Routes to transgenic animals — which cell types are directly transfected in common methods to create transgenic animals?

Introduction / Context:
Transgenic animals are generated by introducing foreign DNA into cells that will contribute to the germline. Two mainstream strategies are direct zygote/embryo manipulation and embryonic stem (ES) cell targeting followed by chimera formation.

Given Data / Assumptions:

• Goal: ensure germline transmission of the transgene.
• Viable routes include pronuclear microinjection into zygotes or vector-mediated delivery to early embryos.
• ES cell methods involve selection in vitro and blastocyst injection, not RBCs.

Concept / Approach:

Direct transfection of eggs/embryos places DNA where it can integrate before extensive cell divisions, minimizing mosaicism. ES cell targeting enables precise genome edits but requires subsequent incorporation into the germline via chimeras. Red blood cells lack nuclei (in mammals) and cannot be transfected for heritable changes.

Step-by-Step Solution:

Verification / Alternative check:

Mouse transgenesis literature documents pronuclear injection and lentiviral transduction of embryos; ES cell gene targeting is also standard but involves an intermediate chimera step.

Why Other Options Are Wrong:

(b) ES cells are used, but not via simple “transfection requires” in every species; the question focuses on primary route. (c) RBCs cannot carry heritable DNA in mammals. (d) Invalid because it includes RBCs.

Common Pitfalls:

Confusing somatic transfection with germline modification; overlooking species differences in ES cell availability.

Final Answer:

Eggs or embryos (e.g., pronuclear zygotes, early embryos)