Morphogenesis and organogenesis in animals MCQ Quiz - Objective Question with Answer for Morphogenesis and organogenesis in animals - Download Free PDF

Concept:

• Planarians are flatworms known for their remarkable regenerative abilities. When they are cut transversely, they can regenerate both a head and a tail, depending on the signaling pathways activated in the respective body parts.
• The Wnt signaling pathway is crucial for tail regeneration, and its activity is typically localized to the posterior region of the severed body segment.
• β-catenin is a key component of the Wnt signaling pathway. Its activation promotes posterior identity, which is essential for tail regeneration.
• β-catenin, which is a key effector in the Wnt signaling pathway, directly influences tail regeneration. When β-catenin is active, it can overcome the inhibition caused by abc, thus promoting tail regeneration.
• Gene abc is hypothesized to play a role in modulating this process, and its overexpression impacts the regenerative outcome.

Explanation:

Role of abc in tail regeneration:

• Overexpression of gene abc in the head piece blocks tail regeneration, suggesting that abc negatively interferes with the activity required for tail formation.
• However, when constitutively active β-catenin is overexpressed along with abc in the severed head piece, tail formation is restored, indicating that β-catenin can override the inhibitory effect of abc.

Pathway depicted in Option 1:

• Option 1 correctly shows that abc acts as an inhibitor in the tail regeneration process, but this inhibition can be counteracted by constitutive activation of β-catenin.
• In this scenario, β-catenin re-establishes posterior identity and allows the regeneration of the tail despite the presence of abc overexpression.

The correct answer is Ventralization of the embryo would occur.

Concept:

• Dorsal-ventral patterning in Drosophila embryos is a critical process that determines the spatial organization of tissues along the dorsal-ventral axis.
• The protein Gurken plays a pivotal role in this process by signaling to the follicle cells surrounding the oocyte, thereby establishing the dorsal-ventral polarity.
• Maternal gene products, including gurken mRNA and protein, are essential for ensuring the correct development of the embryo.

Explanation:

Ventralization of the embryo occurs when there is a maternal deficiency of Gurken:

• Gurken protein normally signals to the follicle cells closest to the oocyte nucleus, leading to the repression of the pipe protein in dorsal cells.
• In the absence of Gurken, pipe expression is not repressed in dorsal follicle cells, and as a result, the ventralizing signals are uniformly present throughout the embryo.
• This disrupts the normal dorsal-ventral polarity, leading to ventralization where the entire embryo develops ventral characteristics.
• Thus, Gurken is absolutely necessary for establishing dorsal identity in the follicle cells and subsequently in the embryo.

The correct answer is C

Concept:

• Fgf8 (Fibroblast Growth Factor 8) is a signaling molecule that plays a critical role in developmental processes, including cell differentiation and tissue patterning. Its concentration gradient is tightly regulated to ensure proper biological function.
• The gradient of Fgf8 is established through two key mechanisms: 
  • Diffusion of Fgf8 from a localized source.
  • Removal of Fgf8 ligand through endocytosis, which involves internalization of the ligand-receptor complex.
• Endocytosis is regulated by proteins such as Rab5C and dynamin. Rab5C is involved in the early stages of endocytosis, while dynamin is crucial for the scission of vesicles from the plasma membrane during endocytosis.
• Altering the activity of Rab5C or dynamin can disrupt the balance between Fgf8 diffusion and removal, leading to changes in the concentration gradient.

Explanation:

• Overexpression of Rab5C enhances endocytosis, which increases the rate at which Fgf8 is removed from the extracellular environment. This results in a steeper Fgf8 concentration gradient, as more Fgf8 is internalized near the source, reducing its availability for diffusion to distant regions.
• Inhibiting dynamin disrupts the endocytic process by preventing vesicle scission. As a result, the removal of Fgf8 through endocytosis is reduced, leading to a shallower gradient because Fgf8 remains in the extracellular space for longer periods and diffuses further from the source.

​ 

Rab5C overexpression enhances endocytosis, leading to faster ligand removal and a steeper gradient (lower Fgf8 levels at distances further from the source), represented by the dashed line.

