Practical transgenesis in fish embryos: into which cellular compartment is DNA typically microinjected to create transgenic fish?

Introduction / Context:
Microinjection is the workhorse for generating transgenic fish (for example, zebrafish, medaka, salmonids). Unlike mouse pronuclear injection, fish embryology and egg architecture often make pronuclei inaccessible or transient. Consequently, investigators inject DNA directly into the cytoplasm of the one-cell embryo near the nucleus to achieve genomic integration in early cell divisions.

Given Data / Assumptions:

• Fertilized fish eggs are large, with a single cell at early stages.
• Pronuclei are not consistently visible or practically targeted in many fish species.
• Cytoplasmic injection allows diffusion toward the nucleus and mosaic integration during early cleavage.

Concept / Approach:
Because of the unique egg anatomy and rapid development, cytoplasmic delivery at the one-cell stage ensures that exogenous DNA is present before or during the first mitoses. This increases the chance of germline transmission from resulting chimeras or founders.

Step-by-Step Solution:

Verification / Alternative check:
Standard zebrafish and medaka transgenesis protocols specify cytoplasmic injection at the one-cell stage; many publications report success using this approach rather than pronuclear injection.

Why Other Options Are Wrong:

• B/C: Pronuclear injection is the norm in mice, not typically feasible in most fish species.
• D/E: Perivitelline and yolk spaces are occasionally used for certain substances but are not standard for DNA intended to integrate efficiently into the genome.

Common Pitfalls:
Applying mouse pronuclear methods to fish; injecting too late, leading to excessive mosaicism and poor germline transmission.

Final Answer:
Cytoplasm of the one-cell embryo