Given below are statements related to expression of some of the pattern‐forming genes in vertebrate limb bud: A. Lmx1b gene is expressed in dorsal mesenchyme B. Shh is expressed in the posterior region. C. Wnt7a gene is expressed in dorsal ectoderm. D. Hoxa13 and Hoxd13 are expressed in the distal region. E. Tbx5 and FGF10 are expressed in the lateral plate mesoderm involved in formation of limb bud. The Nail‐Patella syndrome in human and the syndrome in mouse exhibiting footpads on both dorsal and ventral surfaces of limb are associated with which of the above mentioned gene functions?
A and C only
Understanding Limb Bud Gene Expression
The formation of a vertebrate limb bud is a complex process guided by the precisely timed and spatially controlled expression of various genes. These genes establish the three main axes of the limb: the anterior-posterior axis, the dorsal-ventral axis, and the proximodistal axis. Mutations in these pattern-forming genes can lead to developmental abnormalities or syndromes.
Analysis of Gene Expression Statements
Let's evaluate each statement regarding the expression pattern of pattern-forming genes in the vertebrate limb bud:
- A. Lmx1b gene is expressed in dorsal mesenchyme. This statement is accurate. The Lmx1b gene is indeed expressed in the dorsal mesenchyme of the developing limb bud. It plays a crucial role in specifying dorsal cell fate.
- B. Shh is expressed in the posterior region. This statement is accurate. Sonic hedgehog (Shh) is expressed in the Zone of Polarizing Activity (ZPA) located in the posterior mesenchyme of the limb bud. It is a key signaling molecule for anterior-posterior patterning.
- C. Wnt7a gene is expressed in dorsal ectoderm. This statement is accurate. Wnt7a is expressed in the dorsal ectoderm of the limb bud. It is a signal that helps maintain the expression of Lmx1b in the underlying dorsal mesenchyme and is essential for dorsal-ventral patterning.
- D. Hoxa13 and Hoxd13 are expressed in the distal region. This statement is accurate. Hoxa13 and Hoxd13 are late-acting Hox genes expressed in the most distal part of the developing limb (the autopod, which forms digits). They are involved in proximodistal patterning.
- E. Tbx5 and FGF10 are expressed in the lateral plate mesoderm involved in formation of limb bud. This statement is largely accurate. Tbx5 is expressed in the forelimb field of the lateral plate mesoderm and is essential for forelimb initiation. FGF10 is also expressed in the limb field mesoderm and is a key signal that initiates limb bud outgrowth by signaling to the overlying ectoderm.
Connecting Genes to Syndromes
The question asks which of these gene functions are associated with Nail-Patella syndrome in humans and a mouse syndrome exhibiting footpads on both dorsal and ventral surfaces. Let's consider these syndromes:
- Nail-Patella syndrome: This human genetic disorder is characterized by abnormalities of the nails, patellae (kneecaps), elbows, and iliac horns. It is caused by mutations in the LMX1B gene. Lmx1b is involved in the development of dorsal limb structures. Therefore, statement A is directly related to Nail-Patella syndrome.
- Mouse syndrome with dorsal and ventral footpads: This phenotype, where the dorsal side of the paw develops like the ventral side (forming footpads), indicates a defect in dorsal-ventral patterning. Mutations in the Wnt7a gene cause such a phenotype in mice. Without functional Wnt7a signaling from the dorsal ectoderm, the dorsal mesenchyme does not maintain dorsal identity and adopts a ventral fate. Wnt7a's role is to maintain Lmx1b expression in the dorsal mesenchyme. Thus, mutations in either Wnt7a or Lmx1b can lead to dorsal-ventral patterning defects. Statement C describes Wnt7a expression in the dorsal ectoderm, which is crucial for dorsal development and whose dysfunction causes the mouse syndrome described.
Based on this analysis:
- Statement A (Lmx1b in dorsal mesenchyme) is associated with Nail-Patella syndrome.
- Statement C (Wnt7a in dorsal ectoderm) is associated with the mouse dorsal-ventral patterning syndrome.
Statements B, D, and E are associated with other aspects of limb development (AP axis, distal development, limb initiation) but are not the primary genes linked to Nail-Patella syndrome or the described dorsal-ventral patterning mouse phenotype.
Therefore, the gene functions associated with the mentioned syndromes are described in statements A and C.
Important Questions from Morphogenesis and organogenesis in animals
The individuals considered in this question are having two haploid sets of autosomes and no Y- chromosome. The X∶A ratio of the individuals, the type of organisms chosen, their primary sex and number of Barr bodies expected in their cells are shown in the table below:
X∶A ratio Organism Primary Sex Number of Barr bodies i. 0.5
ii. 2
A. Human
B. Drosophila
I. Male
II. Female
III. Metafemale
a. Zero
b. One
c. Two
d. Three
Select the option below with all correct matches:
Several marine organism release their gametes into the environment, where sperm attraction and subsequent events lead to successful fertilization. With reference to sea urchins, which one of the following statements is NOT true?
The following statements regarding the generation of dorsal/ventral axis in Drosophila was made:
A. Gurken protein moves along with the oocyte nucleus and signals follicle cells to adopt the ventral fate.
B. Maternal deficiencies of either the gurken or torpedo gene cause ventralization of the embryo.
C. Gurken is active only in the oocyte and Torpedo is active only in the somatic follicle cells.
D. The Pipe protein is made in the dorsal follicle cells.
E. The highest concentration of Dorsal is in the dorsal cell nuclei, which becomes the mesoderm.
Which one of the following combination of the above statements is true?
During wing development in chick, if Apical Ectodermal Ridge (AER) is removed, the limb development ceases, on the other hand placing leg mesenchyme directly beneath the wing AER, distal hindlimb structures develop at the end of the wing, and if limb mesenchyme is replaced by non-limb mesenchyme beneath AER, the AER regresses. This may demonstrate that:
A. the limb mesenchyme cells induce and sustain AER.
B. the mesenchyme cells specify the type: wing or limb.
C. the AER is responsible for specifying the type: wing or limb.
D. the AER is responsible for sustained outgrowth and development of the limb.
E. the AER does not specify the type: wing or limb.
Which combination of above statements is demonstrated by the experiment?
The following statements are made regarding the amphibian early-embryonic development:
A. The Nieuwkoop center cells are mesodermal in origin.
B. Chordin, Noggin and Goosecoid are secreted by the Organizer.
C. The default fate of the ectoderm is to become neural tissue.
D. BMP levels are high in the presumptive dorsal mesoderm.
Which one of the following options represents the combination of all correct statements?