Dynamin inhibition reduces endocytosis, allowing Fgf8 to remain longer in the extracellular space, resulting in a shallower gradient with higher concentrations at a distance, shown by the solid line.

This pattern matches the gradient shifts depicted in Option C.

Explanation:

• During development, specific signaling molecules and pathways play critical roles in regulating key processes such as cell differentiation, proliferation, migration, and tissue organization.
• Examples of these signaling pathways include Wnt, Hedgehog, Notch, BMP (Bone Morphogenetic Protein), and FGF (Fibroblast Growth Factor) signaling pathways.
• Each developmental process is tightly regulated by specific signaling molecules, and disruptions in these pathways can lead to developmental abnormalities or diseases.

A.  Dorsal/ventral axis specification in amphibian embryo — Wnt/β-catenin; BMP4; Activin/Nodal​

• Amphibian dorsal-ventral axis formation is regulated by pathways such as BMP and Activin/Nodal.
• Wnt/β-catenin signaling is also involved in dorsal-ventral patterning.

B. Dorsal/ventral axis specification in mammalian limb — Engrailed; Wnt/β-catenin; BMP

The dorsal/ventral axis in the mammalian limb is well known to involve:

• Engrailed-1 (En1) expressed ventrally.
• Wnt7a/Wnt/β-catenin signaling expressed dorsally.
• BMP signaling also plays a key role in limb patterning.

C. Dorsal/ventral axis specification in Drosophila oocyte — FGF; Hh; Dpp

• This option is incorrect for dorsal/ventral axis specification in Drosophila oocyte.
• The dorsal/ventral axis in Drosophila oocyte is primarily regulated by the Toll pathway and Dorsal protein gradients.
• FGF (Fibroblast Growth Factor), Hedgehog (Hh), and Decapentaplegic (Dpp) are involved in other developmental processes (e.g., patterning and segmentation), but not the primary regulators of dorsal/ventral axis in Drosophila oocyte.

D. Anterior/posterior axis specification in mammalian limb — Shh; FGF; Notch

• This option is incorrect.
• Shh (Sonic Hedgehog) plays a central role in anterior-posterior patterning of the limb.
• FGF is important in limb bud outgrowth.
• However, Notch signaling is not primarily involved in anterior-posterior axis specification in mammalian limbs. Notch signaling is more prominent in other developmental contexts like somitogenesis.

The correct answer is Blastema formation

Explanation:

• Axolotls are remarkable amphibians known for their ability to regenerate entire limbs, tails, and parts of their heart and brain after injury.
• The regeneration process involves a specialized structure called the "blastema," which is a collection of proliferating cells formed at the injury site.

Blastema Formation:

• When an Axolotl limb is severed, the wound heals quickly, and a structure called the "epidermal cap" covers the injury site.
• Beneath the epidermal cap, cells from various tissues (such as muscle, cartilage, and skin) dedifferentiate into a more primitive state and form a blastema.
• The blastema contains a pool of proliferating cells capable of differentiating into all the necessary cell types to reconstruct the limb.
• Growth factors and signaling pathways, such as FGF (Fibroblast Growth Factor) and BMP (Bone Morphogenetic Protein), play crucial roles in regulating blastema formation and subsequent limb regeneration.
• This process is highly organized, ensuring the correct spatial patterning and functionality of the regenerated limb.

Other Options:

Transdifferentiation:

• Transdifferentiation refers to the direct conversion of one differentiated cell type into another without reverting to a stem-like state.
• Although transdifferentiation may occur in some organisms during regeneration, it is not the primary mechanism for Axolotl limb regeneration. 

Induced Pluripotency:

• Induced pluripotency involves reprogramming differentiated cells into pluripotent stem cells, similar to embryonic stem cells, through artificial manipulation (e.g., introducing specific genes).
• This process is not naturally involved in Axolotl limb regeneration.

Stem Cell Dedifferentiation:

• While dedifferentiation of cells is a part of blastema formation, the process does not solely rely on stem cells.
• In Axolotl regeneration, differentiated cells from the injured tissue revert to a more primitive state and contribute to the blastema, rather than relying exclusively on stem cells.

