Implications
This review focuses on maternal lactocrine programming of postnatal reproductive tract development in pigs by way of mother’s milk. The importance of nursing from birth on reproductive development and performance is emphasized, with data on both short-term effects in the neonate and long-term effects in adults. Data support the lactocrine hypothesis and milk as a conduit for delivery of maternally derived bioactive factors driving postnatal development. Results reinforce the importance of optimizing conditions that ensure adequate consumption of first milk (colostrum) by nursing young through effective reproductive management in swine production systems. Lactocrine programming has broad implications for human health.
Introduction
Maternal effects on development and reproductive efficiency include environmental conditions and signals provided by the dam that affect developmental trajectory and offspring phenotype (Bagnell and Bartol, Reference Bagnell and Bartol2019). For economically important domestic animals such as the pig (Sus scrofa domesticus), an optimal developmental trajectory is defined by a series of organizational events effecting a developmental program that results in an adult phenotype with potential for maximal fertility and fecundity. Because phenotype is not fixed genetically, but is defined dynamically through the course of development (Bartol et al., Reference Bartol, Wiley, George, Miller and Bagnell2017), a single genotype can produce more than one phenotype. Practically, these observations reinforce the importance of husbandry in management of animal production environments throughout the life cycle.
In the pig, as in other litter-bearing species, prolificacy – the number of offspring per litter – is a function of ovulation rate, fertilization rate and embryo survival (Kemp et al., Reference Kemp, Da Silva and Soede2018). The latter is determined to a significant extent by functional uterine capacity, defined as the maximum number of fetuses that can be carried to term (Bennett and Leymaster, Reference Bennett and Leymaster1989; Vallet et al., Reference Vallet, McNeel, Johnson and Bazer2013a). Clearly, litter size can be no greater than either maternal ovulation rate or uterine capacity (Bennett and Leymaster, Reference Bennett and Leymaster1989). In a mathematical model, changing either ovulation rate or uterine capacity independently did not predict large changes in litter size (Bennett and Leymaster, Reference Bennett and Leymaster1989), indicating that both must change coordinately to achieve a positive effect on prolificacy. To complicate this picture, effective selection for larger litters is associated with lower and more variable piglet birth weights, and increased pre-weaning mortality (Kemp et al., Reference Kemp, Da Silva and Soede2018). However, positive selection for uterine capacity did improve fetal survival and lifetime sow productivity (Freking et al., Reference Freking, Lents and Vallet2016). Thus, conditions that optimize functional uterine capacity should optimize fecundity.
Large litter sizes characteristic of modern swine production challenge the capacity of highly prolific dams to support offspring postnatally (Foxcroft, Reference Foxcroft2012; Kraeling and Webel, Reference Kraeling and Webel2015). In such circumstances, marked pre-weaning piglet mortality often reflects insufficient colostrum (first milk) consumption by piglets that, on a within litter basis, typically vary significantly in size and must compete for access to the udder and teat position (Wu et al., Reference Wu, Wang, Wu, Kim, Chen and Wang2010; Vallet et al., Reference Vallet, Miles, Rempel, Nonneman and Lents2015). Because they are immunologically incompetent at birth, colostrum consumption is essential for piglet survival (Vallet et al., Reference Vallet, Miles and Rempel2013b; Vallet et al., Reference Vallet, Miles, Rempel, Nonneman and Lents2015; Poonsuk and Zimmerman, Reference Poonsuk and Zimmerman2018). Transmission of immunoglobulins and other milk-borne bioactive factors (MbFs) to nursing piglets in colostrum provides protection against infectious diseases and supports maturation of the gastrointestinal tract as they gain immunological competence during the first month of neonatal life (Poonsuk and Zimmerman, Reference Poonsuk and Zimmerman2018). These observations alone encourage management strategies designed to improve colostrum availability and quality in order to minimize pre-weaning losses and optimize fecundity (Rohrer et al., Reference Rohrer, Rempel, Miles, Keele, Wiedmann and Vallet2014; Vallet et al., Reference Vallet, Miles, Rempel, Nonneman and Lents2015; Farmer, Reference Farmer2018). However, beyond neonatal survival, evidence indicating that colostrum consumption on the day of birth (postnatal day = PND 0) has lasting effects on fecundity in adult female pigs (Bartol et al., Reference Bartol, Wiley, Miller, Silva, Roberts, Davolt, Chen, Frankshun, Camp, Rahman, Vallet and Bagnell2013; Vallet et al., Reference Vallet, Miles, Rempel, Nonneman and Lents2015) elevates the importance of colostrum (Vallet et al., Reference Vallet, Miles, Rempel, Nonneman and Lents2015) as a maternal factor affecting reproductive development and performance.
