Regulation of bovine mammary growth by peptide hormones: involvement of receptors, growth factors and binding proteins

ERBB Receptors and Their Ligands in the Developing Mammary Glands of Different Species: Fifteen Characters in Search of an Author

The ERBB tyrosine kinase receptors and their ligands belong to a complex family that has diverse biological effects and expression profiles in the developing mammary glands, where its members play an essential role in translating hormone signals into local effects. While our understanding of these processes stems mostly from mouse models, there is the potential for differences in how this family functions in the mammary glands of other species, particularly in light of their unique histomorphological features. Herein we review the postnatal distribution and function of ERBB receptors and their ligands in the mammary glands of rodents and humans, as well as for livestock and companion animals. Our analysis highlights the diverse biology for this family and its members across species, the regulation of their expression, and how their roles and functions might be modulated by varying stromal composition and hormone interactions. Given that ERBB receptors and their ligands have the potential to influence processes ranging from normal mammary development to diseased states such as cancer and/or mastitis, both in human and veterinary medicine, a more complete understanding of their biological functions should help to direct future research and the identification of new therapeutic targets.

Effect of photoperiod before and during first gestation on milk production and prolactin concentration in dairy heifers

Holstein heifers (n = 45) were subjected to treatments according to a 2 × 2 factorial design where the main effects were the photoperiod treatments during the second isometric (ISO, 52-61 wk of age) and the second allometric (ALLO, 62 wk of age to 8 wk before calving) periods of mammary gland development. During the ISO period, heifers were subjected to either a short-day photoperiod (SDP; 8 h light, 16 h dark; n = 22) or a long-day photoperiod (LDP; 16 h light, 8 h dark; n = 23). During the ALLO period, the photoperiodic treatments were either maintained (SDP:SDP, n = 11; LDP:LDP, n = 11) or switched (SDP:LDP, n = 11; LDP:SDP, n = 12). The treatments ended 8 wk before calving. All animals were then subjected to about 16 h of light per day. Serum prolactin (PRL) concentration during the ISO period was greater in heifers exposed to LDP than in those exposed to SDP. For the first 20 wk of the ALLO period, heifers exposed to LDP had greater serum concentration of PRL than those exposed to SDP. On the other hand, previous exposure to LDP during the ISO period reduced the concentration of PRL compared with those exposed to SDP during that period. During the second 20 wk of the ALLO period, PRL concentration remained greater in the serum of heifers then exposed to LDP than SDP, but serum PRL was greater in heifers exposed to LDP during the ISO period. During the last weeks before calving, when all animals were exposed to LDP, previous exposure to LDP during the ALLO period reduced serum PRL. Early-lactation milk (wk 1-5) and energy-corrected milk (wk 2-6) production were higher in the heifers exposed to SDP than in those exposed to LDP during ALLO. Photoperiod had no effect on milk production after that period. In conclusion, the results do not support to the hypothesis that photoperiod affects mammary gland development during the second allometric phase. However, they confirm that a short-day photoperiod in late gestation enhances milk production in the following lactation in primiparous heifers. Using serum PRL as an indicator of the photoperiodic response, we can conclude that responsiveness to the photoperiodic signal is still conditioned by a previous photoperiod several months after it ends.

Effect of folic acid supplementation on proliferation and apoptosis in bovine mammary epithelial (MAC-T) cells

Folic acid (FA) is known to be an important micronutrient in humans; however, information regarding the effect of FA supplementation on bovine mammary epithelial (BME) cells is insufficient. FA supplementation is reported to increase milk production in dairy cows, but the underlying molecular mechanisms are unknown. This study examined the effects of FA supplementation on the proliferation and apoptosis of a BME cell line (MAC-T). MAC-T cells were treated with various concentrations (deficient in FA (DF) < 0.01 ng/mL; low-level FA (LF) 3.1 ng/mL; normal FA (NF) 15.4 ng/mL; and high-level FA (HF) 30.8 ng/mL) based on serum folate (10-20 ng/mL) in milking cows. HF treatment significantly increased the proliferation of MAC-T cells. Cellular apoptosis was observed mainly in the DF group. The number of apoptotic cells in DF media was significantly higher than that in NF media. The bcl-2/bax mRNA expression ratio was significantly increased in the HF group compared to that in the DF group. FA supplementation significantly increased the ratio of Bcl-2/Bax protein levels in MAC-T cells. FA supplementation increases proliferation and decreases apoptosis in these cells. This study might provide information regarding the molecular mechanism through which FA supplementation is associated with increased milk yield.

