**3.2 Premature progesterone withdrawn/progesterone receptors loss/ responsiveness in premature cervical ripening**

Since 1960, from Cullen and Harkness studies on rat's cervix [78], one knows steroid hormones role in cervical stroma cells specific functions, and complex series of molecular events in pregnancy, and parturition. Steroid hormones actions are proved on HCSMCs surrounding IO [79], through their receptors (ER, PR). Progesterone exerts a dominant role for most of the pregnancy duration to initiate a loss of tissue strength yet maintain competence in the softening phase, which starts early in pregnancy when P4 is high, E2 is low, and normal ripening is at the gestation end [9, 80] to permit fetal delivery after active dilation. The reduced P4/E2 ratio—a parturition hallmark, begins the cascade of accelerated cervical ripening, responsible for the final structural integrity loss, necessary for fetus safe passage. P4/E2 ratio is high during softening; the highest P4 concentration correlated with the least amount of new collagen fibril synthesis, E2 significantly rising new collagen, and progressively increased levels as P4 decreases near term, when P4/E2 ratio is the lowest [81]. P4 controls GAGs modulating ECM [82] and counteracts inflammation [83]. The studies on rats [84] showed that PR genomic receptors modulate the transition from a soft to a ripened cervix in term and PTB. Studies on many mammalian species have shown that softening shift to ripening occurs while blood progesterone is near peak concentrations, being discussed as a responsiveness loss to progesterone, a common final mechanism for mammalian cervix remodeling for term birth preparation [70]. P4 sustains cervical softening and ripening, mediated by a stable stromal cell population, expressing PR-A, and through interactions with resident macrophages, the inflammatory ripening processes are mediated in birth preparation [85]. In ovariectomyinduced systemic progesterone loss, it was not registered hypertrophy, extracellular collagen, or macrophage number changes [35]. Yellon [70] appreciates that the structure and macrophage census in cervix is sufficient for premature ripening and birth before term; PRs are localized on other cells than macrophages, making possible interactions between cells to facilitate the loss of progesterone receptor-mediated actions, as part of a final common mechanism that ripens the cervix in certain preterm etiologies, and in term deliveries.

P4 non-genomic membrane receptors mechanism is dominant in mice, besides genomic mechanism, mediated by isoform A [85] for prepartum softening and ripening, which start before birth, when systemic P4 is near peak concentration [72], through stroma fibroblasts, but not macrophages. Smooth muscles, innate immune cells, and mainly pregnancy-activated lymphocytes have P4 membrane receptors, which fibroblasts are missing. P4 has immune-modulatory proprieties by the 31 kDa protein "progesterone induced-blocking factor", synthesized by human lymphocytes, which inhibits natural killer cells, and modifies cervical, and decidua cytokines balance. RU486/mifepristone mice administration on the 13th day of gestation induced the onset of preterm labor within several hours [43]. This model presents significant ECM disorganization: more and larger spaces between collagen fibers, higher cervical wet weight, a 5% increased water content *vs* LPS infected mice model, with reduced edema. RU-486 group cervix weight being similar to term cervix*.* Premature RU-48 induced ripening results from an acceleration of the process, in place during term ripening, as well as partial activation of proinflammatory, and immune-suppressive process, which is observed during normal postpartum repair, different from LPSinduced PTB, where only the neutrophil stroma population is increased, without parallel monocytes, and eosinophils populations, or systemic blood rise; no blood systemic monocytes levels increased in any studied group [43].

Japanese studies on cultured cervical stroma fibroblasts in cases with refractory PCF and recurrent pregnancy loss [86] have a remarkable discovery: namely, cervical fibroblasts' PRs down-regulation, with impairment of P4 inhibitory effect on lipopolysaccharide-induced inflammatory stimuli, and consecutive PCF. These results suggest that abnormal cervical ripening in CI is caused by down-regulation of P4 signaling at receptor level and provides a novel insight into the molecular mechanism of PTB.
