Organization and growth of mammary epithelia in the mammary gland fat pad

Mammary gland development consists of a series of very highly ordered events involving interactions among a number of distinct cell types. An important aspect of mammary gland development is that the mammary gland consists of a fat pad of mesodermal origin into which epithelial cells of ectodermal origin proliferate. This proliferation of epithelial cells into the mammary fat pad is the subject of this review. The nature of the stroma into which epithelial cells proliferate is of considerable importance in determining the structure of the resulting gland. In mice, white adipose tissue appears to be required for normal mammary development. Transplantation of mammary epithelia to other types of stroma does not support epithelial growth or result in abnormal growth. To date, a synthetic substratum capable of mimicking white adipose tissue has not been developed. Although collagen gel cultures are generally considered superior to glass or plastic substratum in supporting near normal epithelial growth, the technique has not advanced to the point that the in vivo growth pattern is duplicated. Recent research on the generation of chimeric mammary tissue (by transplanting mammary epithelia from rats, cows, and women to the mammary fat pads of athymic nude mice) suggests that there are important species differences in the stromal requirements for mammary gland development. In particular, extensive and expansive growth of rat mammary tissue is observed in mouse mammary fat pads. However, the mouse mammary fat pad appears incapable of supporting expansive growth of bovine or human mammary epithelia. The reason for this difference is not clear. However, human and bovine mammary epithelia may require the presence of more fibrous (collagenous) tissue than rodent mammary epithelia for normal proliferation.     

奶山羊乳腺中胰岛素样生长因子及其受体的表达与作用

为研究胰岛素样生长因子及胰岛素样生长因子受体在奶山羊乳腺组织青春期、妊娠期、泌乳期及退化期表达的差异及变化,探索其在奶山羊乳腺发育过程中的作用,根据奶山羊乳腺发育特点及生长阶段不同分为青春期、妊娠期、泌乳期及退化期,采用激光共聚焦显微镜检测胰岛素样生长因子在奶山羊乳腺各生长阶段的表达及定位,通过定量分析来比较胰岛素样生长因子在奶山羊乳腺各生长阶段表达的差异,描述其在奶山羊乳腺发育过程中的作用.结果显示:胰岛素样生长因子及胰岛素样生长因子受体在奶山羊乳腺法如各时期均有表达,表达水平与乳腺发育密切相关.     

Modeling the effect of energy status on mammary gland growth and lactation

The impact of nutrition on lactation can be separated into acute effects, affecting day-to-day yield, and chronic effects, which govern the persistency of lactation and rate of decline of the lactation curve. A mathematical model of the mammary gland was constructed to investigate both acute and chronic effects. Mammary growth is expressed in terms of the dynamics of populations of active (secreting) and quiescent (engorged) alveoli. The secretion rate of active alveoli is expressed in terms of the energy status of the dam. The model was fitted to data from a 2 x 2 factorial trial in which lactation curves were measured for heifers of two different genotypes (North American and New Zealand Holstein-Friesians) fed two different diets [grass and total mixed rations (TMR)]. Total formation of alveoli during pregnancy and lactation was statistically the same across all groups despite differences between diets, in the rate of formation of alveoli at parturition. The senescence rate of alveoli was significantly higher for heifers fed grass compared with heifers fed TMR, which corresponds to better persistency for heifers fed TMR. Heifers fed TMR had a higher rate of reactivation of quiescent alveoli than heifers fed grass, which also contributes to increased persistence for heifers fed TMR. There was a genotype x diet interaction in the rate of quiescence of active alveoli: the North American-Grass group had a higher rate of quiescence than the other three groups, perhaps reflecting differences in selection pressures between the New Zealand and North American genotypes.     

