Microinjection is a technique used to introduce foreign substances, such as DNA, RNA, proteins, or other small molecules, directly into individual cells. This method involves the use of a fine glass needle, called a micropipette, which is attached to a microinjector system. The micropipette is filled with the substance to be introduced and carefully inserted into the cell under a microscope. The microinjector system then delivers a small volume of the substance into the cell, typically in the cytoplasm or nucleus.

Microinjection methods can be used in various applications, including:

1. Transfection: Microinjection can be used to introduce DNA or RNA into cells for gene expression studies, gene function analysis, or gene therapy research. This method can be highly efficient and is often used when other transfection methods fail or when working with difficult-to-transfect cells, such as primary cells, neurons, or stem cells.
2. Gene editing: Microinjection can be used to deliver gene editing tools, such as CRISPR/Cas9 components, into cells or embryos to generate genetically modified organisms or cell lines. This method allows for precise control over the amount and location of the introduced components, increasing the efficiency and accuracy of the gene editing process.
3. Protein studies: Microinjection can be used to introduce purified proteins or other molecules into cells to study their function, localization, or interaction with other cellular components.
4. Cell biology studies: Microinjection can be employed to manipulate cellular processes, such as injecting ions, metabolites, or drugs into cells to study their effects on cellular function, signaling pathways, or cell cycle progression.
5. Developmental biology: Microinjection is commonly used in the study of early embryonic development, especially in model organisms like zebrafish, Xenopus, and mice. Researchers can introduce DNA, RNA, or morpholino oligonucleotides to manipulate gene expression or investigate gene function during development.

Despite its advantages, microinjection has some limitations, such as the need for specialized equipment and expertise, the potential for damage to the cell membrane or nucleus during the injection process, and the relatively low throughput compared to other transfection methods. However, microinjection remains a valuable tool for researchers in various fields, including molecular and cell biology, developmental biology, and genetics.