Variegation (histology)

In histology, variegation is the property of a tissue or specimen having discrete markings of different colours, and the word is borrowed from the patterned leaves of variegated plants.

The term is descriptive rather than diagnostic, because variegation can arise from fat, blood, pigment, necrosis, fibrosis or simply from differences in staining.

Recognising the cause of a variegated appearance is important because some patterns are benign and characteristic, while others signal haemorrhage, infarction or tumour heterogeneity.

Definition

• Variegation means marked with patches of different colours, and in pathology it usually refers to the naked-eye or low-power appearance of a cut surface.
• The word comes from the Latin varius, meaning varied, and it entered English through horticulture before being applied to diseased tissue.
• Variegation differs from heterogeneity because heterogeneity is a general term for mixed composition, while variegation specifically describes visible colour differences.
• In histology the term may describe a stained slide as well as a gross specimen, and uneven staining is a common technical cause.
• A variegated pattern is usually assessed at low magnification, because the individual patches may be too large to see in a single high-power field.

Causes and patterns

• Fat and blood give the classic yellow-to-red variegation of an adrenal myelolipoma, in which adipose tissue and haematopoietic elements are intermixed.
• Recent haemorrhage produces red or brown patches, while older haemorrhage produces haemosiderin pigmentation that ranges from golden to dark brown.
• Coagulative necrosis produces a pale, firm area that contrasts with the surrounding viable tissue and creates a two-tone appearance.
• Melanin, lipofuscin, bile and haemosiderin are endogenous pigments that can produce brown, black or green variegation.
• Fibrosis and calcification create pale or gritty patches that differ in texture as well as colour from the surrounding tissue.
• In the liver, a nutmeg appearance results from alternating congested and pale areas, and it is a classic example of variegation caused by passive congestion.
• In the lung, patchy consolidation and haemorrhage can produce a mottled surface that reflects the segmental distribution of the disease.
• Tumours such as clear cell renal cell carcinoma and hepatocellular carcinoma often show variegation because of haemorrhage, necrosis and fat.

Microscopic variegation

• At the microscope, variegation can appear as patchy staining of adjacent cells, as in the alternating strong and weak expression of an immunohistochemical marker.
• Mosaic staining patterns are common in viral infections, where infected cells stand out against uninfected neighbours.
• Genetic mosaicism can produce variegated expression of an X-linked gene, as in the patchy sweat-gland staining used to study X-inactivation.
• Uneven hematoxylin and eosin staining can result from thick sections, incomplete deparaffinisation or uneven dye penetration.
• Staining variegation is a technical artefact when it follows section thickness or folding, and a biological pattern when it respects cell boundaries.
• Textural variegation, such as alternating firm and soft areas, is appreciated by palpation and by ultrasound, and it often correlates with the colour pattern.

Clinical significance

• Variegation is a useful sign in gross pathology because it draws attention to mixed lesions such as tumours with necrosis or organs with impaired perfusion.
• In adrenal myelolipoma the variegated cut surface is characteristic enough to suggest the diagnosis before microscopy.
• A variegated liver surface may indicate chronic congestion, steatosis or metastatic disease, and the pattern helps guide sampling.
• Variegated enhancement on contrast imaging can correspond to areas of necrosis or different vascularity within a tumour.
• In dermatology, variegated pigmentation is a warning sign for melanoma, especially when colours include blue, black, red and white.
• Variegated staining in a biopsy can complicate the assessment of a biomarker, so pathologists report the percentage and intensity of stained cells.

History

• Variegation was first a botanical term for leaves with patches of different colours, and nineteenth-century pathologists borrowed it for diseased organs.
• The nutmeg liver was described in the nineteenth century as a sign of chronic passive congestion, and the metaphor is still used in pathology teaching.
• The term myelolipoma was introduced in the early twentieth century for a tumour-like mixture of marrow and fat that typically looks variegated.
• Modern digital pathology has replaced some naked-eye descriptions with quantitative colour and texture analysis, but variegation remains a useful gross descriptor.

Research and controversies

• Whether variegation on imaging should be reported as a separate feature or as part of tumour heterogeneity is debated.
• Computer vision models can classify variegated tissue patches, but their performance depends on staining protocols and scanner calibration.
• The biological meaning of mosaic staining in tumours is still uncertain, because it may reflect genetic subclones, hypoxia or simply uneven fixation.
• Some authors argue that variegation is too subjective a term for modern reporting, while others defend it as a rapid low-power sign that guides sampling.
• A myocardial infarct becomes variegated as the pale necrotic centre is surrounded by a hyperaemic border and by surviving muscle.
• Cerebral infarcts show a mottled appearance when petechial haemorrhage is mixed with pale necrosis, especially after reperfusion.
• Caseous tuberculosis produces a variegated lung surface with grey-white caseous foci, red haemorrhage and anthracotic pigment.
• Metastatic carcinoma in the liver often creates multiple pale nodules that stand out against the brown parenchyma.
• Melanoma is the classic example of variegated pigmentation in dermatology, with irregular patches of brown, black, blue, red and white.
• On imaging, a variegated mass may show areas of enhancement, necrosis and haemorrhage that correspond to the gross colours.
• A variegated specimen should be sampled from each distinct colour zone, because a single block may miss a diagnostic component.
• Teaching collections use variegated specimens to show students how gross colour maps onto microscopic pathology.
• Machine learning applied to gross photographs can classify variegated patterns, but the models are sensitive to lighting and camera calibration.
• The term variegation is imprecise, and modern reports often replace it with specific descriptors such as haemorrhagic, necrotic or fatty.