Cellularity

Cellularity is a biological and histopathological term that describes the proportion of tissue volume occupied by living cells relative to extracellular matrix, stroma, fluid, and other non-cellular components. It provides a measure of how densely cells populate a given tissue.

Assessment is typically descriptive, using terms such as low, moderate, or high cellularity, or expressed as a percentage of the tissue volume. It is evaluated on formalin-fixed, paraffin-embedded sections stained with hematoxylin and eosin and can be supplemented by digital image analysis.

In neoplasia, cellularity refers to the density of tumor cells within a mass or biopsy. High cellularity often reflects a densely packed cell population, which can influence diagnostic interpretation, grading, and treatment planning; conversely, low cellularity may indicate abundant fibrous stroma or necrosis.

Bone marrow cellularity is a well-established clinical parameter that varies with age. It describes the balance between hematopoietic (cellular) marrow and fat (adipocytic) spaces; hypo- or hypercellularity can signal marrow failure, infiltration, or recovery after therapy.

In other tissues, cellularity can be altered in responses such as inflammation, fibrosis, or regeneration. For example, fibrotic tissue has low cellularity, whereas inflammatory lesions may show elevated cellularity due to infiltrating immune cells.

Limitations include sampling error, tissue heterogeneity, fixation and staining variability, and subjective interpretation, though quantitative image analysis and standardized reporting criteria are increasingly used.