The correct answer is Option 2 i.e. Genetic specificity of interaction.

Concept:

Determination:

• This is a term in developmental biology that refers to the point in the developmental process at which a cell or group of cells becomes committed to a particular fate.
• Once a cell's role is determined, the next steps are differentiation and morphogenesis which lead to the formation of a specific type of tissue or organ.
• However, this determination is not always irreversible.

Genetic specificity of interaction:

• This term refers to the concept that the fate of a cell or group of cells is determined by the specific genes they carry.
• Genetic specificity implies that every cell or piece of tissue carries a specific set of genetic information that governs its development and dictates what form and function it will have, regardless of its environment.
• When transplanted to a different location, these cells will still follow their inherent genetic program.

Regional specificity of interaction:

• This concept suggests that a cell's or tissue's developmental behavior and fate might depend on its location within the embryo.
• In other words, cells or tissues will interact with their immediate environment and adjust their developmental path based on these interactions.
• Here, the environment and cell-to-cell interactions can influence the developmental outcome and modify a cell's fate from what was genetically programmed.

Autonomous specification:

• This describes a mode of development in which the fate of a cell is established early on and is independent of interactions with neighboring cells.
• Essentially, certain cells in the early embryo are programmed to follow a specific developmental path, regardless of their surroundings.
• The genetic information in these cells directs them to form specific parts of the organism, even if they are isolated from the rest of the embryo or transplanted into a different region.
• This is similar to the concept of genetic specificity but places more emphasis on the independence of the developmental path from neighboring cell influence.

Explanation

• The transplanted frog tissue interacts with the newt embryo following the laws of its own species-specific developmental program.
• Despite being placed into a different region and species, the transplanted tissue commits to developing into the structure it was originally destined to become, as per its inherent genetic program.
• This shows that the developmental fate is attributed more to the genetic composition of the transplanted cells rather than the influence of their new location.
• The interaction between the gene expression in the transplanted cells and the new host's cells results in the development of frog-like mouth parts in the newt larvae, illustrating the concept of genetic specificity of the interaction.
• This is distinct from the concept of regional specificity of interaction which suggests that the fate of a cell or tissue is affected by its surrounding cells or tissues and will change its developmental path to match its new environment.
• In this case, the cells contributed to the structure they were originally "programmed" to create via genetic specification, not the structure typical of the region to which they were transplanted.

Hence the correct answer is Option 2

The correct answer is Option 3 i.e.β‐catenin will accumulate in the nuclei of all blastula cells leading to animal cells getting specified as endoderm and mesoderm.

Concept:-

• Beta-catenin acts as a transcription factor in wnt signaling.
• Beta-catenin is found throughout the embryo.
• Beta-catenin is Active on the dorsal side.
• GSK-3 inhibits Beta-catenin at the ventral side.
•  During fertilization, Dsh and Wnt 11 protein from the vegetal pole translocated to the dorsal side of the egg.
• Disheveled (Dsh) inhibits Gsk-3, thereby activating Beta-catenin.

Explanation:-

Option 1:- β‐catenin accumulation in the nuclei of large micromeres will be inhibited leading to formation of ectodermal ball.

• GSK-3 inhibits beta-catenin. if GSK-3 is blocked, beta-catenin will be free, which will enter each nucleus of the vegetal pole and will form endoderm and mesoderm.
• ​  Hence, this statement is incorrect.   

Option 2:-  β‐catenin will accumulate in the nuclei of all blastula cells leading to an ectodermal ball.

• If beta-catenin will accumulate, it will form an endoderm and mesoderm, not the ectodermal ball.
•  Hence, this option is incorrect.

Option 3:- β‐catenin will accumulate in the nuclei of all blastula cells leading to animal cells getting specified as endoderm and mesoderm.

• Beta-catenin will get accumulated in the nuclei of all blastula cells which was earlier inhibited by GSK-3. Therefore, beta-catenin will form endoderm and mesoderm.
• Hence, this option is correct.

