Epithelioid cells describe a specific cellular appearance rather than a distinct cell type. In medical contexts, particularly pathology, this term characterizes cells that have adopted a particular shape and arrangement. They morphologically resemble epithelial cells, which line surfaces throughout the body, even though epithelioid cells have a different origin. Their presence often signals specific underlying biological processes, making their identification meaningful.
Understanding Epithelioid Cells
Epithelioid cells are characterized by their large, elongated appearance with abundant, pale cytoplasm. Their cell boundaries can be indistinct, making them seem to merge, and their nuclei are centrally located and ovoid or elongated. These features give them a flattened, brick-like appearance under a microscope, similar to cells in epithelial tissues.
These cells originate from macrophages, a type of white blood cell. Macrophages are immune cells that engulf and digest foreign particles and cellular debris. Under certain conditions, macrophages transform, differentiating into epithelioid cells. This transformation involves significant internal structural changes, including increased cytoplasmic volume and the development of numerous secretory granules and an enlarged Golgi apparatus.
Despite their name and epithelial-like appearance, epithelioid cells do not perform the barrier or secretory functions of true epithelial cells. Instead, they are specialized immune cells that cluster together, reminiscent of epithelial layers.
Epithelioid Cells in Immune Responses
The primary functional role of epithelioid cells is their involvement in chronic inflammatory responses, specifically in the formation of granulomas. Granulomas are organized collections of immune cells that form when the body’s immune system attempts to wall off persistent, difficult-to-eliminate foreign substances or pathogens. This occurs when macrophages cannot completely clear an offending agent, leading to their activation and transformation into epithelioid cells.
Once transformed, these epithelioid cells aggregate and surround the inciting agent, creating a barrier to contain the infection or foreign body. They are found in direct contact with necrotic (dead) tissue at the center of the granuloma, forming a boundary zone. While epithelioid cells exhibit reduced phagocytic activity compared to their macrophage precursors, their collective action in forming granulomas is a defense mechanism. Tuberculosis (Mycobacterium tuberculosis) and sarcoidosis are examples where granulomas rich in epithelioid cells are prominent. In these diseases, epithelioid cell granulomas help contain the infection or inflammation, though prolonged formation can also contribute to tissue damage.
The Diagnostic Role of Epithelioid Cells
Pathologists frequently examine tissue biopsies for the presence and arrangement of epithelioid cells, as their identification provides important diagnostic clues. The characteristic clustering of these cells into granulomas indicates granulomatous diseases. For instance, finding epithelioid cell granulomas in lung tissue can suggest conditions like tuberculosis or sarcoidosis, prompting further specific diagnostic tests.
The presence of epithelioid cells helps pathologists differentiate between types of inflammation, distinguishing chronic granulomatous inflammation from acute processes. Their morphology can sometimes mimic certain cancer cells, such as those in epithelioid sarcoma. In such cases, pathologists use additional tests, including immunohistochemistry, to determine if the cells are true epithelial cancer cells or epithelioid immune cells. While the identification of epithelioid cells is significant, a definitive diagnosis requires correlation with clinical history, other laboratory findings, and specialized staining techniques to confirm the underlying cause.