Structural changes in the contractile apparatus of the myometrium during estrus and pregnancy.

: The ultrastructure of the smooth muscle of the uterus of cats was studied at estrus and at several periods during pregnancy. For the pregnancy studies myometrium was observed at 10, 14, 17, 21, and 50 days postcoitus (PC) as well as in five animals close to term (60-63 days PC). Both thick and thin filaments are observed at estrus in five of the six animals studied and also in all of the animals up to 14 days PC. There is a variable occurrence of thick filaments between 14 and 21 days PC at which time thick filaments are not observed over the implantation site but are observed in myometrial tissue taken away from the nidus. Only thin filaments are observed in the myometrium of the pregnant cat after 21 and before 60 days PC. Prior to term there is again a variable occurrence of thick filaments with thick filaments observed in only one of the five animals studied between 60-63 days PC. These results may implicate the fetoplacental complex in the assembly and disassembly of the thick filaments of uterine smooth muscle. The relationship between thick filament assembly, phosphorylation and the parameters regulating uterine contractility are complex and the variable occurrence of thick filaments at the end of term may implicate factors other than hormonal fluctuations, such as stretch and oxytocics, in this process.