The unexplained behavior of egg retention at the ampulloisthmic junction (AIJ) is a mechanical phenomenon; therefore the key to this process must lie in the anatomy of the oviduct. To investigate the concept that simple anatomical differences between ampulla and isthmus cause AIJ retention, we quantified, in these two regions of the rabbit oviduct, the structural characteristics of the tubal wall that show marked differences: wall thickness and luminal caliber. The oviductal wall greatly increases in thickness and decreases in luminal caliber abruptly at the transition from ampulla to isthmus. These passive mechanical factors predict a dramatically reduced distensibility of the isthmic wall compared to that of the ampulla, and these factors provide us with a simple model to account for the apparent sphincteric activity of the AIJ. Actual measurements of tubal distensibility confirmed our prediction. In addition, egg transport studies following microsurgical removal of the AIJ showed normal transport, an indication that no special structure need exist at the AIJ. We conclude that the contrasting morphology of the ampulla and isthmus may suffice to explain AIJ retention, and no special active mechanism need be postulated.
Knowledge of the structural and functional relationships of the mammalian oviduct is essential to management of clinical infertility. However, egg retention in the ampulla, a phenomenon believed to be critical in normal fertility, has yet to be explained adequately in terms of tubal anatomy.
The ovum is delayed in the ampulla for approximately 1 day in rabbits (Greenwald, 1961), 2 days in rhesus monkeys (Eddy et al., 1975), and up to 3 days in women (Croxatto et al., 1978). This delay appears to be part of an integrated mechanism that allows adequate time for fertilization and early embryo development and prevents premature exposure of the embryo to the endometrial cavity. Although success of in vitro fertilization suggests that the specific milieu of the ampulla is not essential to fertility, there is little doubt that this region of the oviduct provides the optimal environment for these processes and that ampullary delay increases the likelihood of successful conception.
Despite the large amount of experimental work performed mainly on rabbits, the complete nature of am-