Reproduction in Domestic Animals

Lactocrine Programming of Uterine Development 275Fig. 2. Maternal and environmental programming of development.Natural and anthropogenic macroenvironmental factors (oval) impingeupon the maternal system throughout pregnancy and lactation(triangle). Pre-natally, maternal effects on foetal development areprofound and factors of macroenvironmental origin with the potentialto affect development are communicated via the maternal system (left).During this period the uterus serves as the conduit for communicationbetween mother and (potential) offspring. With parturition, communicationbetween mother and offspring continues via the mammarygland and milk (right). Post-natally, macroenvironmental factors mayaffect neonatal development directly and may also be conductedthrough (and modified in) the maternal system. The term lactocrinewas coined to describe a mechanism through which bioactive milkbornefactors are delivered from mother to offspring as a specificconsequence of nursingis clear that factors with the potential to affect uterineER expression and activation, either directly or indirectly,should be expected to have significant effects ondevelopmental programming of these tissues.Relaxin Receptor Expression and Functionalityin the Neonatal Porcine UterusWhile RLX has been studied for over 80 years (Hisaw1926), the cognate receptor for this 6000 Da peptidehormone was identified in 2002 (Hsu et al. 2002). A typeC, leucine-rich, G-protein coupled receptor (LGR)originally designated LGR7, the RLX receptor is nowrecognized as a member of the RLX family of peptide(RXFP) receptors and was recently re-designatedRXFP1 (Bathgate et al. 2006).LGR7 ⁄ RXFP1 is expressed by porcine uterine (Yanet al. 2006b) and cervical tissues from birth (Yan et al.2005). Uterine RXFP1 expression in the pig, which ispredominantly if not exclusively stromal, increases fromPND 0 through PND 14. Immunohistochemical evidencesupports in situ hybridization (ISH) data showingstromal RXFP1 staining similar to that reported foradult primate and human endometrium (Ivell et al.2003). Stromal expression of RXFP1 suggests that directeffects of RLX on epithelial growth and developmentare mediated through a stromally-driven paracrinemechanism (Bagnell et al. 2005). However, the fact thattrophic effects of RLX administered for 2 days fromPND 12, after onset of uterine ER expression, wereinhibited by pre-treatment with ICI 182,780 suggeststhat RLX may also act indirectly via crosstalk with theER system (Pillai et al. 1999; Yan et al. 2006a). Cervicalexpression of RXFP1 in the porcine neonate is higherthan that observed for the uterus (Yan et al. 2005).However, RLX administration for 2 days from birthdecreased cervical RXFP1 expression on PND 2 (Yanet al. 2005), suggesting elements of a negative autoregulatorymechanism governing cervical RXFP1 expression.Preliminary data (Yan and Bagnell 2003; Yanet al. 2008) also indicate that both uterine and cervicalER expression increased on PND 2 following administrationof RLX for 2 days from birth. Since RXFP1expression precedes ER expression in the neonataluterus, which is stimulated by exogenous RLX, subtlebut developmentally critical uterotrophic effects of RLXexposure from birth are likely to be RXFP1-specific.The more pronounced effects of RLX exposure observedduring the second week of neonatal life, after the onsetof ER expression, could reflect amplification of the RLXsignal through cross-talk with the ER (Pillai et al. 1999;Yan et al. 2006a). The fact that RLX can increaseoestrogen-stimulated uterine Hoxa10 expression (Guiet al. 1999) indicates that RLX signalling may also affectER-dependent events driving morphoregulatory geneexpression in the neonatal uterus (Bartol et al. 2006) andother RXFP1-positive, E-sensitive FRT tissues.Like oestrogen, uterotrophic effects of RLX in neonatalpigs are age-specific (Spencer et al. 1993b; Bagnellet al. 2005). When given for 2 days from birth, prior toonset of endometrial ER expression, RLX increaseduterine LE height, but not uterine weight on PND 2(Yan et al. 2006a). In striking contrast, RLX increasedcervical weight by PND 2 when compared with E-treated or control gilts (Yan et al. 2005). In gilts treatedfor 2 days from PND 12, after onset of uterine ERexpression, RLX increased both uterine LE height anduterine weight on PND 14 (Yan et al. 2006a). Data areconsistent with other studies showing that RLXincreases uterine LE height in pre-pubertal gilts (Ryanet al. 2001) and is important for maintenance ofreproductive epithelia in RLX-null mice (Zhao et al.2000). In RLX-deficient rats, proliferation of vaginalepithelial and stromal cells decreased while rates ofapoptosis in these cell types increased more than 10- and3-fold relative to controls (Lee et al. 2005).Image analysis based evaluation of cell proliferation,in which proliferating cell nuclear antigen (PCNA)positive cells were identified in situ and PCNA labellingindices were determined, was used to assess effects of ageat 3-day intervals from birth through PND 15 onpatterns of epithelial proliferation in the neonatalporcine endometrium (Masters et al. 2007). Overall,PCNA labelling index was greater in GE than in LE andwas affected by neonatal age in both cell types. Resultsindicated that the developmental transition from amorphogenetically inactive ‘infantile’ endometrial conditionat birth to a morphogenetically active ‘proliferative’state (Spencer et al. 1993a) has occurred by PND3 in the pig as marked by a dramatic increase inepithelial PCNA labelling index (Masters et al. 2007).This labelling index remained high through PND 6 inboth LE and GE, declined thereafter in both cellcompartments, and increased to PND 15 only in GE.Furthermore, administration of oestrogen or RLX inweek two induced LE and GE proliferation on PND 14Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag