Breast development gives insights into breast disease

Developing ovine mammary terminal duct lobular units have a dynamic mucosal and stromal immune microenvironment

The human breast and ovine mammary gland undergo striking levels of postnatal development, leading to formation of terminal duct lobular units (TDLUs). Here we interrogate aspects of sheep TDLU growth as a model of breast development and to increase understanding of ovine mammogenesis. The distributions of epithelial nuclear Ki67 positivity differ significantly between younger and older lambs. Ki67 expression is polarised to the leading edge of the developing TDLUs. Intraepithelial ductal macrophages exhibit periodicity and considerably increased density in lambs approaching puberty. Stromal macrophages are more abundant centrally than peripherally. Intraepithelial T lymphocytes are more numerous in older lambs. Stromal hotspots of Ki67 expression colocalize with immune cell aggregates that exhibit distinct organisation consistent with tertiary lymphoid structures. The lamb mammary gland thus exhibits a dynamic mucosal and stromal immune microenvironment and constitutes a valuable model system that provides new insights into postnatal breast development.

Development of the human breast

Mammalia are so named based on the presence of the mammary gland in the breast. The mammary gland is an epidermal appendage, derived from the apocrine glands. The human breast consists of the parenchyma and stroma, originating from ectodermal and mesodermal elements, respectively. Development of the human breast is distinctive for several reasons. The human breast houses the mammary gland that produces and delivers milk through development of an extensive tree-like network of branched ducts. It is also characterized by cellular plasticity, with extensive remodeling in adulthood, a factor that increases its susceptibility to carcinogenesis. Also, breast development occurs in distinct stages via complex epithelial-mesenchymal interactions, orchestrated by signaling pathways under the regulation of systemic hormones. Congenital and acquired disorders of the breast often have a basis in development, making its study essential to understanding breast pathology.

Human breast development

This review is intended to give an overview of current knowledge on human breast development. It focuses on the limitations of our understanding on the origins of human breast cancer in the context of this mainly morphological and static assessment of what is known about human breast development. The world literature is very limited and caution is needed in drawing analogies with the mouse. There is an increasing emphasis on research to understand normal stem cells in the breast on the assumption that these are the targets for the initiation of breast cancer. It is thus a priority to understand normal human breast development, but there are major obstacles to such studies mainly due to ethical considerations and to tissue acquisition.

Mena deficiency delays tumor progression and decreases metastasis in polyoma middle-T transgenic mouse mammary tumors

Introduction

The actin binding protein Mammalian enabled (Mena), has been implicated in the metastatic progression of solid tumors in humans. Mena expression level in primary tumors is correlated with metastasis in breast, cervical, colorectal and pancreatic cancers. Cells expressing high Mena levels are part of the tumor microenvironment for metastasis (TMEM), an anatomical structure that is predictive for risk of breast cancer metastasis. Previously we have shown that forced expression of Mena adenocarcinoma cells enhances invasion and metastasis in xenograft mice. Whether Mena is required for tumor progression is still unknown. Here we report the effects of Mena deficiency on tumor progression, metastasis and on normal mammary gland development.

Methods

To investigate the role of Mena in tumor progression and metastasis, Mena deficient mice were intercrossed with mice carrying a transgene expressing the polyoma middle T oncoprotein, driven by the mouse mammary tumor virus. The progeny were investigated for the effects of Mena deficiency on tumor progression via staging of primary mammary tumors and by evaluation of morbidity. Stages of metastatic progression were investigated using an in vivo invasion assay, intravital multiphoton microscopy, circulating tumor cell burden, and lung metastases. Mammary gland development was studied in whole mount mammary glands of wild type and Mena deficient mice.

Results

Mena deficiency decreased morbidity and metastatic dissemination. Loss of Mena increased mammary tumor latency but had no affect on mammary tumor burden or histologic progression to carcinoma. Elimination of Mena also significantly decreased epidermal growth factor (EGF) induced in vivo invasion, in vivo motility, intravasation and metastasis. Non-tumor bearing mice deficient for Mena also showed defects in mammary gland terminal end bud formation and branching.

Conclusions

Deficiency of Mena decreases metastasis by slowing tumor progression and reducing tumor cell invasion and intravasation. Mena deficiency during development causes defects in invasive processes involved in mammary gland development. These findings suggest that functional intervention targeting Mena in breast cancer patients may provide a valuable treatment option to delay tumor progression and decrease invasion and metastatic spread leading to an improved prognostic outcome.

The role of the microenvironment in mammary gland development and cancer

The mammary gland is composed of a diverse array of cell types that form intricate interaction networks essential for its normal development and physiologic function. Abnormalities in these interactions play an important role throughout different stages of tumorigenesis. Branching ducts and alveoli are lined by an inner layer of secretory luminal epithelial cells that produce milk during lactation and are surrounded by contractile myoepithelial cells and basement membrane. The surrounding stroma comprised of extracellular matrix and various cell types including fibroblasts, endothelial cells, and infiltrating leukocytes not only provides a scaffold for the organ, but also regulates mammary epithelial cell function via paracrine, physical, and hormonal interactions. With rare exceptions breast tumors initiate in the epithelial compartment and in their initial phases are confined to the ducts but this barrier brakes down with invasive progression because of a combination of signals emitted by tumor epithelial and various stromal cells. In this article, we overview the importance of cellular interactions and microenvironmental signals in mammary gland development and cancer.