Option 4:- β‐catenin which accumulates in the nuclei of large micromeres will be inhibited leading to animal cells getting specified as endoderm and mesoderm.

• If GSK-3 is blocked, beta-catenin will enter each nucleus of all dorsal cells, it will not get inhibited.
• Hence, this option is incorrect.

The correct answer is Option 2 i.e B and C

Concept:

Dorsal protein

• D-V polarity by the gradient of a transcription factor called Dorsal(gene product is a Morphogen)
  Dorsal protein synthesizes in cytoplasm→ Form ventral axis.
• Translocation of the Dorsal into the nuclei of ventral cells will help in the ventralization of the embryo.

Gurken gene and Torpedo

• Gurken Signal the dorsalization.
• Gurken mRNA is synthesized in oocytes and produces gurken protein whereas torpedo is active only in the somatic follicle cells.
• Gurken protein (ligand) → bind to Torpedo receptor(present in follicle cell)
• Gurken protein can diffuse only a short distance, Gurken protein reaches only those follicle cells closest to the oocyte nucleus, and it signals those cells to become the more dorsal follicle cells.

Cactus gene

• Cactus bound to Dorsal in a cytoplasm.
• After the phosphorylation of the cactus, it releases the Dorsal so that dorsal can enter the nucleus of the ventral cell for its ventralization activity.
• The absence of cactus(maternal effect gene), causes the ventralization of all cells.

Explanation:

Statement A:- INCORRECT

• In embryos that lack Gurken protein, the Dorsal protein will get translocated to the nucleus of the follicle cells which then causes ventralization of the embryo.

Statement B:- CORRECT

• The dorsal gene is a morphogen and it causes the ventral axis by morphogen gradient only, when dorsal protein gets translocated into nuclei of cells.

Statement C:- CORRECT

• The Cactus holds the dorsal in the cytoplasm, when the cactus gets phosphorylated, the dorsal gets free and translocates into nuclei of all cells and ultimately causing ventralization.

Statement D:- INCORRECT

• Dorsal causes the ventral axis not the dorsal. If dorsal will not enter into nuclei, ventralization will stop not dorsalization. 
• Gurken and torpedo are responsible for dorsalization.

Hence, the correct answer is Option 2 (B and C).

The correct answer is Option 3 i.e. Although cell death in the limb is necessary for the formation of digits and joints, it is never mediated by the BMPs, which is only responsible for differentiating mesenchyme cells into cartilage

Concept:

• Limbs do not form just anywhere along the body axis. Rather, there are discrete positions where limbs are generated.
• The mesodermal cells that give rise to a vertebrate limb can be identified by:
  1. Removing certain groups of cells and observing that a limb does not develop in their absence.
  2. Transplanting groups of cells to a new location and observing that they form a limb in this new place.
• LIMB FIELD - 
  • Region, representing all the cells in the area capable of forming a limb on their own, is called the limb field.
  • The cells that make up the limb bud are derived from the posterior lateral plate mesoderm, adjacent somites, and the bud's overlying ectoderm.
• LIMB BUD - 
  • The first visible sign of limb development is the formation of bilateral bulges called limb buds at the presumptive forelimb and hindlimb locations.
• Limb development begins when mesenchyme cells proliferate from the somatic layer of the limb field lateral plate mesoderm (the limb skeletal precursor cells) and from the somites (the limb muscle precursor cells) at the same level.
• These mesenchymal cells accumulate under the ectodermal tissue to create a circular bulge called a limb bud.

Explanation:-

Option 1:- CORRECT

• Limb form in the order stylopod, zeugopod, and autopod which are specified by Hox genes.

​Option 2:- CORRECT

• One of the main functions of the AER is to tell the mesenchyme cells directly beneath it to continue making Fgf10 and due to this positive feedback loop is created.

Option 3:- INCORRECT

• Cell death in the limb is necessary for the formation of digits and joints. It is mediated by BMPs. The effects of BMPs can be regulated by the Noggin protein, and the BMPs can be involved both in inducing apoptosis and in differentiating the mesenchymal cells into cartilage. 