Studies of uterine development in the pig and other domestic ungulate species (Bartol et al., Reference Bartol, Wiley, Spencer, Vallet and Christenson1993; Spencer et al., Reference Spencer, Kelleher and Bartol2019) established that female reproductive tract tissues, including the uterus, remain organizationally plastic during early neonatal life. Data for the pig (Bartol et al., Reference Bartol, Wiley, Spencer, Vallet and Christenson1993), indicating that uterine growth and uterine wall development proceed normally prior to PND 60 in gilts ovariectomized at birth, suggested that extraovarian factors support porcine uterine development in the postnatal period. Colostrum was proposed as a potential source of such uterotrophic support (Yan et al., Reference Yan, Wiley, Bathgate, Frankshun, Lasano, Crean, Steinetz, Bagnell and Bartol2006b; Bartol et al., Reference Bartol, Wiley and Bagnell2008). The term ‘lactocrine’ was coined to describe a mechanism by which MbFs are communicated from mother to offspring in colostrum/milk by consequence of nursing (Bartol et al., Reference Bartol, Wiley and Bagnell2008). The ‘lactocrine hypothesis’ for maternal programming of postnatal development posits that disruption of lactocrine signaling shortly after birth will alter the program and trajectory of development with short-term organizational effects and long-term consequences for adult phenotype. Studies designed to test the lactocrine hypothesis for maternal programming of uterine and reproductive development in the pig are summarized in several recent reviews (Bagnell et al., Reference Bagnell, Ho, George, Wiley, Miller and Bartol2017; Bartol et al., Reference Bartol, Wiley, George, Miller and Bagnell2017; Bagnell and Bartol, Reference Bagnell and Bartol2019). Here, objectives are to provide an overview of this work from a production point of view in the context of maternal programming of postnatal reproductive development and performance.
Milk as a delivery system for bioactive factors
Milk-borne bioactive factors of environmental origin
Maternal effects on development begin at conception and can be influenced in utero during pregnancy, as well as postnatally by environmental exposures that can alter the trajectory of offspring development. These effects can be either positive or negative and ultimately can influence the adult phenotype. There is a large literature in support of environmental effects (i.e., nutrition, climate and chemical or hormonal perturbations) during critical periods affecting the programming in early development of the fetus that led to the concept of developmental origins of health and disease (Barker, Reference Barker1998). In addition, since development continues postnatally, nursing provides a means of extending maternal influence by delivery of MbFs, including environmental agents that can affect neonatal outcomes.
Maternal exposure to environmentally derived endocrine disrupting agents can have lasting consequences on offspring development into adulthood (Bartol and Bagnell, Reference Bartol and Bagnell2012). For example, bisphenol A (BPA), an estrogenic endocrine disrupting agent, was detected in both cow and human milk (Mendonca et al., Reference Mendonca, Hauser, Calafat, Arbuckle and Duty2014). Relatively high BPA in maternal serum–breast milk dyad samples suggested high BPA exposure by consequence of nursing. Postnatal BPA exposure was linked to delays in development of the pituitary–neuroendocrine axis and onset of puberty (Franssen et al., Reference Franssen, Gerard, Hennuy, Donneau, Bourguignon and Parent2016). Toxic environmental agents can also be concentrated in milk. Lipophilic xenobiotics can pass from maternal adipose stores into the maternal circulation and can be concentrated in milk (Lehmann et al., Reference Lehmann, Verner, Luukinen, Henning, Assimon, LaKind, McLanahan, Phillips, Davis, Powers, Hines, Haddad, Longnecker, Poulsen, Farrer, Marchitti, Tan, Swartout, Sagiv, Welsh, Campbell, Foster, Yang, Fenton, Tornero-Velez, Francis, Barnett, El-Masri and Simmons2014). Thus, in comparison to maternal exposure to environmental toxins, neonatal exposure to milk-borne toxicants may occur at higher levels, over shorter time periods when postnatal development of multiple organ systems is occurring. Since milk intake in neonatal pigs is estimated to be up to 30% of body weight (Coalson and Lecce, Reference Coalson and Lecce1973), maternal exposure to environmental endocrine disrupting chemicals and/or toxicants could pose a significant exposure risk to nursing young (Bartol and Bagnell, Reference Bartol and Bagnell2012).
