@parados,
“…..There is no way for a 6um cell to pass through it which is about the size of human sperm.”
Epididymis tubules exist to transport sperms and molecular proteins, not blood cells. In fact, the walls of tubules serve as a barrier against movements between tubule lumen and outside blood circulation (note 1). So, tubules are not designed for cells (other than sperms) to go through.
“I know that is what you meant but you have no evidence of cells in a multicell organism being that small. A red blood cell is 8um.”
As stated above, red blood cells do not go through epididymis tubules. Red blood cells move in blood vessels only, which run outside of tubules. More importantly, look at the size of the sperm I marked with red arrow in the tubule. It was less than one micron wide at head, and about 0.3 micron at tail. The important thing is that all cells in the image were in right proportion to one another, which made life possible.
Note 1: The blood-epididymal barrier quoted from an article in Journal of Reproduction:
The luminal microenvironment of the epididymis is comprised of specific ions, small organic molecules, and proteins that are secreted or absorbed by the epididymal epithelium [1, 8, 9]. The blood-epididymal barrier, which is composed of apical tight junctions between principal cells, forms an impenetrable seal and forces the movement of molecules across these cells by specific receptors, ion and water channels, and solute carrier proteins [1, 10, 11]. Thus, the barrier creates specific environments between the lumen and circulation and within the epithelial cells [12, 13].
