In this review, we describe a new model to explain the regulation of myometrial function during pregnancy and labour. We propose that the myometrium undergoes dramatic changes in phenotype from early pregnancy until the onset of labour, characterized by an early proliferative phase, an intermediate phase of cellular hypertrophy and matrix elaboration, a third phase in which the cells assume a contractile phenotype and the final phase in which cells become highly active and committed to labour. The last phase of myometrial differentiation is postpartum uterine involution, completing the reproductive cycle following pregnancy and labour by returning the uterus to its non-pregnant receptive state. We further propose that phenotypic modulation of the uterine myocytes is the result of integration of endocrine signals and mechanical stimulation of the uterus by the growing fetus. Our previous studies have shown that these signals are important in regulating the onset of labour and we now have indications that they regulate earlier myometrial smooth muscle differentiation. We show that the high rate of myometrial cell proliferation in early pregnancy which reflects important aspects of many smooth muscle populations during development. The proliferative phenotype was associated with dramatic changes in the expression of IGF family proteins and coincided with an up-regulation of the anti-apoptotic pathway. Preliminary evidence suggests that myometrial hyperplasia was controlled by the PI3K-Akt-mTOR signaling pathway. The modulation of the mTOR pathway by rapamycin blocked the proliferative activity of the uterine myocytes. The growth and remodeling of the myometrium during pregnancy was associated with increased synthesis of extra cellular matrix (ECM) proteins and their corresponding integrin receptors. Our results show a decrease in expression of fibrillar collagens and a coordinated temporal increase in expression of components of the basement membrane near term associated with decreased progesterone levels and increased mechanical tension. The phenotypic modulation of uterine smooth muscle cells during pregnancy culminates at term when a myometrium-specific conversion commits these cells to the labour phenotype, characterized by increased excitability, spontaneous activity, responsiveness to agonists and effective coupling of the myocytes. The reinforcement of the ECM-integrin interaction contributes to myometrial hypertrophy and remodeling during late pregnancy and facilitates force transduction during the contractions of labour by anchoring hypertrophied SMCs to the uterine ECM. In addition, we suggest that myometrial cells play an important role in the generation and regulation of uterine inflammation, which is a characteristic feature of parturition. We provide here substantial evidence that myometrial cells can actively participate in the inflammatory process in the uterus by the release of the pro-inflammatory chemokine MCP-1. The increased production of MCP-1 in the term myometrium was associated with uterine occupancy and regulated by progesterone, suggesting that mechanical and endocrine signals integrate to regulate the expression of the chemokine and the induction of labour. A better understanding of the mechanisms regulating myometrial differentiation during pregnancy might inform the development of new therapeutic strategies for the management of preterm labour, which remains a leading cause of neonatal morbidity and mortality. Our data are obtained mostly from the rat, but we believe that they are generally applicable across species.