Milk-borne bioactive factors of maternal origin
In addition to providing nutritional and immunological support for developing offspring, colostrum/milk provides a means for delivery of a wide variety of maternally derived MbFs in support of neonatal growth and development. These include growth factors as well as steroid and peptide hormones found in higher concentrations in milk than in the maternal circulation. Metabolic hormones including leptin, ghrelin, adiponectin and glucocorticoids are transferred from mother to offspring in milk (Power and Schulkin, Reference Power and Schulkin2013) and can affect metabolism, growth and development. Glucocorticoids in milk were linked to more nervous and less confident temperament in both human and non-human primate offspring (Hinde et al., Reference Hinde, Skibiel, Foster, Del Rosso, Mendoza and Capitanio2015).
The value of maternally derived MbFs in support of postnatal development is evident from loss of function studies showing that the absence of specific MbFs in milk has deleterious effects on development. Increased adiposity and altered hypothalamic gene expression were found in wild-type mice fostered to interleukin-6-null dams, in which milk leptin content was twofold higher than in wild-type dams, suggesting that milk composition has programming effects on adiposity (Lager et al., Reference Lager, Asterholm, Schele, Jansson, Nilsson, Jansson, Lonn and Holmang2011). This supported earlier studies indicating that neonatal rats cross-fostered to enable nursing of diabetic dams showed hypothalamic changes and altered expression of genes involved in body weight regulation (Fahrenkrog et al., Reference Fahrenkrog, Harder, Stolaczyk, Melchior, Franke, Dudenhausen and Plagemann2004). Likewise, maternal tumor necrosis factor-α (TNFα) deficiency led to reduced milk chemokine levels and improved adult spatial memory, suggesting a TNFα-regulated lactocrine pathway programming brain development and memory (Liu et al., Reference Liu, Zupan, Laird, Klein, Gleason, Bozinoski, Gal Toth and Toth2014). Similarly, peroxisome proliferator-activated receptor-γ (PPARγ)-null mice produced a toxic milk, high in inflammatory lipids resulting in hair loss and growth retardation in nursing young, illustrating the importance of lactocrine-active PPARγ in protecting nursing offspring (Wan et al., Reference Wan, Saghatelian, Chong, Zhang, Cravatt and Evans2007).
Milk also contains small, non-coding microRNAs (miRNAs) that regulate gene expression by blocking translation and/or promoting messenger RNA (mRNA) degradation. These miRNAs are enclosed as cargo in milk-borne exosomes that also carry mRNA, protein and lipids and provide another means for lactocrine transmission of information. Exosomes, which protect miRNAs from degradation by heat and acidic conditions, are found in milk of several species (Bartol et al., Reference Bartol, Wiley, George, Miller and Bagnell2017) including the pig (Gu et al., Reference Gu, Li, Wang, Liang, Zhong, Wang, Zhou, Chen, Lang, He, Chen, Gong, Gao, Li and Lv2012). These milk-borne miRNAs pass the intestinal barrier and enter the bloodstream to target organ systems in a lactocrine manner. Data for the pig indicate that these milk-borne miRNAs are functionally important for development of the neonatal immune system (Gu et al., Reference Gu, Li, Wang, Liang, Zhong, Wang, Zhou, Chen, Lang, He, Chen, Gong, Gao, Li and Lv2012).
Maternal somatic cell transfer by way of milk is documented in several species as another means of communication with nursing offspring. Porcine milk contains maternal immune cells (Scharek-Tedin et al., Reference Scharek-Tedin, Kreuzer-Redmer, Twardziok, Siepert, Klopfleisch, Tedin, Zentek and Pieper2015) that, when ingested during nursing, can cross the neonatal intestine, enter the bloodstream and populate neonatal organs (Jain et al., Reference Jain, Vidyasagar, Xanthou, Ghai, Shimada and Blend1989). In humans, breast milk contains mammary stem cells that were reported to colonize neonatal tissues with potential to alter postnatal development by way of microchimerism (Barinaga, Reference Barinaga2002). Collectively, these observations establish that milk is more than food (Hinde and German, Reference Hinde and German2012) and lactocrine transmission evolved as a means of delivering a plethora of non-nutritive, MbFs to nursing offspring (Bartol and Bagnell, Reference Bartol and Bagnell2012).
