PRL and meningioma: The arguments underlying the hypothesis are (1) prolactin is secreted by the anterior pituitary gland [29], (2) the plasma concentration of prolactin during pregnancy reaches its peak at term [30], (3) prolactin accelerates the proliferation rate of meningioma-derived cells in vitro [31,32,33], (4) prolactin is both a circulating hormone and paracrine/autocrine factor participating in angiogenesis [34], which is important in the tumorigenesis process of, for example, meningiomas [35], and (5) prolactin has a pivotal role in osmoregulation in various cells and tissues [36,37].