In the picture above one can see the white epididymis transitioning into the vas deferens. Follow the blue arrow. You can see where the vas is cut (red), fulgurated (green) and tied (yellow).
The answer to the question: Two things happen.
There is a blockage after the vasectomy and this cause a mild back pressure. The vas dilates due to this and as a result there is a markedly decreased production of sperm.
The sperm in the vas is also broken down initially into sperm parts and over time eventually becomes a creamy fluid of amorphous debris.
At the time of a vasectomy reversal these changes are seen. A dilated vas on the testicle side of the vasectomy and visual and microscopic changes of the fluid which is released when the vasectomy site is incised.
Fun fact: Only approximately 5-10% of the ejaculate is sperm, the majority is from the prostate so there is very little noticeable change in the volume of the ejaculate after a vasectomy.
The surgeon, the interval of time since the vasectomy and …a sperm granuloma. Of the three important factors of a reversal, the quality of the fluid at the time of the reversal involves a bit of luck.
When the tube closest to the testicle is cut, fluid emanates from the cut edge and then is examined. The quality of the fluid is an important factor in the success of the reversal. So…clear or slightly milky fluid with whole sperm has a higher success rate than say creamy fluid and sperm parts. Having a sperm granuloma or testicles that just happened to protect the quality of sperm after a vasectomy, gives the patient a higher success rate.
Having good fluid to work with at the time of a vasectomy reversal is simply…good luck.
I’m reading a book on the the North African campaign during WW2. There were many qualities in a general that were important in their advancement. One of the biggest qualities that aided advancement in rank was the general’s history of good luck.