Medical doctors primarily study human cells. While they may learn about animal cells in their education, their focus is on understanding the structure, function, and behavior of human cells in the context of health and disease.

Here's why:

* Human-specific diseases: Medical doctors are trained to diagnose and treat diseases that affect humans. These diseases are often caused by problems with human cells.

* Ethical considerations: Research involving animals is tightly regulated and faces ethical considerations. While animal models are crucial for medical research, human cells are the primary focus for clinical practice.

* Human physiology: Medical doctors need to understand the complex interactions of human cells within the body, including their role in organ systems and overall health.

While medical doctors don't focus exclusively on human cells, their understanding of animal cells is usually in the context of how those cells relate to human health and disease.