Transplantability of mammary gland in brown fat pads of mice

Analysis of Mammary Gland Phenotypes by Transplantation of the Genetically Marked Mammary Epithelium

The mammary gland is the only organ to undergo most of its development after birth and therefore particularly attractive for studying developmental processes. In the mouse, powerful tissue recombination techniques are available that can be elegantly combined with the use of different genetically engineered mouse models to study development and differentiation in vivo.In this chapter, we describe how epithelial intrinsic gene function can by discerned by grafting mammary epithelial cells of different genotypes to wild-type recipients. Either pieces of mammary epithelial tissue or dissociated mammary epithelial cells are isolated from donor mice and subsequently transplanted into recipients whose mammary fat pads were divested of their endogenous epithelium. This is followed by phenotypic characterization of the epithelial outgrowth either by fluorescence stereomicroscopy for the fluorescently marked grafts or carmine alum whole mount for the unmarked epithelia.

3D brown adipogenesis to create "Brown-Fat-in-Microstrands"

The ability of brown adipocytes (fat cells) to dissipate energy as heat shows great promise for the treatment of obesity and other metabolic disorders. Employing pluripotent stem cells, with an emphasis on directed differentiation, may overcome many issues currently associated with primary fat cell cultures. In addition, three-dimensional (3D) cell culture systems are needed to better understand the role of brown adipocytes in energy balance and treating obesity. To address this need, we created 3D "Brown-Fat-in-Microstrands" by microfluidic synthesis of alginate hydrogel microstrands that encapsulated cells and directly induced cell differentiation into brown adipocytes, using mouse embryonic stem cells (ESCs) as a model of pluripotent stem cells, and brown preadipocytes as a positive control. Brown adipocyte differentiation within microstrands was confirmed by immunocytochemistry and qPCR analysis of the expression of the brown adipocyte-defining marker uncoupling protein 1 (UCP1), as well as other general adipocyte markers. Cells within microstrands were responsive to a β-adrenergic agonist with an increase in gene expression of thermogenic UCP1, indicating that these "Brown-Fat-in-Microstrands" are functional. The ability to create "Brown-Fat-in-Microstrands" from pluripotent stem cells opens up a new arena to understanding brown adipogenesis and its implications in obesity and metabolic disorders.

Mammary stroma in development and carcinogenesis

Mammary glands of adult human females are secretory organs comprised of interdependent epithelial and mesenchymal cells. These cells constitute an assemblage of collecting ducts that end in terminal duct lobular units with hollow alveolar ductules that can differentiate to produce and expel milk. Systemic and maternal hormones, autocrine and paracrine growth factors, and cytokines regulate virtually all phases of mammary gland development. During organogenesis, epithelial and mesenchymal cells interact to form precursors of the parenchyma and stroma in the mature gland. Organogenesis precedes five stages of postnatal development: puberty, pregnancy, lactation, involution, and menopause. Each stage requires a specific set of morphogenetic changes in glandular structure and function. Cycles of cell proliferation, differentiation, and involution may recur until menopause. In addition, physiological responses such as inflammation and pathological events such as tumorigenesis are remarkable for their similarities to embryonic morphogenesis. Here we take a succinct look at the ever-improving understanding of stroma-epithelial interactions and mesenchyme function in mammary gland biology.

Diverse and active roles for adipocytes during mammary gland growth and function

The mammary gland is unique in its requirement to develop in close association with a depot of adipose tissue that is commonly referred to as the mammary fat pad. As discussed throughout this issue, the mammary fat pad represents a complex stromal microenvironment that includes a variety of cell types. In this article we focus on adipocytes as local regulators of epithelial cell growth and their function during lactation. Several important considerations arise from such a discussion. There is a clear and close interrelationship between different stromal tissue types within the mammary fat pad and its adipocytes. Furthermore, these relationships are both stage- and species-dependent, although many questions remain unanswered regarding their roles in these different states. Several lines of evidence also suggest that adipocytes within the mammary fat pad may function differently from those in other fat depots. Finally, past and future technologies present a variety of opportunities to model these complexities in order to more precisely delineate the many potential functions of adipocytes within the mammary glands. A thorough understanding of the role for this cell type in the mammary glands could present numerous opportunities to modify both breast cancer risk and lactation performance.

Adipose stroma induces branching morphogenesis of engineered epithelial tubules

The mammary gland and other treelike organs develop their characteristic fractal geometries through branching morphogenesis, a process in which the epithelium bifurcates and invades into the surrounding stroma. Controlling the pattern of branching is critical for engineering these organs. In vivo, the branching process is instructed by stromal-epithelial interactions and adipocytes form the largest component of the fatty stroma that surrounds the mammary epithelium. Here, we used microlithographic approaches to engineer a three-dimensional culture model that enables analysis of the effect of adipocytes on the pattern of branching morphogenesis of mammary epithelial cells. We found that adipocyte-rich stroma induces branching through paracrine signals, including hepatocyte growth factor, but does not affect the branching pattern per se. This tissue engineering approach can be expanded to other organs, and should enable piecemeal analysis of the cellular populations that control patterning during normal development.

Unexpected deposition of brown fat in mammary gland during postnatal development

Mammary fat tissue is crucial for mammary ductal morphogenesis in both fetal and adult mice. There are two kinds of adipocytes, the energy-storing white and the energy-dissipating brown adipocyte. The precise identity of the types of adipocyte in the mammary gland has never been investigated but was always assumed to be only white fat. In this study, we show that both white and brown adipocytes are present in the postnatal mammary gland. The amount of brown adipose tissue (BAT) examined by histology and electron microscopy correlates with the transcript levels of uncoupling protein 1, which is a mitochondrial carrier expressed exclusively in BAT. Uncoupling protein 1 mRNAs are the highest during prepuberty, decrease upon puberty, and are finally undetectable in the adult mammary gland. The analysis of a BAT-depleted mouse model showed that depletion of mammary BAT in early postnatal development induces epithelial differentiation. Alveolar structures were formed along all ducts and were functional since they produced beta-casein. However, mammary transplantation experiments indicated that a systemic effect was responsible for epithelium differentiation. Our data suggest that BAT negatively regulates the differentiation of mammary epithelial cells in a systemic manner during prepubertal ductal outgrowth.

