C. elegans embryo uses both autonomous and conditional modes of specification. The following statements are about specification of cell lineages: A. The signals from the P1 blastomere instructs the EMS cell for its further development. B. In the absence of the instructive signal, the EMS cell will divide into two MS cells. C. In the absence of the POP‐1 signal, the prospective MS cell can take up the E fate. D. The MOM‐2 protein from the blastomere neighbouring the EMS cell instructs the dividing EMS blastomere to take up MS fate. Which one of the following options represent both correct statements?
B and C
In the development of the C. elegans embryo, cell fates are determined through a combination of autonomous and conditional specification. Autonomous specification relies on cytoplasmic determinants inherited by the cell, while conditional specification involves signals received from neighboring cells.
The question asks us to identify the correct statements about cell lineage specification in the C. elegans embryo, focusing on interactions involving cells like P1 and EMS, and signals like MOM-2 and POP-1.
Statement Analysis: C. elegans Cell Specification
Statement A Analysis
Statement A says: "The signals from the P1 blastomere instructs the EMS cell for its further development."
While the P1 blastomere is part of the lineage that signals the EMS cell, the direct signal comes from the P1 descendant, P2. The P2 blastomere is the posterior neighbor of the EMS cell at the 4-cell stage. P2 secretes the Wnt signal MOM-2. This signal is received by the EMS cell and is critical for setting up the asymmetry that determines the fates of the EMS daughters (MS and E). So, the signal originates from the P1 lineage but directly from P2. Depending on the strict interpretation, this statement might be considered less precise than saying the signal comes from P2.
Statement B Analysis
Statement B says: "In the absence of the instructive signal, the EMS cell will divide into two MS cells."
The instructive signal from P2 (MOM-2/Wnt) polarizes the EMS cell and causes it to divide unequally, producing an anterior daughter that becomes the MS precursor and a posterior daughter that becomes the E precursor. The MS fate is considered the default fate in the absence of this signal. If the MOM-2 signal is absent or the EMS cell cannot receive it, the EMS cell divides symmetrically, and both daughter cells adopt the MS fate. This statement is consistent with experimental observations.
- Signal (MOM-2 from P2) present: EMS → MS (anterior) + E (posterior)
- Signal absent: EMS → MS + MS
Therefore, statement B is correct.
Statement C Analysis
Statement C says: "In the absence of the POP‐1 signal, the prospective MS cell can take up the E fate."
POP-1 is a β-catenin-activated TCF/LEF transcription factor involved in the Wnt pathway. In the EMS lineage, POP-1 is present at higher levels in the anterior daughter (prospective MS) and lower levels in the posterior daughter (prospective E). High POP-1 levels promote the MS fate, while low POP-1 levels promote the E fate. If POP-1 is absent or its function is reduced (effectively mimicking low POP-1 levels), the cell that would normally have high POP-1 (the prospective MS cell) can switch fate and adopt the E fate, which is characterized by low POP-1. This statement is consistent with the role of POP-1 in determining MS vs. E fates.
Therefore, statement C is correct.
Statement D Analysis
Statement D says: "The MOM‐2 protein from the blastomere neighbouring the EMS cell instructs the dividing EMS blastomere to take up MS fate."
As discussed in Statement B, the MOM-2 signal from the neighboring P2 blastomere does not instruct the EMS cell to take up the MS fate. Instead, the MOM-2 signal polarizes the EMS cell and acts on the posterior side to specify the E fate in the posterior daughter. The anterior daughter takes the MS fate, which is the default fate in the absence of strong Wnt signaling. The signal creates asymmetry, leading to one E cell and one MS cell, but it primarily promotes the E fate in the posterior daughter. Saying it instructs the EMS blastomere to take up the MS fate is inaccurate.
Therefore, statement D is incorrect.
Identifying Correct Statements
Based on the analysis:
- Statement A: Potentially correct depending on interpretation of "signals from P1", but less precise than B or C.
- Statement B: Correct.
- Statement C: Correct.
- Statement D: Incorrect.
The question asks for the option representing both correct statements. Statements B and C are both correct.
Let's look at the options:
- A and B
- B and C
- C and D
- A and D
Option 2 lists B and C, which are identified as correct statements.
Important Questions from Basic concepts of development
Following statements are made regarding animal development:
A. The cell is first specified towards a given fate, suggesting that it would develop into this cell type, even in a neutral environment.
B. Holoblastic rotational cleavage is observed in tunicates.
C. Infolding of sheet of cells is called ingression.
D. Conditional specification can be observed in sea urchin embryos.
Which one of the following options represents the combination of all correct statements?
When 8-cell embryo of tunicates is separated into 4 blastomere pairs and allowed to grow independently in culture medium, then each blastomere pair can form most of the cell types; however, cells for nervous system are not developed. The following statements are formed from the above observations:
A. Nervous system development demonstrated autonomous specification.
B. The other tissue types are formed due to conditional specification.
C. All the tissue types, except nervous tissues that developed, demonstrated autonomous specification.
D. Nervous system development demon-strated conditional specification.
The correct combination of statements that explains the above result is:
The cells of inner cell mass of a blastocyst stage mammalian embryo are
Which one of the following statements about stem cells is correct?
Following statements are being made regarding specification/determination during animal development:
A. During the course of commitment, the cell may not appear different from its nearest or most distant neighbours in the embryo and show no visible signs of differentiation; but its developmental fate is restricted.
B. At the stage of specification, cell commitment is not labile.
C. A cell or tissue is determined when it is capable of differentiating autonomously even when placed into another region of the embryo, or a cluster of differently specified cells in a petri- dish.
D. Cytoskeletal arrangements maintain positioning of nuclei in the syncytium, which enables specification of these nuclei by opposing morphogen gradients namely Bicoid and Caudal in Drosophila.
E. Capacity for “mosaic" development allows cells to acquire different functions as a result of interactions with neighbouring cells.
Which of the above statements are correct?