Relaxin: a prototypical lactocrine-active factor
A series of studies in neonatal gilts established relaxin as a prototypical lactocrine-active factor. Relaxin is a 6-kDa peptide hormone and member of a family of neohormones that evolved to support viviparity and lactation (Ivell and Anand-Ivell, Reference Ivell and Anand-Ivell2017). Well known as a hormone of pregnancy, actions of relaxin also include effects on cervical connective tissue remodeling and growth-promoting effects on the uterus. Evidence for trophic effects of relaxin on the neonatal porcine uterus (Yan et al., Reference Yan, Ryan, Bartol and Bagnell2006a) led to studies designed to identify a source of relaxin in the neonatal pig. Porcine colostrum was identified as this source (Yan et al., Reference Yan, Wiley, Bathgate, Frankshun, Lasano, Crean, Steinetz, Bagnell and Bartol2006b), an observation consistent with detection of relaxin in the milk of other species (Bagnell and Bartol, Reference Bagnell and Bartol2019). Detection of a bioactive factor in milk does not, by itself, constitute evidence for action of that MbF in the neonate. Therefore, criteria for determining the physiological relevance of MbFs in the neonate were defined (Peaker and Neville, Reference Peaker and Neville1991). Observations indicating that relaxin meets the criteria for a lactocrine-active MbF in the pig include (1) detection of biologically active prorelaxin in colostrum (Frankshun et al., Reference Frankshun, Ho, Reimer, Chen, Lasano, Steinetz, Bartol and Bagnell2011); (2) immunoreactive relaxin detected in the neonatal circulation only in pigs allowed to nurse (Yan et al., Reference Yan, Wiley, Bathgate, Frankshun, Lasano, Crean, Steinetz, Bagnell and Bartol2006b); (3) relaxin receptor (RXFP1) expression in porcine uterine (Yan et al., Reference Yan, Wiley, Bathgate, Frankshun, Lasano, Crean, Steinetz, Bagnell and Bartol2006b) and cervical (Yan et al., Reference Yan, Chen, Wiley, Crean-Harris, Bartol and Bagnell2008) tissues from birth; (4) growth-promoting effects of relaxin administered for 2 days from birth on the neonatal uterus (Yan et al., Reference Yan, Ryan, Bartol and Bagnell2006a) and cervix (Yan et al., Reference Yan, Chen, Wiley, Crean-Harris, Bartol and Bagnell2008). Taken together, these studies established relaxin as a prototypical lactocrine-acting factor and supported the idea that milk is an important conduit for communication of MbFs to nursing offspring (Bagnell and Bartol, Reference Bagnell and Bartol2019).
Lactocrine effects on neonatal reproductive development
Uterus
To test the lactocrine hypothesis, a lactocrine-null condition was imposed during the first 48 h of life, by feeding a porcine milk replacer in lieu of nursing, and effects on uterine development were evaluated on PND 2 and PND 14. There was no effect of replacer feeding from birth on uterine weight (Chen et al., Reference Chen, Frankshun, Wiley, Miller, Welch, Ho, Bartol and Bagnell2011) or endometrial histoarchitecture (Miller et al., Reference Miller, Wiley, Chen, Bagnell and Bartol2013) by PND 2. However, in the absence of nursing, uterine glandular and luminal epithelial cell proliferation were reduced and there was a decrease in endometrial stromal estrogen receptor-α (ESR1) localization by PND 2 (Miller et al., Reference Miller, Wiley, Chen, Bagnell and Bartol2013). Effects of replacer feeding for 2 days from birth were evident by PND 14, when imposition of the lactocrine-null condition reduced both endometrial thickness and uterine gland development (Miller et al., Reference Miller, Wiley, Chen, Bagnell and Bartol2013). Antiadenogenic effects, including reduced gland penetration depth, observed at PND 14 in response to replacer feeding for 48 h from birth were similar to those observed in gilts treated daily from birth with the anti-estrogen ICI 182 780 (Tarleton et al., Reference Tarleton, Wiley and Bartol1999). Notably, returning gilts deprived of colostrum for the first 48 h of life to nursing at the end of PND 2 failed to rescue the uterine phenotype of reduced endometrial thickness and glandular development observed in replacer-fed gilts at PND 14 (Miller et al., Reference Miller, Wiley, Chen, Bagnell and Bartol2013) These studies reinforced the importance of lactocrine signaling from birth on uterine endometrial development in the pig.
