Abstract
Wnt signaling is one of the cell's most complex and important signal transduction pathways. This pathway, which is divided into additional sub-pathways, regulates cell growth, motility, polarity, and differentiation during embryonic development as well as stem cell regeneration. In addition, the Wnt cascades are involved in maintaining different aspects of adult homeostasis. The Wnt ligands, which normally initiate this cascade, are secreted glycoproteins that bind specific receptors and co-receptors to execute their intracellular signaling activity. The Wnt pathways have been extensively studied in anchored cells and in tissues. However, recent evidence now shows that the Wnt cascades are functional in the circulation and that these important signals can affect different circulating blood cells by traveling through the bloodstream. Wnt signaling can function in both paracrine and autocrine manner; however, in the current review, we will discuss the exocrine properties of the pathway and address the following topics: the source of Wnt ligands and their ability to travel in the bloodstream; which cell populations express Wnt signaling components; and finally, what are the physiological roles of the Wnt cascade in the different circulating blood cells.