Functional microarray analysis of mammary organogenesis reveals a developmental role in adaptive thermogenesis

The use of DNA microarrays to study vertebrate organogenesis presents unique analytical challenges compared with expression profiling of homogeneous cell populations. We have used a general approach that permits the automated, unbiased identification of biologically relevant patterns of gene expression to study murine mammary gland development. Our studies confirm the utility of this approach by demonstrating the ready identification of cellular processes and pathways of known functional importance in mammary development. Additionally, this approach permitted the identification of genetic pathways with unpredicted patterns of developmental regulation, including those involved in angiogenesis, extracellular matrix synthesis, and the beta-oxidation of fatty acids. Surprisingly, our findings demonstrate that the coordinate regulation of genes involved in the beta-oxidation of fatty acids reflects the presence of an abundant, yet previously unrecognized stromal compartment within the mammary gland that is composed of brown adipose tissue. Our data demonstrate that the amount of brown adipose tissue present in the mammary gland is developmentally regulated; that PPARalpha, Ucp1, and genes involved in fatty acid oxidation are spatially and temporally coregulated during development; that the mammary gland plays a functional role in adaptive thermogenesis; and that the transcriptional control of this adaptive response to cold is itself developmentally regulated.

The mammary fat pad

The mammary fat pad is essential for development of the mammary epithelium, providing signals that mediate ductal morphogenesis and, probably, alveolar differentiation. The "cleared" fat pad is often used as a transplantation site. Considering the crucial role of the fat pad, its properties have received relatively little attention from researchers in the field. Some of the questions whose investigation is pertinent to understanding both normal mammary development and carcinogenesis are outlined in this commentary in the spirit of stimulating enquiry into this important subject. It is clear from a brief perusal of the available literature that until studies are specifically designed to clearly differentiate between functional effects of the fibrous and the adipose stroma, more substantive information about their differential effects on mammary development and tumorigenesis will not be forthcoming.

Adenosine-mediated inhibition of casein production by mouse mammary glands in culture

The present study was carried out to examine whether activation of adenosine receptors by adenosine analogues will affect casein production by mouse mammary epithelial cells. The morphogenesis and functions of epithelial tissue in the mammary gland are influenced by their surrounding adipocytes. Adipocytes are known to release adenosine into the extracellular fluid which can modulate cyclic-AMP levels in surrounding cells through binding to their adenosine receptors. To examine a possible paracrine effect of adenosine, the modulation of casein production in mammary explant culture and mammary epithelial cell (MEC) culture by adenosine receptor agonists has been investigated. We have observed that activation of the A1-adenosine receptor subtype in mammary tissue by an adenosine analogue (-)N6-(R-phenyl-isopropyl)-adenosine (PIA) raised cAMP levels. PIA and another adenosine receptor agonist, isobutylmethylxanthine (IBMX), inhibited casein accumulation both in explants and in MEC cultures in the presence of lactogenic hormones, which suggests that PIA or adenosine can act directly on the epithelial cells. This inhibition does not appear to be caused by elevation of cAMP levels or phosphodiesterase activity. The inhibition of intracellular casein accumulation by PIA and IBMX in explant cultures can be reversed via treatment of pertussis toxin which is known to ADP-ribosylate GTP-binding G alpha i-proteins, indicating that a Gi-protein-dependent pathway may be involved in this inhibition. The results also suggest that local accumulation of adenosine in the extracellular fluids of mammary glands is likely to inhibit the lactogenic response of mammary epithelial cells.

Role of mesenchymal-epithelial interactions in normal and abnormal development of the mammary gland and prostate

Development of the mammary gland (MG) and prostate occurs via mesenchymal-epithelia interactions. Epithelial MG buds are induced in ventral epidermis by mammary mesenchyme, which ultimately specifies the functional expression of the ability to produce milk. Mammary ductal branching is induced by embryonic mammary mesenchyme and is promoted by the mammary fat pad postnatally. These influences of connective tissue on the differentiation of mammary epithelium (ME) begin prenatally, but in adulthood, the connective tissue environment of adult ME profoundly influences epithelial growth, ductal branching, epithelial differentiation, and the ability of adult ME to produce milk. In a similar fashion, prostatic development occurs via mesenchymal-epithelial interactions in which urogenital sinus mesenchyme (UGM) induces epithelial morphogenesis, regulates epithelial proliferation, and evokes the expression of epithelial androgen receptors and prostate-specific secretory proteins. Although prostatic development is induced by androgens, androgenic effects on epithelial development are elicited via androgen receptors of UGM. As in MG, mesenchymal-epithelial interactions in the prostate begin during fetal periods, but continue into adulthood. The responsiveness of adult epithelial cells from various glands to stroma raises the possibility that carcinomas also may be regulated by connective tissue. Indeed, UGM can induce a rat prostatic carcinoma (Dunning tumor) to undergo striking changes in differentiation, which are accompanied by a reduction in growth rate and an apparent loss of tumorigenesis. Although the mechanism of mesenchymal-epithelial interactions remains unknown, the communication between the epithelium and stroma undoubtedly is multifactorial, involving the extracellular matrix, soluble growth or differentiation, and angiogenesis.

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.