mammary glands are anatomical structures that distinguish mammals from all other animals and secrete milk for offspring . these organs are considered composite secretory organs and are made up of various types of cells , including epithelial cells , adipocytes , fibroblasts , and vascular endothelial cells ( macias & hinck , 2012 ) . there are three major stages of mammary gland development : embryonic , pubertal , and reproductive stages . in embryonic mammary gland development , the epithelial placodes of ectoderm formation during early embryo development give rise to the mammary gland structures during morphogenesis ( pispa & thesleff , 2003 ) . the placodes fundamentally regulate epithelial - mesenchymal interactions and generate the rudimentary ductal structure of the gland present at birth . the puberty and reproductive stages of mammary gland development occur continuously through hormonal regulation associated with pubertal growth , pre gnancy , lactation , and involution . at birth , these small ducts can grow allometrically following the growth phase and undergo expansion , called ductal morphogenesis , until puberty . puberty in mammals initiates morphogenesis of branched structures in the mammary gland ; this process is mutually regulated by growth hormone ( gh ) , estrogen , and insulin - like growth factor ( igf)- 1 to generate a ductal tree ( howlin et al . , 2006 ) . during pregnancy , alveoli of the mammary glands , which secrete milk , are generated through combined action of progesterone and prolactin ( prl ) . in the mature structure , short branched ducts are controlled by progesterone ; however , alveologenesis occurs only during pregnancy through induction by prl and progesterone . when the levels of progesterone and prl increase in the serum , branched alveoli are developed , and adipose tissue and blood flow are increased . after achieving milk production , prl is decreased , and involution , defined as remodeling back to the original adult state , occurs . in bovines , colostrum is secreted during late gestation and for the first few days after calving . in this review , we will focus on the igf pathway and its role in developmental processes and involute interactions in bovine mammary glands components . the igf pathway consists of three ligands ( igf - i , igfii , and insulin ) , their receptors ( igf - ir , igf - iir , and insulin receptor ) , high - affinity binding proteins ( igf binding protein [ igfbp ] 16 ) , and their specific proteases ( le roith , 2003 ) ; the complex interactions among these components maintain cellular mechanisms . as shown in fig . 1 , these igf ligands play various roles in mammalian growth , development , and metabolism and act as endocrine factors that prevent apoptotic signaling . mammary epithelial cells are influenced by many components of the igf system , particularly igfbps . the interactions between igfs and igfbps have been shown to inhibit igf - i or igf - ii binding with igf - ir and igf - iir through competitive mechanisms ( kelley et al . , 1996 ) . bovine mammary epithelial cells have been reported to express all three igf receptors ( hadsell et al . , 1990 ) , to synthesize igfbps ( gibson et al . , 1999 ) , and to produced igf - ii ( baumrucker et al . , 1993 ) . igf-1 mrna is expressed in mammary glands of pregnant and lactating cows ( glimm et al . , 1992 ; sharma et al . , 1994 ) , whereas cultured primary mammary gland cells do not exhibit igf-1 expression . recent studies have suggested that igf - i plays a critical role in growth hormone ( gh)-induced mammary ductal growth . the igf and gh interaction is important for regulation of mammalian epithelial cell proliferation and differentiation . igf - i is circulated through its interaction with gh , which is the major inducer of alveolar growth ( kleinberg , 1997 ) . there are correlations between igf - i and gh expression , and these correlations may directly mediate mammary growth ( ruan et al . , 1992 ) . additionally , gh has been shown to induce igf - i expression in mammary tissue ( ruan et al . , 1995 ) . furthermore , igf - i can promote ductal development in implanted mammary glands and synergizes with estrogen to support the generation of terminal end buds ( tebs ) and ductal elongation in ovariectomized mice ( ruan et al . several studies have reported that igf - i and igfr interactions are associated with cell proliferation and apoptosis through inhibition of wnt signaling ( richard - parpaillon et al . , 2002 ; rota & wood , 2015 ) . igf may specifically influence igf-1r owing to its greater binding affinity than igf - ii or insulin ( jones & clemmons , 1995 ) . moreover , igf - iir binds more specifically to igf - ii than igf - i and does not bind with insulin ( massague et al . , 1982 ; rechler & nissley , 1985 ) . in general , mammary gland development is characterized by ductal growth in nonpregnant heifers . while igf - i is highly expressed during the early stages