisms.

146
Q

How does early specification of somite identity contribute to vertebrate development?

A

Early specification ensures that somites form in the correct order and position, crucial for the proper development of the vertebral column and associated structures.

147
Q

What are gastruloids and how are they used in developmental studies?

A

Gastruloids are in vitro models derived from pluripotent stem cells that mimic early embryonic development, allowing researchers to study processes like somitogenesis in a controlled environment.

148
Q

How do different species exhibit variations in the timing of somite formation?

A

Different species show variations in the timing of somite formation due to differences in the regulatory mechanisms of cyclic gene expression, despite using similar molecular pathways.

149
Q

What is the importance of specifying the species when discussing cyclic gene expression?

A

Specifying the species ensures accuracy, as different species may have variations in the number and timing of waves of cyclic gene expression during somitogenesis.

150
Q

What is the role of Sox9 in somite boundary formation?

A

Sox9 labels the caudal half of the somite, indicating segmentation within the somite itself and contributing to the formation of clear boundaries.

151
Q

What challenges do researchers face when studying somitogenesis in large 3D structures?

A

Working with large 3D structures is difficult due to their complexity and size, making it challenging to observe and manipulate specific developmental processes.

152
Q

How can converting a 3D system into a 2D system aid somitogenesis research?

A

Converting a 3D system into a 2D system allows for easier observation and measurement of gene expression patterns and cell movements, facilitating detailed studies of somitogenesis.

153
Q

How does the periodicity of somite formation differ among species like zebrafish, chick, mouse, and human?

A

In zebrafish, somites form every 30 minutes; in chick, every 90 minutes; in mouse, every 2.5 hours; and in humans, every 5-6 hours.

154
Q

What experiment demonstrates the early positional identity of the presomitic mesoderm?

A

Transplanting the PSM from an older to a younger embryo results in the formation of thoracic segments with ribs in the cervical region, showing early specification of segment identity.

155
Q

How does the process of somitogenesis correlate with neural tube closure?

A

The formation of somites occurs simultaneously with the closing of the neural tube, which is a critical process in vertebrate development.

156
Q

How do gene regulatory networks control the periodic budding of somites?

A

Gene regulatory networks involve oscillatory gene expression patterns, such as those regulated by Notch, Wnt, and FGF pathways, which control the timing and formation of somite boundaries.

157
Q

How does the combination of signalling factors influence somite differentiation?

A

Signalling factors from surrounding tissues, such as sonic hedgehog, Wnt, and FGF, influence the differentiation of somites into structures like sclerotome, myotome, and dermatome.

158
Q

How do the Notch, Wnt, and FGF pathways show crosstalk during somitogenesis?

A

These pathways interact and influence each other’s oscillatory behaviour, coordinating the formation and differentiation of somites.

159
Q

What is the role of the caudal-rostral gradient in somitogenesis?

A

The caudal-rostral gradient of FGF, Wnt, and retinoic acid determines the wavefront where somites form, integrating positional information for proper segmentation.

160
Q

How does the expression of cyclic genes vary in different cells during somitogenesis?

A

Cyclic genes exhibit phase-shifted expression patterns, with some cells expressing high levels while adjacent cells have low levels, creating a wave of gene activity.

161
Q

What is the importance of understanding the timing of somite formation across different species?

A

Understanding the timing differences helps elucidate the evolutionary and developmental mechanisms underlying vertebrate segmentation and diversity.

162
Q

How does epithelialisation contribute to the formation of somite boundaries?

A

Epithelialisation, induced by factors like Ephrin-A4, creates clear morphological boundaries between somites, essential for proper segmentation.

163
Q

How does somitogenesis ensure bilateral symmetry?

A

Somitogenesis occurs synchronously on either side of the neural tube, ensuring bilateral symmetry in the segmented structures of the vertebrate body.

164
Q

What are the potential implications of disrupted somitogenesis for human health?

A

Disrupted somitogenesis can lead to congenital deformities such as scoliosis, spina bifida, and other vertebral and muscular abnormalities.

Year 2 Semester 2 Vertebrate Development (5 decks)

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