Option 4:- CORRECT

• Dorsal side-nail Ventral side palm Wnt-7a express in dorsal ectoderm Dorsal-ventral polarity by wnt-7a.
• Wnt-7a→ Lmx-1 gene expression→ specify dorsal fate in the cell of limb
• If wnt-7a deleted→ No dorsal side(both side palm)
• If Lmx-1 deleted→ No dorsal side(both side palm)
• Note: (Lmx-1b Also known as Lim 1)

Hence, the correct answer is Option 3.

The correct answer is Option 2 i.e. B.C and E

Concept:

• Organisms with sex chromosomes have discrepancies in the dosage of genes between the sexes, i.e., there are two copies of X-linked genes in females and one copy of X-linked genes in males.
• Such levels of gene expression in females can be lethal during early development.
• To equalise that effect, organisms have dosage compensation.
• In dosage compensation, one X chromosome in every somatic cell of the female is inactivated. 
• These inactivated X chromosomes appear as a highly condensed mass of chromatin called a Barr Body.

Steps of X-inactivation - 

1. X chromosome counting -
   • The important region on the X chromosome is the XIC region.
   • It is involved in chromosome counting.
   • If any cells have 2 or more than 2 XIC regions that X chromosome inactivation occurs. 
2. Selection of X chromosomes for inactivation -
   • In every cell, one X chromosome is randomly inactivation.
   • The selection of the X chromosome for inactivation is based on the presence of X-Controlling Elements (Xce).
   • Xce is present in the XIC region of the chromosomes.
3. X inactivation -
   • XIST gene is involved in the inactivation of X chromosome.
   • It transcribes an unusually long RNA that is not translated.
   • This mRNA coats the XIC region and coats in both directions of the XIC region. 

Important Points

Statement A: INCORRECT

• X chromosomes are randomly inactivated in every cell during dosage compensation in females. 
• Maternal and paternally derived X chromosomes have an equal chance of getting inactivation. 
• Hence, this is an incorrect statement. 

Statement B: CORRECT

• Oogenesis is the process of gametogenesis in females. 
• During oogenesis, both X chromosomes are active.
• Hence, this is a correct statement.

Statement C: CORRECT

• XIST gene transcribes a non-coding RNA that is very long.
• This RNA binds to the XIC region of the chromosome and coats the chromosomes it produces, thereby rendering that X chromosomes inactive. 
• Hence, this is a correct statement.

Statement D: INCORRECT

• In each cell, the X chromosome is randomly inactivated.
• So, inactivation is not maintained from one generation to the next. 
• Hence, this is an incorrect statement. 

Statement E: CORRECT

• Tsix is a negative regulator of Xist.
• Tsix is antisense to Xist.
• Increase in Tsix RNA represses the effect of Xist. 
• Hence, this is a correct statement. 

Hence, the correct answer is Option 2.

The correct answer is Option 1 i.e. A and B

Concept:

• Limb development is initiated when the mesenchyme cells proliferated from lateral plate mesoderm and from somites and accumulate under epidermal tissues so as to create a circular budge called a limb bud. 
• When mesenchyme cells secrets factors that induce the overlaying ectoderm to form a specialised structure called apical ectodermal ridge (AER).
• This ridge runs along the dorsal margin of the limb bud, it also acts as the major signaling center for the development of the limbs.

Roles of the AER - 

• It helps to maintain the mesenchyme cells in its proliferative phase so as to facilitate linear development of the limb.
• It helps to maintain the expression of molecules that will generate anterior-posterior axis of the limb.
• It also interact with the proteins that are maintain this anterior-posterior axis and dorsal-ventral axis of the limb. 

Important Points

Statement A: CORRECT

• Shh function indirectly through the induction of Gremlin, which is an inhibitor of BMP.
• In the limb mesoderm, BMP suppresses FGF4 expression in the AER.
• So, the overall function of Shh is to stimulate the production of the FGFs in the AER and thus maintain AER function.
• Hence, this is a correct statement.

Statement B: CORRECT

• BMP has a negative and Gremlin has a positive effect on the AER. 
• When the concentration of the FGFs has increased it leads to the repression of the Gremlin. 
• In the diagram, we can see that FGF/Gremlin loop is given in red which indicates that this loop is a repressing loop and hence, it represses Gremlin activity. 
• Hence, this is a correct statement.