of mammogenesis when compared with that during lactation , this protein exhibits potential proliferative functions throughout the entire process of bovine mammary gland development . additionally , igf-1r and gh receptor mrna expression levels are decreased during the lactation stage . these decreases in mrna levels in bovine mammary tissue have been shown to be species specific ( plath - gabler et al . , 2001 ) . different types of igf-1r transcripts have been discovered in bovine mammary glands ( glimm et al . , 1992 ) . igfbps , which are encoded by six similar transcripts , are regulated based on their interactions with igfs and their specific binding proteins ( ballard et al . igfbp bioactivity inhibits the functions of igf - i and igf - ii by competition with igf - ir and igf - iir through igf - i and igf - ii near the cell surface . furthermore , igfs and igfbps can influence interactions with other growth pathways or igf - independent functions ( kelley et al . the interactions between igfbps and igfs are regulated by proteases by affecting the binding affinity of igfbp for igfs . modifications of igfbps , such as protein phosphorylation and binding with other factors , may affect their interactions with other proteins ( mccusker et al . for example , when igfbp-1 binds to integrin , igfbp-3 binding at the cell surface is decreased because the specific affinity of igfbp-3 for igf-1 is reduced when igfbp-3 interacts with cell surface molecules ( conover & powell , 1991 ) . moreover , suppression of igf - i release from igfbp-3 may occur through the interaction between igfs and igfir , thus reducing the activity of igfs . igfbp expression during mammary gland development has also been studied by analysis of the contributions of igfbp-2 and -5 to branching mammogenesis during pregnancy ( wood & yee , 2000 ) . all igfbps exhibit high - affinity binding with igfs ; however , igfbps also have other functions independent of igfs . igfbp-1 and -2 include the tripeptide arg - gly - asp ( rgd ) sequence , which is recognized by cell surface integrins and cell adhesion molecules . igfbp-1 overexpression in chinese hamster cells increases cell migration ( jones et al . , 1993 ) . additionally , igfbp-3 has independent effects on the modulation of bax and bcl-2 , inducing apoptosis in human breast cancer cells through igf - i ( butt et al . , 2000 ) and inhibiting proliferation in type i igf receptor - knockout mice ( valentinis et al . , 1995 ) . recent studies have demonstrated that igfbp-3 regulates transforming growth factor ( tgf)- during programmed cell death through interactions between p53 and igf - independent signaling ( rajah et al . , 1997 ) . additionally , the tgf- receptor can interact with igfbp-3 ( leal et al . , 1997 ) . moreover , igfbp-5 has been shown to activate dna synthesis in osteoblasts through a pathway independent of igf - i ( andress , 1995 ) . both igfbp-3 and -5 contain a putative nuclearlocalization sequence ( nls ) ( radulescu , 1994 ) and have been shown to be expressed in the nuclei using immunelogical localization methods ( jaques et al . these results suggest that igfbps can also contribute to intracellular functions , such as proliferation , apoptosis , and cell migration . thus , these observations also provide insights into whether the high concentrations of igfbp-5 in bovine milk interact with igf - dependent or -independent pathways . in human milk production , igfbp- 2 is more highly expressed than igfbp-1 and igfbp-3 ( erickson et al . , 1993 ; donovan & odle , 1994 ) , and igfbp-1 and igfbp-3 are present at lower concentrations than igfbp-2 in human milk . mrnas encoding igfbp-2 and -3 are expressed in porcine mammary glands ( simmen et al . , 1988 ) . thus , igfbp-2 and - 3 in milk is derived from the mammary glands . in bovine serum , igfbp-2 is highly expressed during the early lactation stage and decreased during the dry period ( vicini et al . , 1991 ) . igfbp-2 is not expressed in maternal serum , but is expressed in lactating mammary glands , suggesting that igfbp-2 is synthesized in mammary glands . igfbp-4 expression has been identified in rat mammary glands , although the concentration is lower than that in serum ( donovan et al . , 1995 ) . igfbp-5 and -6 are not present in milk during the lactation stage . igfs and igfbps may enter milk from the serum by paracellular interactions and secretion by epithelial cells . however , involuting mammary glands exhibit increased levels of igfbp-2 , igfbp-4 , and igfbp-5 mrnas . igf-1 is a survival factor for several different types of cells and is regulated by gh ( travers et al . , 1996 ) . of all the hormonal changes that occur during the involution stage , prl is able to influence only igfbp-5 synthesis , suggesting that the prolactin is a potential inhibitor of igfbp-5 . the igf pathway consists of three ligands ( igf - i , igfii , and insulin ) , their receptors ( igf - ir , igf - iir , and insulin receptor ) , high - affinity binding proteins ( igf binding protein [ igfbp ] 16 ) , and their specific proteases ( le roith , 2003 ) ; the complex interactions among these components maintain cellular mechanisms . as shown in fig . 