Question

Which of the following developmental stages has the greatest nuclear-to- cytoplasmic material ratio? A. Eight-cell zygote B. Morula C. Blastula D. Archenteron

Short answer

C. Blastula

Step-by-step solution

- Understand the Question

Determine which developmental stage has the greatest nuclear-to-cytoplasmic material ratio. The stages to consider are eight-cell zygote, morula, blastula, and archenteron.

- Define Stages

Identify and define each stage. An eight-cell zygote consists of 8 cells. A morula is a solid ball of cells from the division of a zygote. A blastula is a hollow sphere of cells. An archenteron is the primitive gut formed during gastrulation.

- Consider Cell Size and Number

Evaluate the relative number of cells and their sizes in each stage. Smaller cells and fewer cytoplasmic divisions increase the nuclear-to-cytoplasmic ratio.

- Compare Ratios

Analyze which stage has cells that are most numerous and smallest. Divisions occurring without significant growth between them increase the nuclear-to-cytoplasmic ratio.

- Identify the Greatest Ratio

Determine the stage where cells are very small and numerous, meaning the nuclear-to-cytoplasmic ratio is highest. The blastula stage, with many small cells surrounding a hollow cavity, meets this criterion.

Key concepts

Nuclear-to-Cytoplasmic Ratio

The nuclear-to-cytoplasmic ratio (N/C ratio) is a critical concept in embryology. It describes the relationship between the size of the cell's nucleus and the amount of cytoplasm. During early development, this ratio changes significantly.

Initially, when a zygote divides, the cytoplasm is partitioned into smaller and smaller cells. The number of nuclei increases rapidly because of successive cell divisions. However, the total volume of cytoplasm remains approximately the same.

The high N/C ratio signifies that cells have more nuclear material relative to their cytoplasmic content. This is important for gene expression and early development, as smaller cells can be more efficient in regulating DNA processes.

Zygote

A zygote is the initial cell formed when a sperm cell fertilizes an egg cell. It represents the first stage in the development of a new organism.

As the zygote undergoes cleavage, it divides into multiple smaller cells called blastomeres. This process increases the number of cells without a corresponding increase in cytoplasmic volume, hence raising the nuclear-to-cytoplasmic ratio.

In the stage of an eight-cell zygote, the cells are reasonably large because the division has just started. Therefore, its N/C ratio is lower compared to later stages.

Morula

A morula forms after several divisions from a zygote and resembles a solid ball of cells. The term 'morula' comes from the Latin word for mulberry, given its resemblance to the fruit.

During the morula stage, the number of cells is significantly increased but the cells are still packed closely together. The nuclear-to-cytoplasmic ratio is higher than it was in the zygote, yet it continues to rise as development progresses.

Blastula

The blastula is a stage characterized by the formation of a hollow, fluid-filled cavity known as the blastocoel. The transition from the morula to the blastula marks an increase in cell differentiation.

At this stage, the cells are much smaller and more numerous than in the earlier stages. This results in the greatest nuclear-to-cytoplasmic ratio among the developmental stages. The smaller cells at the blastula stage have less cytoplasm surrounding each nucleus, making the N/C ratio the highest at this point.

Archenteron

The formation of the archenteron begins during gastrulation, a crucial phase where the blastula reorganizes into a layered structure. The archenteron is essentially the primitive gut of the developing embryo.

By this stage, the cells have undergone significant differentiation and the overall structure of the embryo becomes more complex.

The nuclear-to-cytoplasmic ratio decreases here as the cells are larger and more specialized compared to the blastula stage. Thus, an archenteron has a lower N/C ratio than the blastula.