When in vitro fertilization techniques became
established, similar findings on the increased competence
of more distally obtained cells were observed with sper-
matozoa from the caput epididymidis fertilizing fewer eggs
in vitro than caudal spermatozoa, whether the eggs were
invested in cumulus or the zona was present or not. The
conditions of capacitation used were those designed to
optimize fertilization by caudal spermatozoa, so again there
is an in-built bias towards these cells. Nevertheless, under
these in vitro conditions when migration through and sur-
vival within the female tract was not necessary, cauda
spermatozoa always had an advantage over those from
the caput in binding to and penetrating the zona and in
binding to and fusing with the oolemma when the zona
was removed. This functional competence was paralleled
by development of motility (acquisition of flagellar beating
and development of coordinated axonemal sliding to pro-
vide forward propulsion) and morphology (compactness
of nuclear and flagellar structures). Therefore, every
sperm function required for fertilization seemed to be deve-
loped in the epididymis: motility, zona binding and mem-
brane fusion.