SEGUNDA ETAPA: DE EJECUCIÓN DEL FUNCIONAMIENTO DEL TALLER:

contractions and aids in the transportation of sperm and ova into the uterine horns. Meanwhile, the endometrial lining begins to degenerate and the glands begin to reduce in size. In metestrus, the lining of the endometrium continues to degenerate, while the amount of fluid in the uterine horns and the uterine contractions decrease. During this stage of the estrus cycle, vaginal smears have leukocytes as well as nucleated and non-nucleated cells. In diestrus, the endometrial layer is reduced in size, and the uterine contractions cease (Kom´arek et al., 2000; Freeman, 2006; Westwood, 2008).

3.3

Pregnancy and Parturition

The reproductive differences between rats and humans are not limited to the struc- ture of their reproductive organs and cycles (see Section 3.1 and Section 3.2), but also extend to pregnancy and parturition. Rats carry multiple offspring and have short gestations of ∼ 22 days, while humans usually have singleton pregnancies of

∼ 39–40 weeks (Sengupta, 2013; Mesiano, 2009). Apart from the number of off- spring, pregnancy in these two species is maintained by different endocrine organs. Pregnancy in humans is maintained by P4 synthesised in the corpus luteum during the first seven weeks of pregnancy, a role which is taken over by the placenta from week 12 to term (Mesiano, 2009). While in the rat, despite the development of several P4-secreting placentae, pregnancy is maintained by the corpora lutea, such that the removal of the ovaries results in labour and delivery of the pups (Maeda et al., 2000).

3.3. Pregnancy and Parturition

3.3.1 Pregnancy

If mating occurs at estrus, the cervix is stimulated and it signals for the release of prolactin by the pituitary gland. The secretion of prolactin inhibits the conversion of P4 to 20α-hydroxyprogesterone, thus maintaining the lining of the uterus through the secretion of high levels of P4 by the corpora lutea (Freeman, 2006). The rescuing and maintenance of luteal function in the absence of pregnancy is called pseudopregnancy, a state that usually persists for∼11 days, before luteolysis occurs (Maeda et al., 2000; Stouffer, 2006).

If, however, the mating at estrus is fertile, fertilisation occurs and blastocysts im- plant ∼ 5 days after mating. Decidualisation in the rat is confined to the site of implantation, unlike in humans where the whole endometrium is decidualised, sug- gesting that the implanting embryos are involved in triggering the decidual reaction in rats (Maeda et al., 2000). While the placentae in rats secrete P4, the major source of the P4 that maintains pregnancy in rats is synthesised by the corpora lutea, which are sustained during the first half of gestation by prolactin and by placental lactogens during the latter half of gestation. In addition, E2 is also in-

volved in maintaining the synthesis of steroid hormones by the corpora lutea (Maeda et al., 2000).

3.3.2 Parturition

Parturition in the rat, and mouse, is preceded by a sharp decline in the levels of circulating P4, a phenomenon that is not observed in human parturition. In fact, labour in humans occurs in the presence of high, and sometimes increasing, levels of P4 (Zakar & Hertelendy, 2007). The systemic withdrawal of P4 that precedes

3.3. Pregnancy and Parturition

Membrane    

phospholipids   _PLA Arachidonic    acid   PGG2   PGH2   PGF2α  

2   CO X   pe ro xi das e   PG F   sy nth as e  

Figure 3.2: Synthesis of PGF2α.PLA2converts membrane phospholipids to arachidonic acid,

which is in turn converted to PGG2by COX enzymes. Peroxide activity results in the formation of

PGH2,an unstable PG that is immediately converted to PGF2α by PGF synthase. Abbreviations:

PLA2: phospholipase A2;COX: cyclooxygenase;PGH2: prostaglandin H2;PGF: prostaglandin F;

PGF2α: prostaglandin F2α.

labour in rats, and mice, is brought about by the luteolytic effects of prostaglandin (PG) F2α (PGF2α). PGF2α is synthesised from arachidonic acid, which is derived

from membrane phospholipids, through the action of phospholipase A2 (PLA2),

cyclooxygenase (COX) enzymes, peroxidase, and prostaglandin F synthase as illus- trated in Figure 3.2 (Michal & Schomburg, 2012, p. 310). There are two types of COX enzymes: one which is normally expressed in various tissues of the body, COX-1; and another COX-2 whose expression can be induced (Vane et al., 1998). The effects of PGF2α are mediated by the PGF2α receptor (FP), a G-protein cou-

pled receptor. Through FPs on corpora lutea, PGF2α induces luteolysis resulting in

the systemic withdrawal of P4 and the onset of labour (Ratajczak & Muglia, 2008; Sugimoto et al., 2015). PGF2α−FP signalling has also been shown to stimulate

contractility through the increase of intracellular Ca2+ (Funk et al., 2009).

The role of PGF2α in the initiation of labour in mice was confirmed in studies of

genetically altered mice; Sugimoto et al. (1997) demonstrated that labour could not be initiated in FP knock-out mice, FP-/-, as luteolysis and P4 withdrawal did not occur. In addition to the maintenance of high circulating levels of P4 inFP-/- mice, OTR expression was not upregulated in uterine tissues as is consistent with the onset of labour. In mice lacking PLA2,and mice lacking the constitutive COX-1 enzyme,

P4 levels remained elevated and labour was delayed, suggesting that PLA2,COX-1,

PGF2α,and FP are required for luteolysis and the systemic withdrawal of P4 in mice 48

3.3. Pregnancy and Parturition

at term (Ratajczak & Muglia, 2008; Funk et al., 2009; Sugimoto et al., 2015).

Condon et al. (2004) proposed that the signal for the initiation of parturition at term in mice originates from the fetus through the augmented production of surfactant protein A (SP-A) from maturing pup lungs. SP-A is a component of pulmonary surfactant, which is necessary for breathing following birth as it reduces surface tension at the interface between air and water in lung alveoli (Khubchandani & Snyder, 2001). SP-A interacts with amniotic fluid macrophage Toll-like receptors culminating in their activation and migration into the uterus, thus increasing the production of interleukin-1β (IL-1β), as well as activating a nuclear factor (NF)-

κB. The increase in IL-1β production and activation of NF-κB upregulates the expression of PLA2,and induces the expression of the COX-2 enzyme leading to the

increased production of PGF2α,luteolysis, and the withdrawal of progesterone. In a subsequent study, Montalbano et al. (2013) showed that SP-A-deficient, SP-A-/-, mice delivered at term during their first pregnancies. In subsequent pregnancies, however, the onset of labour was delayed and was accompanied by a decrease in the expression of IL-1β as well as other pro-inflammatory markers, suggesting that initiation of the parturition cascade by fetal SP-A may not occur in first pregnancies, but in subsequent ones. Recently, using genetically altered mice, Gao et al. (2015) showed that mice lacking both SRC-1 and SRC-2, coactivators involved in regulating the transcription of the gene that encodes SP-A, had delayed parturition as a result of the inability of the pups to produce SP-A. Gao et al. (2015) further demonstrated that wild-type female mice that were mated with SRC-1/SRC-2-deficient males had delayed luteolysis and elevated levels of circulating P4 at term, suggesting that fetal signals to initiate parturition require mature lungs that can produce adequate amounts of surfactant lipoproteins such as SP-A.