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?
A and D
Animal Development Statements Analysis
Let's analyze each statement regarding animal development to determine which ones are correct.
Statement A: Cell Specification
Statement A says: "The cell is first specified towards a given fate, suggesting that it would develop into this cell type, even in a neutral environment."
This statement describes the process of specification in developmental biology. Specification is the initial, reversible stage of cell commitment. A cell is specified if it is capable of differentiating autonomously (developing into its normal cell type) when placed in a neutral environment. If it is placed in a different environment (like near different cells in another region of the embryo), its fate can still be altered. This description accurately reflects the definition of specification. So, statement A is correct.
Statement B: Holoblastic Rotational Cleavage
Statement B says: "Holoblastic rotational cleavage is observed in tunicates."
Holoblastic cleavage refers to cleavage where the entire egg is divided. Rotational cleavage is a specific pattern where the first division is meridional, but in the second division, one blastomere divides meridionally while the other divides equatorially, and the orientation of cleavage planes relative to the animal-vegetal axis differs between blastomeres. This type of cleavage is characteristic of mammals and nematodes (like C. elegans). Tunicates, which are urochordates, typically exhibit bilateral cleavage or somewhat radial cleavage, not rotational cleavage. Therefore, statement B is incorrect.
Statement C: Infolding of Sheet of Cells
Statement C says: "Infolding of sheet of cells is called ingression."
This statement describes a process during gastrulation. Infolding of a sheet of cells into the embryo is specifically called invagination. Ingression, on the other hand, involves the migration of individual cells from an epithelial sheet into the blastocoel. Examples include the formation of primary mesenchyme in sea urchins or neural crest cell migration. Since infolding is not ingression, statement C is incorrect.
Statement D: Conditional Specification in Sea Urchin Embryos
Statement D says: "Conditional specification can be observed in sea urchin embryos."
Conditional specification is a mode of cell fate determination where the fate of a cell is influenced by interactions with neighboring cells. This contrasts with autonomous specification, where fate is determined by internal factors. Sea urchin embryos are classic examples where conditional specification plays a significant role, particularly in the determination of micromere fate and their influence on macromeres and vegetal plate formation. If micromeres are removed or rearranged, the development of other cells is affected. Thus, statement D is correct.
Identifying Correct Statements
Based on the analysis:
- Statement A is correct.
- Statement B is incorrect.
- Statement C is incorrect.
- Statement D is correct.
The combination of all correct statements is A and D.
Important Questions from Basic concepts of development
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?
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?
