Cytology versus histology: cells versus tissue architecture
Cytology looks at individual cells taken from a lump, a surface or a body fluid, while histology looks at intact tissue and its architecture (the way the cells are arranged).
Remember
Cytology looks at individual cells, so the tissue architecture is lost; histology examines intact tissue, so the architecture can be seen. Cytology is generally quicker but more limited.
Memory aid
cytology means cells and histology means tissue
Use the meaning of each name to keep the two methods apart: cytology is about cells and histology is about tissue.
Samples of diseased material are examined under the microscope in two main ways. Histology (also called histopathology) looks at a thin slice of intact tissue, usually from a biopsy (a piece of tissue that has been removed) or an operation. Because the tissue is intact, the tissue architecture can be seen: how the cells are arranged and how they relate to each other and to the structures around them.
Cytology (also called cytopathology) looks at free cells, or small clusters of cells, that have been taken from a lump, a surface or a body fluid. One method is fine-needle aspiration, in which a thin needle is used to suck out cells from a lump. Another is a smear, where the cells are spread on a slide and examined. A familiar example is the cervical screening smear (the Pap test), in which a sample of cells is collected from the cervix, the neck of the womb. Because the cells are separated from their surroundings, the tissue architecture is lost.
Cytology is generally quicker and less invasive than histology (a thin needle or a smear is used in place of a surgical biopsy), which is why it is often used as a first test. The cost is that the disease has to be judged from the appearance of the cells alone. This makes cytology more limited, and a definite diagnosis may need a tissue sample.
In current NHS cervical screening, the sample is tested first for high-risk human papillomavirus (HPV, the virus that can cause cervical cell changes), and the cells are examined (cytology) if the HPV test is positive. A screening sample is a collection of loose cells, not a piece of tissue, so examining the cells belongs to cytopathology. If a biopsy is taken later, that tissue goes to histopathology.
Key facts
- Cytology
- looks at free cells or small clusters of cells taken from a lump, a surface or a body fluid.
- Histology
- looks at a thin slice of intact tissue, so the tissue architecture can be seen.
- Trade-off
- cytology is generally quicker and less invasive, but the tissue architecture is lost.
- Cervical screening
- the sample is a collection of loose cells, so examining the cells belongs to cytopathology.
- Limits of cytology
- the sample may have to be repeated, or tissue taken for histology, when architecture is needed.
- Histopathology and cytopathology
- histopathology is the study of the cellular changes within tissues that are caused by disease, whereas cytopathology examines sampled cells to determine the cause or the nature of disease.
Exam traps
- Cytology is usually quicker and less invasive than histology, not slower and more invasive, because a fine needle or a smear can be used instead of a surgical biopsy.
- Cytology is not generally more accurate than histology: with the tissue architecture lost, it is more reliant on how the individual cells look, so it has more limitations.
- Cytology is not kept for use after histology has failed. It is often the first test, for example a needle sample of a lump, and it may decide whether histology is needed.
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