Statement C: INCORRECT

• As we can see in the given diagram FGF/Shh loop is shown in black which indicates that this loop causes an increase in the expression of the shh.
• FGFs 4, 9 and 17 activate the expression of the Shh in the cells.
• Hence, this is an incorrect statement. 

Statement D: INCORRECT

• Gremlin is an antagonist to BMP and it is required for maintenance of Shh and FGF signals throughout the limb patterning.
• Hence, this is an incorrect statement.

Hence, the correct answer is Option 1. 

The correct answer is Option 1 i.e. A, B, C, D and E

Concept:

• A crucial morphogenetic step during brain development is the production of neural tubes. Early in development, neurulation starts in vertebrates like humans.
• The neural tube is the embryonic forerunner of the central nervous system, which is made up of the brain and spinal cord, in the growing chordate (including vertebrates).
• As the neural folds rise, the neural groove gradually becomes more profound.
• Eventually, the folds consolidate in the middle of the groove to form the closed neural tube.
• In humans, the fourth week of pregnancy is often when the neural tube closes (the 28th day after conception).
• The foundation of the nervous system is formed by the ectodermal wall of the tube.
• The neural canal is located in the tube's middle. It is crucial for the brain and spine development of the fetus.

Explanation:

Statement A:- CORRECT

• During primary neurulation, the cells that surround the neural plate instruct the neural plate cells to multiply, invaginate, and pinch off from the surface to form a hollow tube.

Statement B:- CORRECT

• The neural tube develops during secondary neurulation from a solid cord of cells that enters the embryo and then cavitates to form a hollow tube. Different vertebrate classes utilize these processes of formation to different extents. In fish, neurulation only occurs secondarily.
• pinch off from the surface to form a hollow tube.

Statement C:- CORRECT

• Primary neurulation forms the front portions of the neural tube in birds, whereas secondary neurulation forms the neural tube caudal to the twenty-seventh somite pair (i.e., everything posterior to the hindlimbs).

Statement D:- CORRECT

• After examining mouse and human embryos, researchers discovered that the third to fifth sacral vertebrae are the location of the caudal neuropore's final closure in both species.
• The neural tube is often entirely closed by stage 13 (4 weeks).
• Similar to chick and rabbit embryos, a rapid initial drop in caudal neuropore length was discovered in human embryos.

Statement E:- CORRECT

• Anencephaly is a fatal condition that occurs when the anterior neural tube sections are not closed.
• In this situation, the forebrain continues to be in contact with the amniotic fluid and eventually deteriorates.
• Development of the fetal forebrain stops and the vault of the skull is not formed.

Hence, all statements are correct.

The correct answer is Option 1 i.e. A ‐ IV, B ‐ III, C ‐ II, D ‐ V, E ‐ I, F ‐ VI 

Concept:

• The embryo's body wall gives rise to tiny protrusions called limb buds, which eventually become the limbs.
• Cellular interactions between the mesenchymal cells that make up the limb bud's core and the ectoderm that surrounds it are necessary for positioning and patterning the limb.

Explanation:

EMT (Epithelial to Mesenchymal  transition)

• The coelomic epithelium undergoes a limited epithelial-to-mesenchymal transition (EMT), which occurs only in the fields of the presumed limbs.
• Tbx5 and Fgf10, two genes known to govern limb start, are at least partially responsible for controlling this EMT.
• Early mesenchymal limb progenitors, the foundation for future limbs, are created by inducing epithelial to mesenchymal transition (EMT) in the somatopleural epithelium. 

Mesenchyme

• Undifferentiated animal embryonic connective tissue known as mesenchyme gives rise to the majority of tissues, including skin, blood, and bone.
• Almost every organ in the developing embryo is formed as a result of interactions between the mesenchyme and epithelium.
• Mesenchyme is a component of an embryo's mesoderm.

AER (Apical Ectodermal Ridge)

• AER is the thickening of the ectoderm at the apex of the developing limb bud.

