De Graaf's thread: the human epididymis.

The epididymis consists of a single, highly coiled and convoluted tubule that Antoine De Graaf, the famous 17th-century anatomist, likened to a thread thickening to a string. The uncoiled tubule is several meters long and sperm in transit through it become functionally mature under the under the influence of the tubule lumen's microenvironment. The regulation of that microenvironment and the manner by which it influences sperm maturation have been the topic of investigation for many years, though the study of the human epididymis directly is fraught with problems related to sample availability and condition. Nevertheless, investigations using a variety of mammalian tissue sources, human included, have resulted in significant advances in our understanding of both the biology and pathology of the organ. The epididymal functions of transporting, concentrating, maturing, and storing sperm are important to male fertility and their absence or significant impairment can be a factor in male infertility.