Histopathology (or histology) involves the examination of sampled whole tissues under the microscope. Three main types of specimen are received by the pathology laboratory.
- Larger specimens include whole organs or parts thereof, which are removed during surgical operations. Examples include a uterus after a hysterectomy, the large bowel after a colectomy or tonsils after a tonsillectomy.
- Pieces of tissue rather than whole organs are removed as biopsies, which often require smaller surgical procedures that can be performed whilst the patient is still awake but sedated. Biopsies include excision biopsies, in which tissue is removed with a scalpel (e.g. a skin excision for a suspicious mole) or a core biopsy, in which a needle is inserted into a suspicious mass to remove a slither or core of tissue that can be examined under the microscope (e.g. to investigate a breast lump).
- Fluid and very small pieces of tissue (individual cells rather than groups of cells, e.g. within fluid from around a lung) can be obtained via a fine needle aspiration (FNA). This is performed using a thinner needle than that used in a core biopsy, but with a similar technique. This type of material is usually liquid rather than solid, and is submitted for cytology rather than histology (see Cytopathology).
Specimens received by the pathology laboratory require tissue preparation then are treated and analysed using techniques appropriate to the type of tissue and the investigation required. For immediate diagnosis during a surgical procedure a frozen section is performed.