Exogenous porcine somatotropin stimulates mammary development in late-pregnant gilts

The goal of this project was to determine if increasing insulin-like growth factor-1 (IGF-1) concentrations in late pregnancy can stimulate mammogenesis in gilts. Yorkshire × Landrace gilts of a similar body weight (BW; 196.2 ± 6.2 kg) on day 89 of gestation were separated in 2 groups, namely, controls (CTL, n = 17) that were injected with sterile water, and porcine somatotropin-treated (pST, n = 20) that received injections of 5 mg of pST (Reporcin). Injections were given daily from days 90 to 109 of gestation and gilts were slaughtered on day 110 to collect mammary glands for compositional analyses. Blood samples were obtained on days 89, 96, 103, and 109 of gestation to measure IGF-1, free fatty acids (FFA), urea, glucose, and insulin concentrations. Treated gilts gained more BW (22.7 vs. 18.2 kg, P < 0.05) and lost more backfat (P < 0.05) than CTL gilts during the treatment period. There was a treatment × day effect (P < 0.01) on IGF-1, glucose, and urea concentrations. Concentrations of IGF-1 increased 4-fold (P < 0.01) in pST compared with CTL gilts on days 96, 103, and 109 of gestation. Insulin values were also greater on days 96 (P < 0.01) and 103 (P = 0.01), and tended to be greater (P < 0.10) on day 109 of gestation in pST gilts. Glucose was greater in pST than CTL gilts on days 96 (P < 0.01), 103 (P < 0.01), and 109 (P = 0.01). Concentrations of urea were lower (P < 0.01) on days 96, 103, and 109 of gestation in gilts receiving pST injections, and FFA was not altered by treatment on any sampling day (P > 0.10). Injections of pST did not affect mammary extraparenchymal tissue weight (P > 0.10) but increased mammary parenchymal mass (1922 vs. 1576 ± 124 g, P < 0.05). The composition of parenchymal tissue was also altered by treatment. Mammary parenchyma from pST gilts contained more (P < 0.05) protein, DNA and RNA and less fat (P < 0.05) and dry matter (P < 0.01) than that from CTL gilts. These findings provide a clear demonstration that increasing circulating IGF-1 in late-pregnant gilts can stimulate mammary development both in terms of total parenchymal mass and of parenchymal tissue composition.

Serotonin as a homeostatic regulator of lactation

Serotonin (5-HT), a neurotransmitter produced in mammary epithelial cells (MECs), acts via autocrine-paracrine mechanisms on MECs to regulate milk secretion in a variety of species. Recent studies in dairy cows reported that 5-HT ligands affect milk yield and composition. We determined the mRNA expression of bovine 5-HT receptor (5-HTR) subtypes in bovine mammary tissue (BMT) and cultured bovine MECs. We then used pharmacologic agents to evaluate functional activities of 5-HTR subtypes. The mRNAs for five receptor isoforms (5-HTR1B, 5-HTR2A, 5-HTR2B, 5-HTR4, and 5-HTR7) were identified by conventional reverse transcription PCR, real-time PCR, and in situ hybridization in BMT. In addition to luminal MEC expression, 5-HTR4 was expressed in myoepithelium, and 5-HTR1B, HTR2A, and HTR2B were expressed in small mammary blood vessels. Studies to date report that there are multiple 5-HTR isoforms in mammary tissue of rodents, humans, and cattle. Inhibition of the 5-HT reuptake transporter with selective 5-HT reuptake inhibitors (SSRIs) disrupted tight junctions and decreased milk protein mRNA expression in mouse, human, and bovine mammary cells. Selective 5-HT reuptake inhibitors act to increase the cellular exposure to 5-HT by preventing reuptake of 5-HT by the cell and eventual degradation. Increasing 5-HT concentration in milk via inhibiting its reuptake (SSRI), or by increasing the precursor for 5-HT synthesis 5-hydroxytryptophan, accelerated decline in milk synthesis at dry-off. We conclude that the 5-HT system in mammary tissue acts as a homeostatic regulator of lactation.