Colostrum composition fluctuates over the course of porcine lactation. Consumption of colostrum can be delayed if sows fail to initiate lactation, fail to produce enough colostrum for the litter or if access to colostrum consumption is compromised by within litter competition for access to the udder (Vallet et al., Reference Vallet, Miles and Rempel2013b). In pigs, timing of colostrum intake coincides with a period of gut permeability to colostral macromolecules, which are typically present in high concentrations at birth and decline over the next 24 to 48 h in association with the loss of gut permeability, termed gut closure (Poonsuk and Zimmerman, Reference Poonsuk and Zimmerman2018). In the tammar wallaby, timing of milk intake, milk composition and rate of milk production influence growth of pouch young and offspring phenotype dramatically (Trott et al., Reference Trott, Simpson, Moyle, Hearn, Shaw, Nicholas and Renfree2003). Therefore, it was of interest to determine whether timing of colostrum consumption or duration of nursing in pigs affected lactocrine-mediated development of uterine and cervical tissues.
Studies of the neonatal porcine uterine transcriptome indicated that matrix metalloproteinases (MMPs) and tissue inhibitors of the MMPs (TIMPs) were affected by both age and lactocrine signaling between birth and PND 2 (Rahman et al., Reference Rahman, Camp, Prasad, McNeel, Levy, Bartol and Bagnell2016). The MMP2 and MMP9 gelatinases remodel the extra cellular matrix and are co-expressed with TIMPs, which regulate MMP activity. Uterine (Chen et al., Reference Chen, Frankshun, Wiley, Miller, Welch, Ho, Bartol and Bagnell2011;Ho et al., Reference Ho, Rahman, Camp, Wiley, Bartol and Bagnell2017) and cervical (Frankshun et al., Reference Frankshun, Chen, Barron, Ho, Miller, Rahman, Bartol and Bagnell2012) proMMP9 (latent) and MMP9 (active) as well as uterine TIMP protein abundance was greater in nursed gilts when compared to replacer-fed animals. However, none of these proteins were detectable in porcine reproductive tissues when nursing was delayed by 12 h (Ho et al., Reference Ho, Rahman, Camp, Wiley, Bartol and Bagnell2017). In addition, duration of nursing is important since extending nursing from 30 min to 12 h from birth, increased active and latent MMP9 proteins in reproductive tissues to levels comparable to those observed for gilts nursed for 2 days from birth (Ho et al., Reference Ho, Rahman, Camp, Wiley, Bartol and Bagnell2017). By contrast, uterine MMP2 levels were detected but unchanged by age at first nursing or duration of nursing, indicating that not all uterine protein production is lactocrine-sensitive. Uterine MMP2 and MMP9 activities, detected by zymography, mirrored immunoblotting data. In other studies, a single feeding of colostrum was effective in supporting cervical (Camp et al., Reference Camp, Wiley, Boulos, Rahman, Bartol and Bagnell2014) and endometrial cell proliferation at 12 h postnatal (George et al., Reference George, Rahman, Miller, Wiley, Camp, Bartol and Bagnell2018). In addition, there was no effect of method of delivery of a single dose of colostrum, either by nursing, bottle feeding or orogastric gavage, on uterine developmental markers at 12 h postnatal (George et al., Reference George, Rahman, Miller, Wiley, Camp, Bartol and Bagnell2018). Collectively, these data indicate that both age at first nursing and duration of nursing are important in neonatal porcine female reproductive tract development, and that lactocrine effects can be detected in nursing piglets within 12 h of birth (Bagnell and Bartol, Reference Bagnell and Bartol2019).
Nursing and the neonatal uterine transcriptome
Global analysis of neonatal porcine gene expression in response to age and lactocrine signaling from birth to PND 2 was investigated by RNA sequencing (RNAseq) (Rahman et al., Reference Rahman, Camp, Prasad, McNeel, Levy, Bartol and Bagnell2016). With respect to age, more than 3200 uterine genes in nursed gilts and over 4500 genes in lactocrine-null gilts were differentially expressed on PND 2 when compared to uterine gene expression in uterine tissues obtained from gilts at birth. With respect to lactocrine effects, more than 890 differentially expressed genes were identified on PND 2 when nursed and milk replacer-fed gilts were compared. Bioinformatic analyses of biological processes revealed age-sensitive pathways that included ESR1 and hedgehog signaling cascades. Lactocrine-sensitive pathways in the neonatal porcine uterus identified on PND 2 included those involved in response to wounding, cell adhesion, the plasminogen activator network and coagulation (Rahman et al., Reference Rahman, Camp, Prasad, McNeel, Levy, Bartol and Bagnell2016).
