The ecology of nidation.

Nidation defined as a process beginning with coitus and ending with the implanting of the egg involves a sequence of interdependent events which include transport of semen through the uterus transport of ova do wn the Fallopian tubes and a concurrent modification of the tubal and u terine milieu. These occurrences facilitate zygote transport and produc e a suitable environment for implantation of the fertilized ovum. 5 stages of nidation include priming - beginning just before proestrus and including mating and ovulation lasting 2 or 3 days and merging with sensitization - lasting 4 days and ending about the time the fertilized ovum enters the uterus. Uterine sensitivity occurs next in which stimulation and induction occur. Induction initiates decidualization wh ich begins with the appearance of transformed stromal cells and continues on to placentation. The decidual tissue becomes the maternal component of the placenta. Implantation occurring during the early stages of decidualization includes penetration of the nidus and envelopment of the blastocyst. A constant feature of the uterine environment throughtout these processes is progesterone which is continuously secreted by the corpora lutea. Reviewing the process of nidation serves to provide means of achieving control of human fertility by interfering with any of the many factors upon which implantation depends during any of the several phases which precede it.