Stretch-induced uterine myocyte differentiation during rat pregnancy: involvement of caspase activation.

Proliferation, differentiation, and apoptosis are three major processes by which the pregnant uterus maintains homeostasis to accommodate the growing fetus. We demonstrated previously that caspase activation in the pregnant rat myometrium at midgestation coincides with the transition from uterine hyperplasia to hypertrophy. We hypothesized that this transition was induced by stasis of myometrial blood flow (and subsequent hypoxia/ischaemia insult) resulting from acute myometrial stretch induced by a growing embryo. Therefore, we measured the expression of active caspase 3 and two hypoxia markers (transcription factor HIF1A and pimonidazole hydrochloride) in pregnant rat myometrium. To investigate the effect of gravidity we used unilaterally pregnant rats. Caspase 3 was activated only in the gravid horn of the unilaterally pregnant animals on Gestational Days 12-15. This activation was associated with high levels of HIF1A and pimonidazole immunostaining, which were limited to the circular myometrial layer of the gravid horn, indicative of hypoxia within this tissue. To isolate the effect of myometrial stretch applied by the growing fetus, we inserted an expandable polymer tube (intra-uterine expandable tube [IUET]) into the empty horn of Day 13 and Day 20 unilaterally pregnant rats. Tissue was collected 2, 14, and 24 h later. In the IUET-stretched empty horn, cleaved caspase 3 was activated at midgestation (Day 14), but not at late gestation (Day 21). We speculate that hypoxia resulting from mechanical stretch may activate caspase 3 within the pregnant myometrium only in the context of a specific endocrine environment.