Effect of dry period management on mammary gland function and its endocrine regulation in dairy cows

The objective of this study was to evaluate the effect of shortening the dry period on the mammary gland and the hormonal regulation of its functions. Holstein cows (n = 18) were assigned to a short dry period (SDP; 35 d; n = 9) or a conventional dry period (CDP; 65 d; n = 9). All cows were fed the same diets, with the exception that, during the dry period, the SDP cows received only the pre-calving diet for 35 d, whereas the CDP cows were fed a high-fiber diet from 65 to 28 d before calving and then received the same pre-calving diet as the SDP cows. Mammary gland functional capacity was evaluated at 70 days in milk, and mammary biopsies were taken in early and midlactation. Dry period length averaged 64.3 ± 1.1 and 31.9 ± 1.0 d for the CDP and SDP cows, respectively. The SDP cows had a lower milk yield and a lower energy-corrected milk yield compared with the CDP cows. The SDP cows also had a lower dry matter intake from wk 5 to 20 of lactation and tended to have lower plasma concentrations of β-hydroxybutyrate from wk 1 to 4. Prepartum serum progesterone and estradiol concentrations were unaffected by the dry period management. Serum growth hormone concentrations and milking-induced prolactin release were similar in both groups. However, during the period when the CDP cows were dry but the SDP cows were still being milked (wk -9 to -6), serum prolactin concentrations were higher in the SDP cows than in the CDP cows. The SDP cows had a lower milk BSA content than the CDP cows after the dry period and similar milk lactose concentrations, suggesting that their mammary tight junctions were closed following parturition and, therefore, that the later stage of their lactogenesis was not impaired by SDP management. In early and midlactation, mammary cell apoptosis and proliferation rates as well as mammary expression of genes involved in the function of this tissue were unaffected by the dry period management strategy. For cows in their second lactation, mammary gland functional capacity at 70 d in milk tended to be lower in the SDP cows. In conclusion, even though SDP management decreased milk production during the subsequent lactation, it did not affect mammary cell activity. Although direct evidence is still lacking, decreased mammary cell growth during the dry period is likely responsible for this negative effect. The higher prolactin concentrations in lactating cows during late gestation could be involved in this effect. More research is needed to test these hypotheses.

Short Communication: Intramammary Infusion of IGF-I Increases Bromodeoxyuridine Labeling in Mammary Epithelial Cells of Prepubertal Heifers

When dairy heifers are fed to gain more than 900 g of body weight/d, they have less mammary parenchymal DNA at puberty but more insulin-like growth factor-I (IGF-I) in serum. This negative relationship between serum IGF-I concentration and mammary epithelial cell proliferation is in disagreement with the extensively reported role of IGF-I as a stimulator of mammary epithelial cell proliferation. Despite the large body of evidence suggesting that an increase in IGF-I concentration should lead to an increase in mammary epithelial cell proliferation of prepubertal heifers, it had not been previously tested. Our objective was to determine if intramammary infusions of IGF-I would stimulate mammogenesis in prepubertal heifers in vivo. After 7 d of treatment, bromodeoxyuridine was infused intravenously and heifers were slaughtered 3 h later. Samples from 3 regions of the mammary parenchyma were collected, fixed, sliced, and incubated with bromodeoxyuridine monoclonal antibody to identify cells in the S-phase of the cell cycle. Intramammary infusion of IGF-I increased the percentage of epithelial cells in the S-phase by 52% (6.4 vs. 4.2%, +/- 0.3%). Proliferation was similar in all 3 parenchymal regions, and the response to IGF-I was similar in each region. We conclude that local IGF-I increases proliferation of mammary parenchymal epithelial cells in prepubertal heifers.