Modeling the interaction of milking frequency and nutrition on mammary gland growth and lactation

A mathematical model of biological mechanisms regulating lactation is constructed. In particular, the model allows prediction of the effect of milking frequency on milk yield and mammary regression, and the interaction of nutrition and milking frequency in determining yield. Possible interactions of nutrition with milking frequency on alveolar dynamics are highlighted. The model is based upon the association of prolonged engorgement (as a consequence of milk accumulation) of active secretory alveoli with changes in gene expression that result in impairment and, ultimately, cessation of milk secretion. The emptying of alveoli at milking, following alveolar contraction induced by oxytocin, prevents this process and also allows quiescent alveoli to reactivate. Prolonged engorgement results in apoptosis of the secretory cells and, hence, regression of the mammary gland. Milk yield is linked to alveolar populations, with secretion rates being modulated by nutrition and udder fill effects. The model was used to investigate different management scenarios, and is in agreement with experimental results. The model shows that while milking frequency drives alveolar population, and therefore potential milk production, actual production varies considerably with nutrition. A significant portion of the loss associated with once-daily milking was due to udder fill rather than loss of secretory tissue. The model showed qualitative agreement with experimental data, on the acute and chronic effects of temporary once-daily milking.     

Myoepithelial cell differentiation markers in prepubertal bovine mammary gland: effect of ovariectomy

We reported previously that ovariectomy alters prepubertal development of mammary myoepithelial cells (MC) by mechanisms that are not well understood. Therefore, in the present study, we analyzed expression of 2 myoepithelial differentiation markers, α-smooth muscle actin (SMA) and the common acute lymphoblastic leukemia antigen (CD10), in mammary parenchymal tissue from intact (INT) and ovariectomized (OVX) heifers. On d 40, Holstein heifers underwent either an ovariectomy (OVX; n=16) or a sham (INT; n=21) operation. At 55, 70, 85, 100, 130, and 160 d of age, tissues were collected, and multispectral imaging was used to quantify immunofluorescent staining for myoepithelial cell (MC) markers. Fluorescent intensity (FI) of the markers was normalized against a control sample. In the basal epithelial layer, CD10 FI was less and SMA FI was greater in OVX than INT. The ratio of SMA to CD10 FI, as a proxy indicator for MC differentiation, was greater in tissue from OVX compared with INT heifers after 55 d of age. The staining for SMA was frequently more intense along the basal aspect of cells, whereas CD10 expression was localized on the apical surface of the MC. In mammary tissue from both INT and OVX heifers, we observed basal cells that were negative for both CD10 and SMA, some of which appeared to span the distance from basement membrane to the ductal lumen. Interestingly, we also observed CD10+ cells adjacent to the ductal lumen, a situation that was more prevalent in OVX than in INT heifers. Also, ovariectomy affects MC expression of both SMA and CD10, as well as the pattern of MC development. Myoepithelial cells are known to limit parenchymal growth in other species. Involvement of MC as regulators of prepubertal bovine mammary development is worthy of further investigation.     

Autotransplantation of the goat mammary gland

The modified technique for transplanting one mammary gland of a lactating goat to the neck, with the mammary (pudic) artery and vein anastomosed to the carotid artery and jugular vein respectively, is described. This technique was successful in 6 operations and there were no significant differences between the milk yield of the transplanted and control (in situ) glands at any stage after operation. These results are compared with those from an earlier series in which no special provision was made for lymphatic drainage and in which anticoagulant therapy and treatment of the gland prior to attachment were not standardized.     

Role of prolactin, growth hormone and insulin-like growth factor 1 in mammary gland involution in the dairy cow

Bovine mammary involution, an important process for subsequent lactations, is characterized by loss of epithelial cells by apoptosis, but its hormonal regulation is still not well defined. Prolactin (PRL) and growth hormone (GH) play a specific role on rat mammary gland apoptosis, through insulin-like growth factor 1 (IGF-1) and the IGF binding protein (IGFBP) system. The purpose of our investigation was to determine the possible role of PRL, GH, and IGF-1 on cell survival and on IGFBP-5 expression in the bovine mammary gland. Mammary gland explants were cultured in the presence of cortisol, 17beta-estradiol, progesterone, insulin, PRL, GH, and IGF-1 and with the same treatment but without PRL, GH or IGF-1, respectively. After 24 h of culture, we determined the level of apoptosis through evaluation of DNA laddering in the oligonucleosomal fraction and examined IGFBP-5 messenger RNA (mRNA) expression. The results show a high level of DNA laddering and an increase in IGFBP-5 mRNA content in mammary explants cultured in the absence of PRL, GH, or IGF-I with respect to explants treated with all hormones. Moreover, explants cultured in presence of PRL, GH, or IGF-I show a low level of DNA laddering and IGFBP-5 expression with respect to explants cultured without any hormones. These data demonstrate a relationship between levels of apoptosis and IGFBP-5 mRNA expression in the bovine mammary gland and confirm the involvement of this binding protein programmed cell death and its relationship with the main lactogenic hormones.     