Post-transcriptional regulation of gene expression by miRNAs is a mechanism that could be responsible, in part, for global uterine gene expression differences observed in response to age and nursing. Small non-coding miRNAs target mRNAs and can decrease mRNA stability and block translation. Consequently, effects of age and nursing from birth on the porcine uterine miRNA transcriptome were evaluated in tissues obtained on PND 2 (George et al., Reference George, Rahman, Camp, Prasad, Bartol and Bagnell2017) using the same neonatal uterine tissues on which RNAseq analyses were performed (Rahman et al., Reference Rahman, Camp, Prasad, McNeel, Levy, Bartol and Bagnell2016). Integration of miRNAseq and mRNAseq data enabled target prediction analyses designed to identify potential miRNA–mRNA interactions. Results showed that about 10% of age- and lactocrine-sensitive differences in uterine gene expression could be explained by differential uterine miRNA expression (George et al., Reference George, Rahman, Camp, Prasad, Bartol and Bagnell2017). Biological processes predicted to be affected by age and nursing in uterine tissues on PND 2 as defined by the miRNA–mRNA interactome included cell-to-cell signaling, cell and tissue morphology, and cell growth and proliferation (George et al., Reference George, Rahman, Camp, Prasad, Bartol and Bagnell2017). Observations were consistent with morphogenetic activities associated with uterine growth and endometrial development in the early neonatal period (Bartol et al., Reference Bartol, Wiley, Spencer, Vallet and Christenson1993).
Cervix
Cervical histology of gilts nursed from birth to PND 2 was similar to that of replacer-fed gilts. However, by PND 14, imposition of the lactocrine-null state from birth reduced cervical crypt depth and luminal epithelial height when compared to gilts nursed over the same period (Camp et al., Reference Camp, Wiley, Boulos, Rahman, Bartol and Bagnell2014). Cervices from replacer-fed, PND 14 gilts were histologically similar to cervices from newborn pigs. Similar to observations for the uterus (Miller et al., Reference Miller, Wiley, Chen, Bagnell and Bartol2013), returning replacer-fed gilts to nursing on PND 2 failed to rescue the PND 14 cervical phenotype in that both cervical crypt and stromal cell proliferation at PND 14 were reduced to levels comparable to those reported for gilts fed milk replacer from birth (Camp et al., Reference Camp, Wiley, Boulos, Rahman, Bartol and Bagnell2014).
In other studies designed to evaluate short-term effects of nursing in the cervix and to develop a more efficient bioassay protocol for assessment of lactocrine effects in the neonate, a single feeding of hour 0 colostrum or milk replacer was given at birth, followed by milk replacer feeding through 12 h postnatal. The single feeding of colostrum, but not replacer, increased cervical cell proliferation by 12 h postnatal (Camp et al., Reference Camp, Wiley, Boulos, Rahman, Bartol and Bagnell2014). In addition, when delivered orally, IGF1 found naturally in relatively high concentrations in pig milk (Simmen et al., Reference Simmen, Simmen and Reinhart1988), increased cervical cell proliferation and markers of IGF1 action, including phosphorylated AKT and anti-apoptotic B-cell lymphoma 2(BCL2), by 12 h postnatal in both colostrum and replacer-fed gilts. Taken together, these data showed that nursing supports cervical development in neonatal pigs, and that IGF1 is a potential lactocrine-active factor for reproductive tract development as illustrated in a 12-h bioassay system to identify lactocrine active MbFs.
Testis
Evidence for lactocrine effects on development of the male reproductive system comes from studies on neonatal testicular development in boars (Rahman et al., Reference Rahman, Lovich, Lam, Camp, Wiley, Bartol and Bagnell2014). Development and proliferation of two major cell types in the pig testis, Sertoli (McCoard et al., Reference McCoard, Lunstra, Wise and Ford2001) and Leydig cells (Franca et al., Reference Franca, Silva, Chiarini-Garcia, Garcia and Debeljuk2000), occurs within the first month of neonatal life. In addition, Sertoli cell number, determined before puberty (Franca et al., Reference Franca, Silva, Chiarini-Garcia, Garcia and Debeljuk2000), influences testicular size and sperm production (McCoard et al., Reference McCoard, Lunstra, Wise and Ford2001), indicating that the neonatal period is critical for porcine testicular development.