Prospects for zero days dry

Recent studies in high-producing dairy cows have demonstrated the dry period requirement is greatly influenced by parity and management practice. Multiparous cows that were continuously milked and treated with bST demonstrated negligible production losses in the next lactation. First-lactation heifers, however, demonstrated large reductions in milk yield. These reductions were not overcome by using bST or IMF the next lactation. No studies have been performed examining use of LDPP or SDPP in combination with bST and IMF on the dry-period requirement. Cows that are continuously milked demonstrated higher feed intakes during the peripartum period, which may greatly improve metabolic health. Future studies must examine potential benefits of continuous milking on health in the next lactation.

Mammary secretion of oestrogens in the cow

Two experiments in vivo and one experiment in vitro were conduced to examine the mechanisms involved, which lead to mammary secretion of oestrogens and its importance for milk production and udder health in cows. In experiment 1 in six cows of the White-Black breed on day 268 of pregnancy catheters were inserted into uterine vein of pregnant horn, the abdominal aorta and the caudal superficial epigastric (milk) vein. Blood samples for estimation of oestrone, oestrone sulphate, oestradiol-17alpha and -17beta by RIA were obtained daily from day 7 pre-partum until day 1 post-partum. Only the concentration of oestradiol-17beta was statistically higher (P< or =0.01) in mammary venous plasma than in aortal and uterine plasma. In experiment 2 forty late-pregnant cows were divided into two groups according to their milk production in the previous lactation: group 1 (n=20) high-yielding cows (>6500kg milk per lactation), and group 2 (n=20) low-yielding cows (<3700kg milk per lactation). Blood samples for measurement of oestradiol-17beta by RIA were collected from milk and tail veins every fourth day during a period from day 20 prior to parturition to day 4 post-partum. The concentration of oestradiol-17beta was significantly higher (P< or =0.01) in the milk vein than in the peripheral plasma from day 12 pre-partum to parturition. In high-yielding cows the level of oestradiol-17beta in mammary venous blood was significantly higher (P< or =0.01) than in low-yielding cows. In six cows with pathological udder oedema ante-partum the concentration of oestradiol-17beta in milk vein was significantly higher (P< or =0.05) than in control cows. There were no statistically significant differences in the levels of oestradiol-17beta in cows with clinical mastitis (n=10) during 2 weeks after parturition and without it (P> or =0.05). In an in vitro experiment, homogenates of mammary tissue collected on day 7 pre-partum from two cows were incubated with 3H-androstendione. After incubation the samples were extracted and 3H-oestradiol-17beta was separated by HPLC. 3H-oestradiol-17beta was formed in a total yield of 37%. These results indicate that oestrone, oestrone sulphate and oestradiol-17alpha are not secreted by bovine mammary gland. Furthermore, the secretion of oestradiol-17beta starts about day 12 pre-partum and is associated with milk yield and udder oedema. Preliminary in vitro study suggests the synthesis of oestradiol-17beta by mammary tissue.

Molecular cloning of bovine (Bos taurus) cDNA encoding a 94-kDa glucose-regulated protein and developmental changes in its mRNA and protein content in the mammary gland

