Lumicrine signaling: Extracellular regulation of sperm maturation in the male reproductive tract lumen

The behaviors of cells, tissues, and organs are controlled by the extracellular environment in addition to their autonomous regulatory system. Dysfunction of extracellular regulatory mechanisms affects not only the development and survival of organisms but also successful reproduction. In this review article, a novel extracellular regulatory mechanism regulating the mammalian male reproductive ability will be briefly summarized. In terrestrial vertebrates, spermatozoa generated in the testis are transported through the lumen of the male reproductive tract and become functionally mature during the transport. Recent studies with gene-modified animals are unveiling the luminal extracellular environment of the reproductive tract to function not only as the pathway of sperm transport and the site of sperm maturation but also as the channel for cellular communication to regulate sperm maturation. Of special interest is the molecular mechanism of lumicrine signaling, a transluminal secreted signal transduction in the male reproductive tract lumen as a master regulator of sperm maturation and male reproductive ability. The general significance of such transluminal signaling in the context of cell biology will also be discussed.