Parturition

The maternal placenta also produces relaxin, a hormone produced by the ovaries. Relaxin causes relaxation of the cervix during parturition. 

Precisely what initiates parturition in humans is not known. Unlike the situation in sheep, there is no induction 

Pre- and postmature childbirths are each estimated to occur in almost 10% of pregnancies in the United States. The physiological processes of parturition (birth) must be precisely timed and effective to avoid harm to the mother and fetus. Like pregnancy, parturition relies on the sending and receiving of numerous hormonal signals between mother and fetus and is therefore especially susceptible to toxic insult. 

The development of commercial substitutes for breastmilk has lessened the frequency of breastfeeding in humans, but this does not apply to domesticated and wild animals. Much attention is given to the issue of toxicant transfer from mother to offspring via breast milk (Chapter 34), yet reproductive toxicologists must also consider the impact that impaired lactation or lactational behavior may have when evaluating changes in the fecundity of a population. 

Progesterone is the hormone of pregnancy, relevant effects of which include  

A decrease in the contractile activity of the myometrium which is necessary for implantation. This is achieved by inhibition of prostaglandin formation. 

Development of the endometrium to form the placenta after implantation. 

It would be expected that, since the foetus is a foreign body, it should be recognised as such by the immune system and rejected. Furthermore, foetal tissue is part of the placenta, so that maternal and foetal tissue are intimately structurally associated. This is no evidence that the maternal immune system is suppressed during pregnancy since the mothers do not suffer from an abnormally high number of infections. 

Four suggestions have been made as to why foetal cells are not rejected, although there is no firm evidence for any  

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Reproductive System. The primary PGs are intimately involved in reproductive physiology (67). PGE2 and PGP2Q, are potent contractors of the pregnant utems and intravenous infusion of either of these compounds to pregnant humans produces a dose-dependent increase in frequency and force of uterine contraction. PGI2 and TXA2 have mild relaxant and stimulatory effects, respectively, on uterine tissue. The primary PGs also play a role in parturition, ovulation, luteolysis, and lactation and have been impHcated in male infertility. 

Gebaren, n. childbirth, parturition. Gebarmutter, /. uterus, womb. 

The first SPRM to reach the advanced stage of clinical development for treatment of endometriosis and uterine fibroids, asoprisnil, is expected to receive FDA approval this year. The therapeutic effect of asoprisnil stems from its PR antagonist/ antiproliferative activity in the endometrium and breast. Unlike classical PR antagonists however, this compound does not induce labor in animal models of pregnancy and parturition. Recent structural studies... 

AVP plays a central role in water homeostasis of terrestrial mammals, leading to water conservation by the kidney. OT is primarily involved in milk ejection, parturition and in sexual and maternal behaviour. Both hormones are pqDtides secreted by the neurohypophysis, and both act also as neurotransmitters in the central nervous system (CNS). The major hormonal targets for AVP are the renal tubules and vascular myocytes. The hormonal targets for OT are the myoepithelial cells... 

Although the OT system may be regarded as a key regulator of labour, in OT-deficient mice, parturition remains unaffected. Moreover, OTR knockout mice do deliver in a normal fashion, but the offspring die during the very first days of life, due to starving, as the milk ejection reflex is absent in these animals. These experiments show that the OT system is not essential for labour or reproductive behaviour (at least in mice), but for the milk ejection reflex, which is fundamental for litter survival. 

Placental transfer of methyl parathion was demonstrated following oral administration to pregnant rats 1-3 days before parturition (Ackermann and Engst 1970). 

Data from a single study in dogs suggest that hepatic first-pass metabolism may limit systemic availability of the parent compound following oral exposure (Braeckman et al. 1983). Placental transfer of methyl parathion was demonstrated in pregnant rats 1-3 days before parturition. Thirty minutes after administration, both methyl parathion and methyl paraoxon were found in fetal brain, liver, and muscle methyl parathion, but not methyl paraoxon, was detected in placenta and maternal liver (Ackermann and Engst 1970). Methyl parathion binds reversibly to serum albumin, but is readily distributed to the tissues (Braeckman et al. 1980, 1983). 

Lung surfactant is composed mainly of lipid with some proteins and carbohydrate and prevents the alveoli from collapsing. Surfactant activity is largely attributed to dipalmitoylphosphatidylcholine, which is synthesized shortly before parturition in full-term infants. Deficiency of lung surfactant in the lungs of many preterm newborns gives rise to respiratory distress syndrome. Administration of either natural or artificial surfactant has been of therapeutic benefit. 

Relax in 53 amino acids insulin-like (two protein chains) E. coli Facilitates childbirth Prepares endometrium for parturition and reduces fetal distress Pig relaxin shown to be clinically effective... 

It is well established that the concentration of oxytocin in plasma is increased in a pulsatile manner prior to, and during, parturition in a wide... 

The pivotal role played by oxytocin and its receptor during parturition has prompted many research groups to seek an effective antagonist of these effects. The starting point for these endeavours was the endogenous agonist. 

In mart, an overlooked feature is the occurrence of mucoid-like plugs in the foetal nostrils (Schaeffer, 1910). The presence of this blockage can be confirmed by endoscopic inspection in utero these plugs seem likely to affect free amniotic flow, since they appear to be reinforced by a folded membranous gathering at the nasal vestibule (PI. 4B). A degree of restriction of fluid access to the VN aperture, which is immediately caudal to the nostril aperture, and is patent in foetal life, may be a protective feature (Jordan, 1972). The timing of the dissolution of these sealant devices prior to parturition is regrettably not known. 

Later in intra-uterine life, the human infant is susceptible to early chemical prompting, but again the affector route is not known with certainty. Neonatal discrimination in favour of familiar (maternal) amniotic fluid is demonstrable, suggesting that the foetus already has active chemosensory capacities (Schaal, 1998). Smell and taste are operative in the near full-term foetus since it shows detection of about 120 mg/day maternal intake of anethole (as anise condiments) within a few days before parturition this exposure induced subsequent preferential responses by babies to anethole (Schaal et ai, 2000). The human neonate is not likely to have its organ as a fully functioning chemosensor,... 

The developmental changes seen in the immediate postnatal period in altricial rodents and especially in the early stages of marsupials, are an expected outcome of their shortened gestational period, early parturition and consequential dependent status. Regrettably, the relative contribution of the main and accessory chemosensory route(s) cannot be fully assessed. The lesser importance of the AOS (by some tests)... 

Parathion was added to diets in form of carefully prepared dust. Dietary levels in p.p.m. are as parathion. All values are average for number of survivors. Omissions in female series cover periods of parturition and... 

Clapham DE, Lechleiter JD, Girard S 1993 Intracellular waves observed by confocal microscopy from Xenopus oocytes. Adv Second Messenger Phosphoprot Res 28 161-165 Martin C, Chapman KE, Thornton S, Ashley RH 1999 Changes in the expression of myometrial ryanodine receptor mRNAs during human pregnancy. Biochim Biophys Acta 1451 343—352 McLean M, Smith R 2001 Corticotrophin-releasing hormone and human parturition. Reproduction 121 493-501... 

Hudson, R., Muller, A. and Kennedy, G.A. (1995) Parturition in the rabbit is compromised by daytime nursing The role of oxytocin. Biol. Reprod. 53, 519-524. 

Hudson, R., Cruz, Y., Lucio, R.L., Ninomiya, J. and Martinez-Gomez, M. (1999) Temporal and behavioral patterning of parturition in rabbits and rats. Physiol. Behav. 66, 599-604. 

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