If the blastomeres of a 4-celled sea urchin embryo are isolated, each blastomere can form a pluteus larvae. This is an example of:
conditional specification
Sea Urchin Blastomere Development
The question describes an experiment where blastomeres from a 4-celled sea urchin embryo are separated. Upon isolation, each individual blastomere develops into a complete pluteus larva. This observation is a classic example in developmental biology that demonstrates a specific type of cell fate determination.
Understanding Cell Specification
Cell specification is the process by which cells acquire positional identity during development. There are generally two main types of specification:
- Autonomous Specification: In this mode, cell fate is determined by cytoplasmic determinants that are segregated into different cells during cleavage. The fate of a cell is fixed early and is independent of its environment or interactions with other cells. If cells are isolated, they will develop according to their predetermined fate, typically forming only the part of the embryo they were destined to become.
- Conditional Specification: In this mode, cell fate is determined by interactions with neighboring cells (cell-cell signaling) or signals from the environment. The fate of a cell is not rigidly fixed early on and can be influenced or changed by external cues. If cells are isolated or their position is changed, their fate can change, potentially allowing them to form structures they wouldn't have formed in the intact embryo.
Analyzing the Sea Urchin Experiment
In the case of the 4-celled sea urchin embryo, when the blastomeres are isolated, each one has the potential to form a complete larva. This means that at the 4-cell stage, the fate of these blastomeres is not autonomously determined to form only a specific quadrant or part of the future larva. Instead, the potential of each cell is broader, and its fate is normally restricted or specified by its interaction with the other blastomeres in the intact embryo.
When isolated, the cell is freed from these restrictive signals and can express its full developmental potential, leading to the formation of a complete organism. This plasticity and dependence on cellular environment and interactions is the hallmark of conditional specification.
Evaluating the Options
autonomous specification: Incorrect, because if specification were autonomous, isolated blastomeres would form only parts, not complete larvae.conditional specification: Correct, as the ability of isolated cells to form complete larvae demonstrates that cell fate is determined by interactions within the embryo, not fixed autonomously at this stage.determination: Determination is the commitment of a cell to a fate, but the experiment shows that at the 4-cell stage, the cells are specified conditionally, not autonomously determined in a way that restricts their full potential upon isolation.mosaic development: This term is often associated with autonomous specification, where the embryo develops as a mosaic of independently determined cells. The sea urchin example contradicts this.
Therefore, the observation that isolated 4-celled sea urchin blastomeres can each form a complete pluteus larva is a clear example of conditional specification.
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?