Effect of plane of nutrition on growth and mammary gland development in Holstein heifers

Forty-one Holstein and 6 Holstein crossbred heifers, 6 to 8 mo of age, were used to determine the effect of plane of nutrition on growth and mammogenesis prior to and during puberty. Animals were fed to gain 611 g (low), 737 g (medium), and 903 g (high) by a diet of cracked corn and chopped alfalfa-grass hay. Mammary biopsies were carried out in vivo to determine if they provide acceptable information on mammary composition (based upon morphometric evaluation) in comparison with dissected glands. Results indicated that NRC (1978) recommendations for average daily gain of Holstein heifers between 6 and 16 mo may be too generous. At puberty (first estrus), age and wither height decreased linearly with increasing plane of nutrition, whereas body weight and hip height were not affected by plane of nutrition. Five heifers were slaughtered at the beginning and 18 at the conclusion of the trial. Increasing plane of nutrition resulted in fatter mammary glands with decreased concentration of DNA, whereas total mammary DNA did not differ among treatment groups. In this study, morphometric evaluation of mammary tissue obtained through biopsies did not yield useful information in comparison to chemical analysis of dissected glands.     

共轭亚油酸对乳腺脂类代谢和乳腺发育的作用及其机理

共轭亚油酸(Conjugated Linoleic Acid,CLA)是亚油酸具有共轭双键的一组同分异构体,由瘤胃细菌在生物氢化过程中合成,具有抗癌、抗炎症、抗动脉粥样硬化等多种生物学功能。CLA在代谢方面也有调节作用,能影响全身代谢,影响肝脏、乳腺以及富含脂肪的相关组织代谢。在此介绍了CLA的来源、结构、对动物乳腺脂类代谢和乳腺发育的作用及其机理。     

Release of cholesteryl esters by the mammary gland of the pre-partum cow

In five cows that were regularly milked before parturition, cholesteryl esters were continuously released into the mammary fluid; their concentration in the fluid was initially high, but decreased a few days before parturition when mammary secretion of fluid and triglyceride was increasing. The composition of fatty acids in the cholesteryl esters of mammary fluid and in blood plasma was different, suggesting mammary synthesis of cholesteryl esters.     

Extraction of plasma triacylglycerols by the mammary gland of the lactating cow

The fatty acid content of triacylglycerols (TG) in arterial and mammary venous blood plasma has been analysed in five cows that were close to peak lactation. The fatty acids in arterial TG were extracted by the mammary gland in amounts 14:0 less than 16:0 less than 18:0 and 18:0 greater than 18:1n - 9 greater than 18:2. The difference of extraction between palmitate and stearate was similar in different TG species. The fatty acid 18:1n - 7, which was largely the trans isomer, was extracted more than 18:1n - 9. Significant amounts of phytanic acid were acylated in plasma TG, but not extracted by the mammary gland.     

