Relationship Between Lower Uterine Segment And CRH Concentrations In Retroplacental Blood And Cord Blood During Parturition At Term

Parturition is a complex process involving different compartments with changes on the maternal side (myometrium and cervix) and on the fetal side (fetus itself, fetal membranes) leading to delivery of the fetus. The mechanism of onset of labor and the interrelationship between these compartments remains elusive and is likely multi-factorial, with many physiological pathways being involved. The procedure requires that the uterus, which has been maintained in a relative state of quiescence during pregnancy, develops coordinated contractility and that the cervix dilates in a manner that allows passage of the fetus through the birth canal and maturation of those fetal organ systems necessary for extra-uterine survival . Of the multiple mechanisms implicated, it was suggested that the fetus may be in control of the timing of birth via activation of its own hypothalamic- pituitary- adrenal(HPA) axis. Current thinking is that placental corticotropin- releasing hormone (CRH) promotes activation of the fetal hypothalamic- pituitary- adrenal axis, which in turn stimulates the production of cortisol by the fetal adrenal gland, followed by activation of a cascade of events that suppress the mechanisms responsible for uterine quiescence. It exhibits a higher level in maternal plasma during pregnancy, and decreases rapidly after parturition.This paper studies the relationship between the lower uterine segment and CRH concentrations in the retroplacental blood of the maternal-fetal interface and cord blood during parturition at term. There were 42 women undergoing cesarean section sampled and divided into three groups: 15 cases before labor,15 cases in latent phase and 12 cases in active phase. Thelengths of lower uterine segment were measured in the operation and CRH concentrations obtained through radioimmunoassay. Excel statistical software was used for statistical analysis.The results show that CRH concentrations increase after the onset of labor both in retro-placental plasma and in cord blood. P values below 0.05. But the difference of the CRH concentration was not significant between in latent phase and in active phase, and the change of CRH in cord blood was resemble with that in retro-placental blood .The CRH concentration showed positive linear correlation in the two locations. We have also determined that the length of lower uterine segment increase with labor progressing, which showed significant difference among three groups. P values below 0.05. The length of lower uterine segment changed significantly in a direct linear relationship with the concentration of CRH.Placental production of CRH is proposed as an early event regulating the cascade of events of labor. It exerts its effects by binding to specific G protein-coupled receptors, two major subtypes of which have been recognized,CRH-R1 and CRH-R2. CRH activates maternal and fetal HPA axis. Activation of the fetal HPA axis increase output of dehydroepiandro- sterone, dehydroepiandrosterone sulfate, androstenedione, and cortisol . In placenta DHEA is synthetized to estrogen, which taking part in labor onset. Cortisol contributes to fetus development and maturation,and the specialized ability of which is to promote placental CRH production intrauterine tissues, forming a positive feedback loop. CORT binds to glucocorticoids receptors by competing with progesterone, which leading"functional pregesterone withdrawal". CRH increases PGHS expression and decreases PGDH activity,which increases output of PG. Besides CRH increases oxytocin receptors expression of intrauterine tissues.During active labor, the uterus is transformed into two distinct part. The actively upper segment becomes thicker as labor advances. The lower segment of the uterus and the cervix is relatively passive compared with the upper segment. Herein lies the importance of the division of the uterus into an actively contracting upper segment and a more passive lower segment that differ not only anatomically but also physiologically. For the contents of uterus to be diminished, particularly early in labor when the entire uterus is virtually a closed sac with only a minute opening at the cervical os, the musculature of the lower segment must stretch. This permits increasingly more of the intrauterine contents to occupy the lower segment, and the upper segment distends and the cervix dilates. The fibers of the lower segment becomes stretched with each contraction of the upper segment. The successive lengthening of the muscular fibers as labor progresses is accompanied by thinning. The difference between the upper segment and the lower segment is presumed related to the different expression of CRH receptors at the upper segment and the lower segment.During pregnancy the cervix must remain unyielding and reasonable riyid. Coincident with the initiation of parturition,the cervix must soften, yield, and become more readily dilatable to allow a timely passage of the fetus at term. The cervical modifications involve changes that occur in collagen, connective, and its ground substance. The mechanisms and effects on each step in ripening are not clear, but this process seems to be controlled at least in part by hormones, including progesterone, estrogen, relaxin andandrogens. Autocrine and paracrine mediators like cytokines, prostaglandins, Platelet activating factor, and nitric oxide produced by different isoforms of its synthases have been shown to take part in physiologic cervical ripening thereby forming a complex network with short circuits and parallel pathways. CRH stimulates production of estrogen, prostaglandin and cytokines, hastening cervical softening indirectly.