Nursing for 2 days from birth increased Sertoli cell proliferation when compared to boars fed a commercial pig milk replacer over the same period (Rahman et al., Reference Rahman, Lovich, Lam, Camp, Wiley, Bartol and Bagnell2014). In a similar manner, Sertoli cell number and GATA4 protein abundance were greater in nursed boars on PND 2. There was no effect of age or nursing on Leydig cell-associated testicular protein levels, including the steroidogenic enzyme P450scc or insulin-like factor 3. However, testicular RXFP1 expression increased from birth to PND 2 in replacer-fed, but not in nursed, boars. This was thought to be due, in part, to the absence of milk-borne relaxin in replacer-fed animals (Yan et al., Reference Yan, Wiley, Bathgate, Frankshun, Lasano, Crean, Steinetz, Bagnell and Bartol2006b). Relaxin is detectable in colostrum and in the circulation of nursed pigs (Yan et al., Reference Yan, Wiley, Bathgate, Frankshun, Lasano, Crean, Steinetz, Bagnell and Bartol2006b) and administration of exogenous relaxin decreased RXFP1 expression in the neonatal porcine uterus and cervix (Yan et al., Reference Yan, Chen, Wiley, Crean-Harris, Bartol and Bagnell2008). Given that relaxin increased Sertoli cell proliferation in vitro (Cardoso et al., Reference Cardoso, Nascimento, Royer, Porto and Lazari2010), the absence of milk-borne relaxin in replacer-fed boars could remove inhibition of testicular RXFP1 expression and alter testicular development. Potential for maternal lactocrine programming of testicular function remains to be explored.
Long-term lactocrine effects in the adult
Lactocrine-null conditions, imposed experimentally by feeding milk replacer in lieu of nursing from birth, altered the developmental program in neonatal porcine reproductive tract tissues (Bagnell et al., Reference Bagnell, Ho, George, Wiley, Miller and Bartol2017; Bartol et al., Reference Bartol, Wiley, George, Miller and Bagnell2017). However, lactocrine deficiency can also occur naturally through maternal (e.g., mastitis and agalactia) as well as neonatal factors (e.g., within litter competition for teat position, birth rank and low birth weight) (Wu et al., Reference Wu, Wang, Wu, Kim, Chen and Wang2010). An immunoglobulin immunocrit assay, developed to monitor immunoglobulin transfer from mother to offspring during nursing (Vallet et al., Reference Vallet, Miles and Rempel2013b), was established as an indirect measure of colostrum intake in nursing pigs (Vallet et al., Reference Vallet, Miles, Rempel, Nonneman and Lents2015). The lactocrine hypothesis predicts that minimal colostrum consumption on PND 0, indicated by low serum immunocrit and lactocrine deficiency in nursing gilts, will be associated, ultimately, with reduced adult uterine capacity. A retrospective study of 381 gilts showed that low serum immunocrit on the day of birth was linked to reduced lifetime fecundity and live litter size across four parities (Bartol et al., Reference Bartol, Wiley, Miller, Silva, Roberts, Davolt, Chen, Frankshun, Camp, Rahman, Vallet and Bagnell2013). Subsequently, in a large prospective study, PND 0 immunocrit was obtained from 16 762 piglets and subsets of these gilts were assigned to study a variety of reproductive parameters (Vallet et al., Reference Vallet, Miles, Rempel, Nonneman and Lents2015). Results showed that low PND 0 immunocrit was associated with reduced growth and increased age at puberty. In addition, in a group of 799 females, low immunocrit on the day of birth was associated with reduced number of piglets born alive, consistent with the initial report (Bartol et al., Reference Bartol, Wiley, Miller, Silva, Roberts, Davolt, Chen, Frankshun, Camp, Rahman, Vallet and Bagnell2013). Litter size differences for adult females with low v. high PND 0 immunocrit were approximately 1.4 piglets per litter (Vallet et al., Reference Vallet, Miles, Rempel, Nonneman and Lents2015). In addition, high PND 0 immunocrit in neonatal gilts was linked to improved lactational performance when these females reached adulthood, suggesting lactocrine effects on programming of mammary gland function.
