pilocytic

Pilocytic is an adjective used in biology and medicine to describe structures or cells that have hair-like or filamentous projections. The term derives from the Latin pilus, meaning hair, and is applied to emphasize a characteristic piloid or hair-like appearance in cellular processes.

In histology, pilocytic describes cells with slender, elongated processes that resemble hair filaments. The term is frequently used in relation to piloid astrocytes, a defining cell type in certain brain tumors. These cells typically exhibit GFAP positivity and an interwoven, delicate cytoplasmic extension pattern that contributes to a pale, threadlike appearance on light microscopy.

Pilocytic astrocytoma is the most well-known clinical instance of pilocytic pathology. It is a benign, World Health Organization Grade I brain tumor, most common in children and young adults. It often originates in the cerebellum but can arise along the optic pathway or hypothalamic region. Symptoms may include headaches, balance problems, or vision disturbances, depending on location. Magnetic resonance imaging usually shows a cystic lesion with a mural nodule. The preferred treatment is surgical resection, which is frequently curative when a gross total resection is achievable. The prognosis is generally favorable, with long-term survival rates high after complete removal.

Histologically, pilocytic astrocytoma shows piloid (hair-like) cytoplasm in tumor cells, Rosenthal fibers, low mitotic activity, and a prominent reticulin network around tumor tissue. Molecularly, fusions involving BRAF, such as BRAF-KIAA1549, are common and support diagnosis in appropriate contexts.