• One of the main functions of the AER is to tell the mesenchyme cells directly beneath it to continue making Fgfio.

• In this way, a second positive feedback loop is created wherein mesodermal Fgfio tells the surface ectoderm to continue to make Fgf8, and the surface ectoderm continues to tell the underlying mesoderm to make Fgfio.

Progress Zone

• The highly proliferative mesenchyme that fuels limb bud growth known as progress zone (undifferentiated zone).
• The zone is located below the apical ectodermal ridge of the vertebrate limb bud and is made up of mesodermal cells (see figure below).
• It works by giving cells the positioning information they need to grow into limbs.

ZPA (Zone of polarizing Activity)

• The cells found within the most posterior region of the limb bud constitute the ZPA, since it patterns cell fates along the anterior-posterior ais.
• when grafted to the anterior of a second limb, causes mirror-image duplications.

Autopod

• The limb has three separate domains: the proximal stylopod, the zygopod, and the distal autopod, after induction of the AER in the potential limb field and outgrowth of the appropriate bud.
• These domains are determined by Hox genes.
• Hox genes play a role in specifying the location where the limbs will form.
• They play a second role in specifying whether a particular mesenchymal cell will become stylopod, zeugopod or autopod.

1. Hox9 and Hox10 paralogues specify→ the stylopod,
2. Hox11 paralogues specify→ zeugopod, 
3. Hox12 and Hox13 paralogues specify→ autopod

Therefore, the correct answer is A ‐ IV, B ‐ III, C ‐ II, D ‐ V, E ‐ I, F ‐ VI 

The correct answer is Option 1 i.e. Gastrulation begins with the invagination of bottle cells, followed by coordinated involution of the mesodermal precursors and the epiboly of the prospective ectoderm

Concept:

• The amphibian development involves the following events:-
  • Fertilization
  • Cleavage
  • Morula
  • Blastula   
  • Gastrula
• The gray crescent cells that make up the blastopore, which is where cells move into the embryo's core, are responsible for its dorsal lip.
• The beginning of gastrulation is indicated by the invagination of cells into the area of the embryo that was previously occupied by the middle of the gray crescent. This results in:
  1. a pore (the blastopore) that will eventually become the anus.
  2. an accumulation of cells that forms the Spemann organizer.
•  The mostly mesodermal Spemann organizer will:
  • grow into the notochord to the backbone, the notochord.
  • induce the ectoderm above it to start forming neural tissue r ather than skin.
•  The Spemann organizer is formed by the Nieuwkoop center.
•  The Nieuwkoop center refers to the blastula's dorsalmost vegetal cells, which are capable of triggering the organizer.
• β-catenin was the main candidate for the element that creates the Nieuwkoop center in these vegetal cells

Important Points

Option 1 - CORRECT

• Gastrulation is the process that starts with the invagination of bottle cells and is followed by various events such as:-
  • epiboly
  • vegetal rotation
  • involution and cell migration
  • convergence and extension

Option 2 - INCORRECT

• Nieuwkoop Center induces the organizer but in the option, it's written vice versa.

Option 3 - INCORRECT

• In general, this statement is correct but not on a specific basis.
• β-catenin forms the Nieuwkoop center, then further the Spemann organizer is formed by the Nieuwkoop center.​

Option 4 - INCORRECT

• If BMP is present then ectoderm will form the epidermis
• If BMP is absent or inhibited by chordin, noggin, and follistatin which are organizer proteins, then ectoderm will form the nervous system.
• Therefore, if BMP inhibitors are absent, BMP will be active which will further bind ectoderm and eventually form the epidermis.

Explanation:

• Neuroectoderm gives rise to brain, spinal cord, cranial and spinal nerves, retina and posterior pituitary, while somatic ectoderm give rise to epidermis, cutaneous derivatives, olfactory organs, anterior pituitary, etc.
• For inducing the neural plate to become neural tube it is necessary that chordamesoderm cells must remain in physical contact of neural plate cells.
• When neuroectoderm is transplanted to epidermal region, the donor tissue cannot give rise to neural tube.