Spatial and temporal expression of insulin-like growth factor-I, insulin-like growth factor-II and the insulin-like growth factor-I receptor in the sheep fetal mammary gland

The mammary gland is an example of a tissue of epidermal origin that depends for the development of its characteristic morphology on underlying mesenchymal cells. The interaction between mesenchyme and epithelium appears to be mediated by polypeptide growth factors. In situ hybridization has been used to study, in the mammary gland of female sheep fetuses, the distribution of mRNA for the mammary mitogens, insulin-like growth factor (IGF)-I and IGF-II, and the IGF-I receptor, from 10 to 20 weeks of intrauterine life (term is approximately 22 weeks). At 10 weeks, secondary ducts had formed from the primary duct. By week 20, the gland had increased in volume and complexity, showing primitive lobules embedded in intralobular connective tissue disposed around main ducts. IGF-I and IGF-II mRNA were expressed in cells of the intralobular connective tissue underlying the epithelium, while the IGF-I receptor was expressed in epithelium. Quantitation by absorbance measurements showed that mRNA expression increased with pregnancy stage for IGF-I and IGF-II, but not significantly for the IGF-I receptor, and that IGF-II was more highly expressed than IGF-I. A role for the IGF system in mediating mesenchymal epithelial interactions in mammary development is indicated.     

Interspecies variation in mammary gland growth rate: relationship to gestation length

Growth of the mammary gland is measured by several indices including total wet weight, dry fat-free tissue, and deoxyribonucleic acid. The latter is a superior measure of true growth because it represents changes of cell numbers. Sufficient data have been generated to determine the relationship among species of mammals between gestation length and differences in rates of mammary growth. Exponential growth equations were estimated for eight mammalian species with gestation lengths from 16.5 d for the hamster to 280 d for the cow. The form of the most appropriate equation was Y = AeBx, where Y is mammary deoxyribonucleic acid or dry fat-free tissue, x is day of gestation, e is the base of natural logs, and A and B are constants. The A term was related to body weight (W) and the B-term to gestation length (G). Resulting equations were deoxyribonucleic acid (mg) = .0547W.803 e1.98 G-.98x and dry fat-free tissue (mg) = 2.35W.779 e.719 G-.77x. First-order rate constants of mammary growth ranged in a reverse order from a high of .141 d-1 in hamsters to a low of .008 d-1 in cows; in other words, mammary deoxyribonucleic acid in hamsters doubled in 4.9 d but in the bovine it took 87 d to double.     

IGFBP-5在乳腺发育过程中的生物学作用

细胞生长的调节主要由IGFs起作用,IGFBPs能够通过蛋白水解、翻译后修饰以及同细胞表面和细胞外基质的相互作用来加强或抑制IGFs的作用,从而调节细胞的增殖、存活和分化.IGFBPs也具有独立的作用方式.IGFBP-5与乳腺发育特别是乳腺退化关系密切,IGFBP-5的表达可能是细胞凋亡发生时的普遍事件,尤其是在乳腺退化的初始阶段.     

Phosphorylation of casein by the lactating mammary gland: a review.

The lactating mammary gland synthesizes and secretes large amounts of phosphoproteins that mainly are associated with the casein fraction of milk. The free amino acids and inorganic phosphate of blood serve as building materials for casein, and the final product appears in milk as a colloidal-sized particle, the casein micelle. According to our present concept, the biosynthesis of casein occurs in two steps: synthesis of the polypeptide chain, followed by phosphate addition. Phosphate groups are transferred to the nascent casein by a protein kinase localized in the Golgi apparatus. The enzyme uses adenosine 5'-triphosphate as the phosphate donor and requires divalent cations. Neighboring amino acids may be important in determining which serine residues in casein are phosphorylated. This review discusses historical and current research on the phosphorylation of casein.     

Protein kinases in the lactating mammary gland

Protein kinases in the cytosol of whole bovine lactating mammary gland were separated by phosphocellulose chromatography. Five protein kinases (identified as histone kinase and casein kinases A, B, C, and D) were characterized and compared with kinases from other tissues. The histone kinase activity was identified as cyclic AMP-dependent protein kinase. The casein kinases differed in their activities toward a variety of proteins (alpha s1-casein, native and dephosphorylated beta-casein, and alpha-lactalbumin) and peptides. Based on substrate specificity studies and the inhibitory effects of heparin, 2,3-diphosphoglycerate, and guanosine triphosphate, kinases B, C, and D were tentatively identified as glycogen synthase kinase-3, casein kinase I, and casein kinase II, respectively.