We isolated and sequenced cDNA clones encoding a 94-kDa glucose-regulated protein (GRP94) from a cDNA library constructed using bovine (Bos taurus) mammary gland poly(A)(+) RNA. The coding nucleotide sequence and the deduced amino acid sequence of bovine GRP94 shared 94.2-88.4% and 98.1-96.5% identity with those of other mammalian species, respectively. The primary structure contained a carboxyl-terminal signal sequence for retention in the endoplasmic reticulum, six potential sites for N-linked glycosylation and two potential adenosine 5'-triphosphate binding sites, similar to other mammalian and avian GRP94 homologues. In Northern blot hybridization using a cDNA probe from the bovine GRP94 cDNA sequence, a transcript 3.0 kb in size was detected. We measured the amounts of GRP94 and its mRNA in mammary glands from cows at various developmental stages of hormonally induced lactation. The highest level of GRP94 mRNA, determined by dot blot analysis, was detected in the developing stage. In contrast to the mRNA level, the amount of protein, determined by immunoblot analysis using rabbit antiserum raised against GRP94 purified from bovine brain, was higher in lactating stages than in others. The increased level of GRP94 mRNA during the developing stage and the maintenance of GRP94 protein during lactation suggest that the synthesis of GRP94 is regulated during mammary development and differentiation, and also that the protein is involved in a function related to lactation.

Involvement of growth factors in the regulation of pubertal mammary growth in cattle

Pubertal mammary growth in heifers is dependent on interactions of many hormones and growth factors of which some are stimulatory while others are inhibitory. Although estrogen and growth hormone (GH) are of primary importance, more recent studies have suggested a role for both systemic and mammary tissue-specific growth factors. Growth factors may act as mediators of estrogen and GH or through specific effects of their own. These growth factors include insulin (INS), IGFs (IGF-I and IGF-II), epidermal growth factor (EGF), FGFs (FGF-1 and FGF-2), TGFs (TGF-alpha and TGF-beta's, amphiregulin (AR), platelets derived growth factor (PDGF), and mammary derived growth factor-1 (MDGF-1). Using mammary epithelial cells derived from prepubertal heifers and cultured in three-dimensional collagen gels as an in vitro model, we have investigated the mitogenic effects of a number of different growth factors (IGF-I, des(1-3) IGF-I, IGF-II, INS, EGF, TGF-alpha, AR, FGF-1, FGF-2, and TGF-beta 1). As expected, IGF-I, des(1-3)IGF-I, IGF-II and INS all stimulated proliferation of mammary cells with des(1-3)IGF-I being the most potent and INS the least potent. The mitogenic effect of IGF-I could be inhibited by both IGFBP-2 and IGFBP-3 showing that these binding proteins modulate the bioactivity of IGF-I in the mammary gland at the cellular level. Regulation of IGF availability by IGFBPs in the extracellular environment therefore is critical for IGF action in the mammary gland. Proliferation of mammary epithelial cells was also stimulated by growth factors of the EGF family, i.e. EGF, TGF-alpha and AR, however, not as much as growth factors from the IGF family. Members of the fibroblast growth factor family showed various mitogenic activities. FGF-1 stimulated DNA synthesis while FGF-2 in concentrations above 10 ng/ml inhibited DNA synthesis. TGF-beta 1 at very low concentrations stimulated proliferation slightly whereas higher concentrations strongly inhibited proliferation of mammary epithelial cells and inhibited mitogenesis induced by growth factors of both the EGF- and IGF family. This shows that TGF-beta 1 is a very potent regulator of pubertal mammary growth.

Course of epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I) in mammary secretions of the goat during end-pregnancy and early lactation

The epidermal growth factor (EGF) plays a crucial role in mammogenesis in many species. In ruminants, studies are limited, as EGF does not occur in peripheral plasma and specific analytical systems do not exist. Therefore a heterologous radioimmunoassay based on rhEGF was set up to monitor EGF in mammary gland secretions from goats during end-pregnancy and early lactation. IGF-I was measured with an established radioimmunoassay. Samples were collected from 13 goats for 25 days ante-partum and 25 days post-partum. Mammary gland secretions were obtained ante-partum by removing a small amount of the udder secretions (control half) or milking (stimulated half). Post-partum normal milk samples were collected. Blood samples were drawn by jugular venipuncture for the same period. EGF was found to occur in different molecular weight forms in the mammary glands. For routine measurements these proteins were extracted with acetone and not further separated. IGF-I and EGF concentrations in mammary secretions and similarly IGF-I in blood were high ante-partum and decreased slightly towards birth. IGF-I but not EGF is found in the peripheral plasma. Whereas IGF-I concentrations in blood were quite constant post-partum, IGF-I and EGF dropped in mammary secretions close to the detection limits. The decrease was more pronounced in the stimulated half than in the control half. The data support a synergistic role for EGF and IGF-I for mammogenesis. Both factors are further influenced by the milking stimulus and thus the functional state of the udder.

