Mammalian sperm collected from the testis are immobile and unable to bind to and then fertilize the oocyte. They acquire these properties gradually while they travel through the epididymal tubule. During this transit, the sperm lose their cytoplasmic droplets and their plasma membrane undergoes profound changes, including addition, removal, or transformation of proteins and lipids. These gradual surface modifications occur in response to variations in the luminal fluid composition and are thought to be responsible for their final maturation.
One of the most striking changes is the transfer to the sperm membrane of very hydrophobic proteins found in the epididymal fluid and of proteins that normally bear glycosyl phosphatidyl inositol (GPI) tails. Because such proteins normally exist in a membrane environment and are not free in the fluid, it has been suggested that a transport system, for example, vesicles (named epididymosomes) or lipid micelles, might exist in the epididymis. The presence of membrane vesicles in seminal plasma of human and mammals (named prostasomes) has been well documented, and it has been suggested that these vesicles are derived mainly from the accessory glands, such as the prostate or the seminal vesicles.
We have recently provided evidence that such vesicles exist in the cauda epididymal fluid of rams, and in vitro experiments have shown that such vesicles, which are also present in the bovine cauda epididymis and seminal plasma, can transfer some proteins to the sperm in zinc-and pH-dependent manners.
In this study, we definitively establish that an intraluminal system of membrane vesicles occurs naturally in the ram cauda epididymis. We have isolated, purified, and biochemically characterized these vesicles from the epididy-mal fluid and the seminal plasma, and we defined biochemical markers that help us to determine their origin. Their relationship to the vesicles present in the seminal plasma and to exosome vesicles secreted by other types of epithelium is discussed, as well as their role in the transport and transfer of proteins to the sperm membrane.