Histology

Histology is the branch of biology that studies the microscopic structure of tissues, and it is also called microscopic anatomy or microanatomy.^[1]

The word comes from the Greek histos, meaning tissue or web, and logia, meaning study, and it entered medical language in the nineteenth century.

Histology is the microscopic counterpart of gross anatomy, and the two fields describe the same body at different scales.^[2]

Definition and scope

Histology examines how cells are organised into tissues, how tissues form organs, and how structure relates to function.
It is distinct from cell biology because histology preserves the spatial relationships between cells, whereas cell biology often studies isolated cells.
Histology overlaps with pathology, and histopathology is the branch that studies diseased tissue.
Histochemistry, immunohistochemistry and in situ hybridisation are histology-based methods that identify specific molecules in tissue sections.
The field is both descriptive and quantitative, because modern histology measures cell counts, area fractions and staining intensity.

A tissue is a group of cells with a common origin and a common function, together with the extracellular matrix they produce.
Animal tissues are traditionally divided into four basic types, and most organs are built from combinations of these types.
Plant tissues are classified differently, because plants have cell walls and a different developmental logic from animals.
The tissue level is one of the classic levels of biological organisation, between the cell and the organ.
Histology studies normal tissue, while pathology studies tissue altered by injury, infection, degeneration or neoplasia.

Epithelial tissue covers surfaces, lines cavities and forms glands, and its cells are closely apposed with little extracellular space.
Connective tissue supports, binds and separates other tissues, and it is characterised by abundant extracellular matrix.
Muscle tissue is specialised for contraction, and it is divided into skeletal, cardiac and smooth muscle.
Nervous tissue is specialised for signalling, and it consists of neurons and glial cells.
Blood is classified as connective tissue because its cells are suspended in an extracellular matrix, the plasma.
Bone and cartilage are specialised connective tissues with mineralised or resilient matrices.
Each basic tissue type has subtypes, and the classification is based on morphology, function and extracellular matrix composition.

Plant tissues include dermal, vascular, ground and meristematic tissues, and each has a different role in the plant body.
Dermal tissue forms the epidermis and periderm, and it protects the plant and limits water loss.
Vascular tissue consists of xylem and phloem, which conduct water and sugars respectively.
Ground tissue includes parenchyma, collenchyma and sclerenchyma, and it provides storage, support and photosynthesis.
Meristematic tissue contains dividing cells and is responsible for growth at root and shoot tips and in the cambium.

Histopathology is the microscopic study of diseased tissue, and it is central to the diagnosis of cancer and inflammatory disease.
Histotechnology is the technical field that prepares tissue for microscopy, including fixation, embedding, sectioning and staining.^[3]
Histotechnologists, histology technicians and biomedical scientists perform these procedures in hospital and research laboratories.
Frozen sections are prepared during surgery so that a pathologist can give a rapid diagnosis and guide the extent of resection.
Histology is also used in autopsy, in transplant assessment and in research on animal models of human disease.

Most tissues must be fixed before sectioning, and fixation preserves structure by cross-linking or precipitating proteins.
The most widely used fixative for light microscopy is 10 per cent neutral buffered formalin, which is about 4 per cent formaldehyde.
Glutaraldehyde is used for electron microscopy because it cross-links proteins more extensively and preserves ultrastructure.
After fixation, tissue is dehydrated in graded alcohols and cleared in xylene before embedding in paraffin wax.
Hard tissues such as bone may need decalcification before paraffin sectioning, and this can damage some antigens.
Frozen sections avoid paraffin embedding and are used when rapid diagnosis or sensitive antigens require it.
Microtomes cut sections a few micrometres thick, and ultramicrotomes cut sections below 100 nanometres for electron microscopy.
Haematoxylin and eosin is the routine stain, with haematoxylin colouring nuclei blue and eosin colouring cytoplasm pink.
Special stains such as periodic acid-Schiff, Masson trichrome and reticulin highlight specific tissue components.
Immunohistochemistry uses antibodies to localise proteins, and it has become essential for tumour classification.

The compound light microscope remains the workhorse of histology, and it resolves structures down to about 0.2 micrometres.
Phase contrast and differential interference contrast improve contrast in unstained or lightly stained specimens.
Fluorescence microscopy detects fluorophores and is used for immunofluorescence and for live-cell imaging.
Confocal microscopy provides optical sectioning and three-dimensional reconstruction of thick specimens.
Transmission electron microscopy resolves organelles and viruses, while scanning electron microscopy shows surface topography.
Super-resolution microscopy and digital pathology are extending histology beyond the classical resolution limit and glass slide.

Robert Hooke described cells in cork in 1665, and Antonie van Leeuwenhoek observed living cells with his microscopes.
Marie François Xavier Bichat in the early nineteenth century distinguished twenty-one tissues, and histology became a systematic discipline.
Rudolf Virchow linked cellular pathology to histology in the mid-nineteenth century, arguing that disease arises in cells.
The microtome and paraffin embedding in the late nineteenth century made reproducible sections possible.
Immunohistochemistry in the 1970s and digital pathology in the 2000s transformed the practice of histology.

Digital pathology replaces glass slides with whole-slide images, but validation, storage and regulatory approval remain uneven.
Artificial intelligence can detect tumours in histological images, yet its performance depends on staining and scanner variation.
Three-dimensional histology and tissue clearing challenge the traditional two-dimensional section as the reference standard.
Antigen retrieval and antibody specificity remain technical controversies that affect reproducibility across laboratories.
Ethical issues around human tissue use, consent and biobanking are increasingly important as histological data become digital and shareable.

References