Histology

Histology is the scientific study of the structures of cells using microscopy, either a light or electon microscope. It's used in both biology and medicine, particularly in pathology (histopathology). It's vital in the study of oncology and the diagnosis of cancer.
The beginnings of histology are in the 17th century, where new techniques such as staining allowed scientists to see individual organs within the cell. By the 18th century, it was a specialized field in its own right.
Histology at Wikipedia