Intramammary infusion of insulin or long R3 insulin-like growth factor-I did not increase milk protein yield in dairy cows

Two experiments investigated the regulation of milk protein synthesis in well-fed cows (n = 4) using 1) a hyperinsulinemic-euglycemic clamp and 2) intramammary infusion of insulin or long R3 insulin-like growth factor-I plus supplementary amino acids. In experiment 1, insulin was infused at 1.0 microg x kg BW(-1) x h(-1) to increase circulating levels fourfold, and euglycemia was maintained by infusion of glucose. An insulin clamp increased the yields of casein and whey protein both with and without supplementary amino acids. Plasma concentrations of insulin-like growth factor-I were increased and insulin-like growth factor binding protein-2 decreased during insulin clamp, while both insulin and insulin-like growth factor-I in milk were elevated by this treatment. Milk concentrations of insulin peaked on day 4, but insulin-like growth factor-I concentrations in milk peaked on day 1 of the insulin clamp. In experiment 2, intramammary infusion of insulin had no effects on any measured variables, while yields of milk, protein, and fat were slightly lower following long R3 insulin-like growth factor-I treatment. This could be associated with an increase in somatic cell count, which occurred following long R3 insulin-like growth factor-I treatment. Results from experiment 1 suggest insulin and insulin-like growth factor-I are likely candidates responsible for the increased milk protein yields during the insulin clamp. However, in experiment 2 neither hormone enhanced milk protein yield when administered using an intramammary technique.

Regulation of local synthesis of insulin-like growth factor-I and binding proteins in mammary tissue

Our objective was to investigate the mammary expression of insulin-like growth factor-I (IGF-I) and IGF-binding proteins in prepubertal heifers and regulation of IGF-I by bovine somatotropin (bST) and feeding level. Twenty-four prepubertal Friesian heifers were divided into six blocks according to genotype and starting date for the experiments. Within blocks, heifers were assigned to daily bST treatment (0 or 15 mg/d) at low or high feeding level (0.55 kg/d or 1.1 kg/d average daily gain, respectively) for 5 wk so that the mean body weight and standard error was approximately equal for all four treatment groups. At high feeding level, content of IGF-I protein in mammary tissue extracts was increased 46% by somatotropin compared with placebo. Somatotropin tended to increase abundance of IGF-binding protein-3 (40 to 43 kD) in mammary extracts. High feeding level increased abundance of a 24-kD binding protein and reduced abundance of IGF-binding protein-2 (32 kD) in mammary extracts. High feeding level reduced abundance of IGF-binding protein-1 mRNA in mammary tissue, but there was no significant effect of feeding level or somatotropin on mRNA levels of other IGF-binding proteins. These results suggest that effects of somatotropin treatment and feeding level on the prepubertal mammary gland are mediated in part by alterations in local synthesis of IGF-I and IGF-binding proteins.

Growth hormone and mammary development

Classic studies in rodents conducted in the 1950s showed that growth hormone (GH) is essential for mammary development both in the pubertal phase and during pregnancy. Since then, a considerable number of experiments have been carried out in ruminants to investigate the role of GH for regulation of normal mammary development and to examine the possibility of enhancing mammary growth by administration of GH. The available evidence demonstrates that GH treatment stimulates mammary growth before puberty, but the data do not convincingly support the idea that the effect is translated into increased milk yield. GH treatment during late pregnancy seems to stimulate both mammary growth and milk yield during lactation. The limited data concerning the effect of GH on mammary growth during lactation indicate that mammary growth is unaffected by GH treatment in early lactation, whereas GH seems to increase the amount of mammary parenchyma in mid-lactation. The mechanism of action of GH remains a puzzle, but the effect of exogenous GH most likely involves insulin-like growth factor-I (IGF-I). Full understanding of the role of endogenous GH for regulation of normal mammary development requires more knowledge about the interaction between GH and IGF-I and the interplay between the GH-IGF-I axis and locally produced factors, including receptors, binding proteins, and growth factors.