1 , these igf ligands play various roles in mammalian growth , development , and metabolism and act as endocrine factors that prevent apoptotic signaling . mammary epithelial cells are influenced by many components of the igf system , particularly igfbps . the interactions between igfs and igfbps have been shown to inhibit igf - i or igf - ii binding with igf - ir and igf - iir through competitive mechanisms ( kelley et al . , 1996 ) . bovine mammary epithelial cells have been reported to express all three igf receptors ( hadsell et al . , 1990 ) , to synthesize igfbps ( gibson et al . , 1999 ) , and to produced igf - ii ( baumrucker et al . , 1993 ) . igf-1 mrna is expressed in mammary glands of pregnant and lactating cows ( glimm et al . , 1992 ; sharma et al . , 1994 ) , whereas cultured primary mammary gland cells do not exhibit igf-1 expression . recent studies have suggested that igf - i plays a critical role in growth hormone ( gh)-induced mammary ductal growth . the igf and gh interaction is important for regulation of mammalian epithelial cell proliferation and differentiation . igf - i is circulated through its interaction with gh , which is the major inducer of alveolar growth ( kleinberg , 1997 ) . there are correlations between igf - i and gh expression , and these correlations may directly mediate mammary growth ( ruan et al . , 1992 ) . additionally , gh has been shown to induce igf - i expression in mammary tissue ( ruan et al . , 1995 ) . furthermore , igf - i can promote ductal development in implanted mammary glands and synergizes with estrogen to support the generation of terminal end buds ( tebs ) and ductal elongation in ovariectomized mice ( ruan et al . several studies have reported that igf - i and igfr interactions are associated with cell proliferation and apoptosis through inhibition of wnt signaling ( richard - parpaillon et al . , 2002 ; rota & wood , 2015 ) . igf may specifically influence igf-1r owing to its greater binding affinity than igf - ii or insulin ( jones & clemmons , 1995 ) . moreover , igf - iir binds more specifically to igf - ii than igf - i and does not bind with insulin ( massague et al . , 1982 ; rechler & nissley , 1985 ) . in general , mammary gland development is characterized by ductal growth in nonpregnant heifers . while igf - i is highly expressed during the early stages of mammogenesis when compared with that during lactation , this protein exhibits potential proliferative functions throughout the entire process of bovine mammary gland development . additionally , igf-1r and gh receptor mrna expression levels are decreased during the lactation stage . these decreases in mrna levels in bovine mammary tissue have been shown to be species specific ( plath - gabler et al . , 2001 ) . different types of igf-1r transcripts have been discovered in bovine mammary glands ( glimm et al . , 1992 ) . igfbps , which are encoded by six similar transcripts , are regulated based on their interactions with igfs and their specific binding proteins ( ballard et al . , 1989 ) . igfbp bioactivity inhibits the functions of igf - i and igf - ii by competition with igf - ir and igf - iir through igf - i and igf - ii near the cell surface . furthermore , igfs and igfbps can influence interactions with other growth pathways or igf - independent functions ( kelley et al . the interactions between igfbps and igfs are regulated by proteases by affecting the binding affinity of igfbp for igfs . modifications of igfbps , such as protein phosphorylation and binding with other factors , may affect their interactions with other proteins ( mccusker et al . for example , when igfbp-1 binds to integrin , igfbp-3 binding at the cell surface is decreased because the specific affinity of igfbp-3 for igf-1 is reduced when igfbp-3 interacts with cell surface molecules ( conover & powell , 1991 ) . moreover , suppression of igf - i release from igfbp-3 may occur through the interaction between igfs and igfir , thus reducing the activity of igfs . igfbp expression during mammary gland development has also been studied by analysis of the contributions of igfbp-2 and -5 to branching mammogenesis during pregnancy ( wood & yee , 2000 ) . all igfbps exhibit high - affinity binding with igfs ; however , igfbps also have other functions independent of igfs . igfbp-1 and -2 include the tripeptide arg - gly - asp ( rgd ) sequence , which is recognized by cell surface integrins and cell adhesion molecules . igfbp-1 overexpression in chinese hamster cells increases cell migration ( jones et al . , 1993 ) . additionally , igfbp-3 has independent effects on the modulation of bax and bcl-2 , inducing apoptosis in human breast cancer cells through igf - i ( butt et al . , 2000 ) and inhibiting