Disruption of uterine receptivity to implantation during the periattachment period of pregnancy can lead to reduced reproductive performance in pigs (Bazer et al., Reference Bazer, Spencer, Johnson and Burghardt2011). On pregnancy day (PxD) 13, elongated porcine conceptuses initiate attachment to uterine luminal epithelium. Documented negative effects of neonatal lactocrine deficiency on live litter size in adult female pigs (Bartol et al., Reference Bartol, Wiley, Miller, Silva, Roberts, Davolt, Chen, Frankshun, Camp, Rahman, Vallet and Bagnell2013; Vallet et al., Reference Vallet, Miles, Rempel, Nonneman and Lents2015) prompted study of the impact of lactocrine deficiency on the endometrial transcriptome during the periattachment period of early pregnancy on PxD 13. Global transcriptomic analysis revealed more than 1100 differentially expressed endometrial mRNAs at PxD 13 in high v. low immunocrit gilts (George et al., Reference George, Ho, Prasad, Keel, Miles, Vallet, Bartol and Bagnell2019). In addition, in terms of miRNA–mRNA interactions, target prediction analysis revealed 5 differentially expressed miRNAs predicted to target over 60 differentially expressed mRNAs in the endometrium of high v. low immunocrit gilts on PxD 13. These endometrial mRNAs and related miRNA–mRNA interactions were associated with lactocrine-sensitive gene families for which predicted functions included solute transport, endometrial receptivity and immune response (George et al., Reference George, Ho, Prasad, Keel, Miles, Vallet, Bartol and Bagnell2019). Taken together, these observations showed that impairment of reproductive performance in lactocrine-deficient, adult female pigs is reflected by alterations in endometrial gene expression in the periattachment period of early pregnancy.
Conclusions
Studies designed to test the lactocrine hypothesis for maternal programming of reproductive development and uterine capacity in the pig indicate that, beyond ensuring postnatal survival through the passive transmission of immune competence from mother to nursing offspring (Poonsuk and Zimmerman, Reference Poonsuk and Zimmerman2018), lactocrine communication via colostrum affects uterine developmental trajectory and, ultimately, determines functional uterine capacity in adults. Genetically, selection for uterine capacity over 11 generations increased live litter size by approximately 1.6 piglets (Freking et al., Reference Freking, Lents and Vallet2016). This kind of genetic advantage could be effectively negated by failure to ensure adequate colostrum consumption by nursing piglets at birth. Reduction in live litter size for neonatally lactocrine-deficient gilts was estimated at 1.4 piglets per litter with no effect of parity (Vallet et al., Reference Vallet, Miles, Rempel, Nonneman and Lents2015). Such permanent impairment of reproductive performance in adult, neonatally lactocrine-deficient female pigs is significant. Observations emphasize the importance of developmentally critical interactions between genotype and the maternally provisioned lactocrine environment in programming uterine capacity and reproductive efficiency. While MbFs responsible for lactocrine programming of reproductive development remain to be defined, husbandry guidelines aimed at optimization of genetic and environmental conditions affecting porcine uterine capacity can be proposed, as summarized in Figure 1. Evidence for lactocrine programming of uterine development demands studies designed to identify MbFs responsible for lactocrine signaling and related mechanisms regulating organizational processes and events that ultimately determine adult uterine capacity and fecundity in the pig.
Figure 1 Programming porcine uterine capacity. Uterine capacity is determined by an interaction of genotype with maternally provisioned environmental conditions affecting mammogenesis, lactogenesis and lactocrine programming of postnatal uterine development. With conceptus genotype established, interactions between developing conceptuses and the intrauterine environment determine patterns of conceptus development, survival and fetoplacental lactogenic potential. In turn, endocrine conditions of pregnancy define patterns of mammogenesis and lactogenesis. Nursing ensures lactocrine transmission of MbFs. Lactocrine signaling affects the neonatal uterine developmental program, the trajectory of uterine development and uterine capacity. Reproductive performance of female piglets that do not receive sufficient colostrum is permanently impaired. Therefore, management strategies designed to improve colostrum quality and availability are important for optimization of uterine capacity. Practical actions to optimize uterine capacity and maternal lactocrine programming of postnatal development (boxes 1 to 3) include (1) selection for uterine capacity, number of piglets born alive, pre-weaning survival rate and number of pigs weaned; (2) adoption of breeding and management practices designed to optimize mammogenesis, lactogenesis, milk yield and colostral/milk quality; and (3) implementation of management strategies designed to ensure adequate colostrum availability to neonatal piglets. Adapted with permission from Bartol and Bagnell (Reference Bartol and Bagnell2012). MbF = milk-borne bioactive factor.
Acknowledgments
Authors thank our collaborators, scientific and animal care staff, graduate students and undergraduate students who contributed to this research. Research was supported by USDA-National Research Initiative (2007-35203-18098 and 2013-67016-20523) to FFB and CAB, NSF EPS-1158862 (to FFB) and the Steinetz Charitable Lead Unitrust (to CAB).
C. A. Bagnell 0000-0002-0058-8233
F. F. Bartol 0000-0001-5872-7489
Declaration of interest
Authors declare no conflict of interest.
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