Biology of somatotropin in growth and lactation of domestic animals

Impressive progress has been made during the past 15 years in our understanding of the biology of somatotropin (ST) in domestic animals. In part, this progress was sparked by advances in biotechnology that made feasible the production of large quantities of recombinant bovine ST (bST) and porcine ST (pST). The availability of recombinant bST and pST resulted in an exponential increase in investigations that explored their role in growth and lactation biology, as well as evaluated their potential for commercial use. Collectively, these studies established that administration of bST to lactating dairy cows increased milk yield, and treatment of growing pigs with pST markedly stimulated muscle growth and reduced fat deposition. In addition to these "efficacy" studies, a substantial number of investigations examined the mechanisms by which ST affects lactation and growth of domestic animals. This review summarizes the diverse physiological effects ST has on growth and lactation and discusses the underlying mechanisms that mediate these effects in domestic animals.

Ontogeny and epithelial-stromal interactions regulate IGF expression in the ovine mammary gland

Although the insulin-like growth factors (IGF-I and -II) have been implicated in the stimulation of mammogenesis, little is known of their regulation in the mammary gland. In this study we removed epithelial tissue from one of the two mammary glands of 1-week-old ewe lambs and examined IGF-I and -II mRNA expression during postnatal development in both the intact mammary gland and in the gland cleared of epithelial tissue. Expression of IGF-I mRNA was highest at 6 and 10 weeks of age, coincident with the prepubertal phase of rapid mammary growth, then declined and remained low until expression increased during late pregnancy. IGF-I mRNA was more abundant in the mammary fat pad adjacent to parenchyma (MFP) than in the contralateral fat pad that had been surgically cleared of epithelium (CFP). The level of IGF-II mRNA in parenchyma was highest at 1-23 weeks of age due to an increase in the abundance of specific mRNAs. Expression was lower in the fat pads, with generally higher levels in the intact MFP than the CFP, and in these tissues IGF-II expression was shown to increase with age between 6 and 23 weeks. We also investigated the influence of the ovary and estrogen on the expression of IGFs. While IGF-I mRNA abundance was unaffected by ovariectomy, exogenous estrogen resulted in higher levels of expression in the MFP of ovariectomized ewes and tended to increase its level in the parenchyma of intact ewes. Ovariectomy increased IGF-II mRNA within mammary parenchyma whereas estrogen suppressed levels in both the parenchyma and MFP. These findings demonstrate that IGF-I and -II mRNAs are expressed locally within the developing ovine mammary gland and are regulated by stage of ontogeny, ovarian hormones, and epithelial stromal interaction.

Insulin-like growth factors in milk and mammary gland

Milk contains insulin-like growth factor I and II (IGF-I and IGF-II) and four IGF binding proteins (IGFBP). Their concentrations are highest prepartum and early postpartum, coinciding with maximal proliferation of mammary cells and the time when the infant gut is the least developed. This has lead to the suggestion that IGFs may be important for the growth and development of the mammary gland and that IGFs in milk play a role in promoting development of the gastrointestinal tract of the newborn. IGF-I and, to a lesser extent, IGF-II can stimulate milk yield and blood flow in goats when infused directly into the mammary gland, suggesting they may also be important in supporting established lactation. Thus IGFs may have a dual function in the mammary gland, establishing and maintaining the maternal mammary system and, once secreted into milk, supporting gastrointestinal development in the newborn.

