Saturday, August 25, 2018

"a new, even more patriarchal political conceit . . . the egg was merely a passive receptacle waiting"

Convincing evidence has instead revealed that human sperm are passively transported over considerable distances while travelling through the womb and up the oviducts. So much for Olympic-style racing sperm!

In fact, of the 250 million sperm in the average human ejaculate, only a few hundred actually end up at the fertilisation site high up in the oviduct. Sperm passage up the female tract is more like an extremely challenging military obstacle course than a standard sprint-style swimming race. Sperm numbers are progressively whittled down as they migrate up the female tract, so that less than one in a million from the original ejaculate will surround the egg at the time of fertilisation. Any sperm with physical abnormalities are progressively eliminated along the way, but survivors surrounding the egg are a random sample of intact sperm.

Many sperm do not even make it into the neck of the womb (cervix). Acid conditions in the vagina are hostile and sperm do not survive there for long. Passing through the cervix, many sperm that escape the vagina become ensnared in mucus. Any with physical deformities are trapped. Moreover, hundreds of thousands of sperm migrate into side-channels, called crypts, where they can be stored for several days. Relatively few sperm travel directly though the womb cavity, and numbers are further reduced during entry into the oviduct. Once in the oviduct, sperm are temporarily bound to the inner surface, and only some are released and allowed to approach the egg.