Direction of uterine contractions during estrus in ewes: a reevaluation.

In four ewes direction of propagation of uterine contractions was evaluated using an electromyographic technique during 15 entire estrous periods; 120,101 propagations were analyzed. When horns were considered separately 89.9% of all propagations were classified into three main modes: ascending (34.3%), descending (59.85%), and divergent (5.85%). When both horns were analyzed simultaneously, horn's synchronicity was observed in most instances; on this basis, eight modes of propagation were identified of which three accounted for two-thirds of all propagation: synchronous descending (24.9%), reciprocal propagation (descending on one horn and then ascending on contralateral horn) (22.1%), and isolated descending propagation (18.0%). A time-dependent pattern of propagation throughout estrus was clearly identified, the percentage of ascending propagations reaching a minimum (16.5%) and the percentage of descending propagation reaching a maximum (77%) at peak uterine motility level. By considering both direction of uterine propagation and cervical mechanical activity, a new hypothesis concerning two aspects of sperm transport (speed and mechanism) was formulated. It is hypothesized that the high prevalence of descending propagations is important to reduce sperm cell population by selecting the most vigorous spermatozoa; such selection is possible when the mechanical cervical activity is low or absent (cervix open); when cervical mechanical activity is high (cervix closed), it is suggested that both descending and ascending propagations participate in sperm transport by back and forth motion of luminal fluid within the uterine lumen.