The insulin-like growth factor and epidermal growth factor families in mammary cell growth in ruminants: action and interaction with hormones

Selective breeding and improved management have had major effects in increasing peak milk yields but relatively little effect on lactation persistency. In ruminants, cell loss appears to be largely responsible for the decline in milk yield. Little is known about the longevity of individual cells, but, in lactating dairy cows, few epithelial cells are in the S phase (DNA synthesis) of the cell cycle. The IGF and epidermal growth factor families are direct mitogens, stimulating DNA synthesis in cultures of ruminant mammary epithelial cells. Receptors that mediate the effects of these growth factors, the type 1 IGF receptor and the epidermal growth factor receptor, respectively, are present at similar levels in membranes prepared from the mammary glands of nonpregnant and pregnant sheep. Binding capacity falls by parturition and remains low during lactation. These findings suggest that the drive to mammary development in pregnancy comes from control of growth factors, and, in the case of IGF, modulating binding proteins, a control exerted by hormones, which, in general, are not themselves mitogens. A paracrine or autocrine mode of action and, therefore, local growth factor synthesis, are more likely to be important than systemic concentrations of growth factor. Stimulatory growth factors produced locally by the mammary gland include IGF-I, IGF-II, transforming growth factor-alpha, and amphiregulin. More information is needed on the control of stimulatory and inhibitory growth factors and on how growth factors control the cell cycle. Knowledge of these processes could result in strategies to improve lactation persistency by increasing secretory cell renewal or reducing cell loss during lactation.

Animal models for the study of milk secretion

Milk secretion is regulated by a complex interaction of galactopoietic hormones which is not yet fully understood. Recent studies have demonstrated that this systemic control is modulated within the mammary gland by local mechanisms responsive to the frequency and completeness of milk removal. New insights into the endocrine and local (paracrine and autocrine) regulation of milk secretion have come from the adaptation of traditional endocrinological techniques to take advantage of new molecular tools, and from technical advances in other fields. This paper reviews recently developed animal models for the study of milk secretion and describes their application to provide new information into the roles of two key galactopoietic hormones, growth hormone and prolactin, and the modulation of their actions by local, intramammary mechanisms.

Relationship between frequent milking or suckling in early lactation and milk production of high producing dairy cows

Groups of 9 or 10 cows were assigned to one of three treatments 1) machine-milking three times daily, 2) machine-milking six times daily, and 3) suckling three times daily in addition to machine- milking three times daily. Treatments were conducted during the first 6 wk postpartum; thereafter, all cows were milked three times daily. During treatment, milk production was highest for suckled cows and lowest for cows milked three times daily. The DMI were similar for suckled cows and cows milked three times daily but higher for those milked six times daily. Body weight loss was greatest for suckled cows and least for cows milked three times daily. During wk 7 to 18 postpartum, cows milked six times daily exhibited a carry-over effect on milk production that was greater than that of other groups, During treatment, plasma growth hormone and IGF-I concentrations were elevated for suckled cows and, to a lesser extent, for cows milked six times daily. Prolactin and oxytocin similarly increased, but insulin decreased in suckled cows and, to a lesser extent, in cows milked six times daily. Posttreatment differences persisted for insulin and IGF-I, but not for the other hormones. Increased frequency of udder emptying increased milk production, and suckling was superior to machine-milking. High milk production was associated with elevated growth hormone, IGF-I, prolactin, and oxytocin, although cause and effect could not be established. The failure of suckled cows to increase feed intake to match output requires further investigation.

Relationships between nutrition, puberty and mammary development in cattle

The available studies concerning the relationships between nutrition, puberty and mammary development demonstrate the importance of pubertal mammary growth for the future development and ultimate milk-producing capacity of the mammary gland. A relationship between reproductive development and mammary development is also evident and mammary development at puberty is clearly influenced by the feeding level at that time. The role of specific nutrients has not been thoroughly investigated, but results suggest that specific fatty acids may be involved in the regulation of mammary growth. Mammary growth during puberty is affected by oestrogen and GH, but their respective roles and mechanisms of action have not yet been clarified.