proliferation in type i igf receptor - knockout mice ( valentinis et al . , 1995 ) . recent studies have demonstrated that igfbp-3 regulates transforming growth factor ( tgf)- during programmed cell death through interactions between p53 and igf - independent signaling ( rajah et al . , 1997 ) . additionally , the tgf- receptor can interact with igfbp-3 ( leal et al . , 1997 ) . moreover , igfbp-5 has been shown to activate dna synthesis in osteoblasts through a pathway independent of igf - i ( andress , 1995 ) . both igfbp-3 and -5 contain a putative nuclearlocalization sequence ( nls ) ( radulescu , 1994 ) and have been shown to be expressed in the nuclei using immunelogical localization methods ( jaques et al . these results suggest that igfbps can also contribute to intracellular functions , such as proliferation , apoptosis , and cell migration . thus , these observations also provide insights into whether the high concentrations of igfbp-5 in bovine milk interact with igf - dependent or -independent pathways . in human milk production , igfbp- 2 is more highly expressed than igfbp-1 and igfbp-3 ( erickson et al . , 1993 ; donovan & odle , 1994 ) , and igfbp-1 and igfbp-3 mrnas encoding igfbp-2 and -3 are expressed in porcine mammary glands ( simmen et al . , 1988 ) . thus , igfbp-2 and - 3 in milk is derived from the mammary glands . in bovine serum , igfbp-2 is highly expressed during the early lactation stage and decreased during the dry period ( vicini et al . , 1991 ) . igfbp-2 is not expressed in maternal serum , but is expressed in lactating mammary glands , suggesting that igfbp-2 is synthesized in mammary glands . igfbp-4 expression has been identified in rat mammary glands , although the concentration is lower than that in serum ( donovan et al . , 1995 ) . igfbp-5 and -6 are not present in milk during the lactation stage . igfs and igfbps may enter milk from the serum by paracellular interactions and secretion by epithelial cells . however , involuting mammary glands exhibit increased levels of igfbp-2 , igfbp-4 , and igfbp-5 mrnas . igf-1 is a survival factor for several different types of cells and is regulated by gh ( travers et al . , 1996 ) . of all the hormonal changes that occur during the involution stage , prl is able to influence only igfbp-5 synthesis , suggesting that the prolactin is a potential inhibitor of igfbp-5 . in bovine mammary glands , mammary gland development occurs through complex regulation of the igf pathway . although igf pathway ligands ( igf - i , igf - ii , and insulin ) affect various cellular mechanisms , including proliferation , migration , and apoptosis , the roles of these ligands in mammary gland development depend on associations with their specific receptors , e.g. , igfbps and igf - r . 2 describes the general interactions among igfbps , ligands , and their specific receptors in mammary epithelial cells . igfs can not interact with their receptors when igf - i and igf - ii bind to igfbps owing to competitive interactions between igf and igfbps . igfbps are degraded by proteases , whereas igfs are free to bind with igf receptors . igfbps are present in bovine serum , whereas igfbps are expressed in mammary epithelial cells . in addition , igfbps are secreted in bovine milk derived from mammary gland epithelial cells . furthermore , at the involution stage , igfbps are decreased in milk and epithelial cells , along with prl and gh , resulting in degradation of mammary glands . mammary glands are considered highly sensitive to hormones and igf signal transduction during the developmental stage . therefore , improving our understanding of and igfbp interactions and secretion during mammary gland development may facilitate increased milk productivity through induction of mammogenesis .
the insulin - like growth factor ( igf ) pathway is a key signal transduction pathway involved in cell proliferation , migration , and apoptosis . in dairy cows , igf family proteins and binding receptors , including their intracellular binding partners , regulate mammary gland development . 
 igfs and igf receptor interactions in mammary glands influence the early stages of mammogenesis , i.e. , mammary ductal genesis until puberty . 
 the igf pathway includes three major components , igfs ( such as igf - i , igf - ii , and insulin ) , their specific receptors , and their high - affinity binding partners ( igf binding proteins [ igfbps ] ; i.e. , igfbp16 ) , including specific proteases for each igfbp . 
 additionally , igfs and igfbp interactions are critical for the bioactivities of various intracellular mechanisms , including cell proliferation , migration , and apoptosis . 
 notably , the interactions between igfs and igfbps in the igf pathway have been difficult to characterize during specific stages of bovine mammary gland development . in this review 
 , we aim to describe the role of the interaction between igfs and igfbps in overall mammary gland development in dairy cows .