Quantitative analysis of three-dimensional human mammary epithelial tissue architecture reveals a role for tenascin-C in regulating c-met function

Remodeling of the stromal extracellular matrix and elevated expression of specific proto-oncogenes within the adjacent epithelium represent cardinal features of breast cancer, yet how these events become integrated is not fully understood. To address this question, we focused on tenascin-C (TN-C), a stromal extracellular matrix glycoprotein whose expression increases with disease severity. Initially, nonmalignant human mammary epithelial cells (MCF-10A) were cultured within a reconstituted basement membrane (BM) where they formed three-dimensional (3-D) polarized, growth-attenuated, multicellular acini, enveloped by a continuous endogenous BM. In the presence of TN-C, however, acini failed to generate a normal BM, and net epithelial cell proliferation increased. To quantify how TN-C alters 3-D tissue architecture and function, we developed a computational image analysis algorithm, which showed that although TN-C disrupted acinar surface structure, it had no effect on their volume. Thus, TN-C promoted epithelial cell proliferation leading to luminal filling, a process that we hypothesized involved c-met, a proto-oncogene amplified in breast tumors that promotes intraluminal filling. Indeed, TN-C increased epithelial c-met expression and promoted luminal filling, whereas blockade of c-met function reversed this phenotype, resulting in normal BM deposition, proper lumen formation, and decreased cell proliferation. Collectively, these studies, combining a novel quantitative image analysis tool with 3-D organotypic cultures, demonstrate that stromal changes associated with breast cancer can control proto-oncogene function.

Oncogenic pathways in hereditary and sporadic breast cancer

Cancer is a genetic disease. Breast cancer tumorigenesis can be described as a multi-step process in which each step is thought to correlate with one or more distinct mutations in major regulatory genes. The question addressed is how far a multi-step progression model for sporadic breast cancer would differ from that for hereditary breast cancer. Hereditary breast cancer is characterized by an inherited susceptibility to breast cancer on basis of an identified germline mutation in one allele of a high penetrance susceptibility gene (such as BRCA1, BRCA2, CHEK 2, TP53 or PTEN). Inactivation of the second allele of these tumour suppressor genes would be an early event in this oncogenic pathway (Knudson's "two-hit" model). Sporadic breast cancers result from a serial stepwise accumulation of acquired and uncorrected mutations in somatic genes, without any germline mutation playing a role. Mutational activation of oncogenes, often coupled with non-mutational inactivation of tumour suppressor genes, is probably an early event in sporadic tumours, followed by more, independent mutations in at least four or five other genes, the chronological order of which is likely less important. Oncogenes that have been reported to play an early role in sporadic breast cancer are MYC, CCND1 (Cyclin D1) and ERBB2 (HER2/neu). In sporadic breast cancer, mutational inactivation of BRCA1/2 is rare, as inactivation requires both gene copies to be mutated or totally deleted. However, non-mutational functional suppression could result from various mechanisms, such as hypermethylation of the BRCA1 promoter or binding of BRCA2 by EMSY. In sporadic breast tumorigenesis, at least three different pathway-specific mechanisms of tumour progression are recognizable, with breast carcinogenesis being different in ductal versus lobular carcinoma, and in well differentiated versus poorly differentiated ductal cancers. Thus, different breast cancer pathways emerge early in the process of carcinogenesis, ultimately leading to clinically different tumour types. As mutations acquired early during tumorigenesis will be present in all later stages, large-scale gene expression profiling using DNA microarray analysis techniques can help to classify breast cancers into clinically relevant subtypes.

The origin of vimentin expression in invasive breast cancer: epithelial-mesenchymal transition, myoepithelial histogenesis or histogenesis from progenitor cells with bilinear differentiation potential?

Vimentin expression is a rather rare finding in invasive breast cancer, and is associated with high tumour invasiveness and chemoresistance. It is currently explained by two different biological theories: direct histogenetic derivation from myoepithelial cells, and epithelial-mesenchymal transition (EMT) reflecting the end-stage of breast cancer dedifferentiation. In this study we aimed to obtain further insights into the biological hallmarks of these vimentin-expressing breast cancers. We applied immunohistochemistry for vimentin and 15 other differentiation markers to a series of 364 invasive breast cancer cases, using tissue microarray technology. 7.7% of all tumours expressed vimentin. Almost all of these cases (19/21) were Grade 3 invasive ductal carcinomas, and the majority (13/21) of these were associated with a ductal in situ component. Vimentin expression was also seen in the respective in situ components and correlated positively with the expression of SMA, CD10, CK 5, p53, Mib-1 and EGFR. A negative correlation was seen for the expression of CK 8/18 and the oestrogen receptor. Vimentin-expressing carcinomas revealed a significantly higher average absolute number of cytogenetic alterations per case, but a significantly lower frequency of chromosome 16q losses compared to vimentin-negative cases. Our present results demonstrate that, despite analogies between vimentin-positive breast cancers and myoepithelial cells in their expression of differentiation-related proteins, neither myoepithelial histogenesis nor EMT can exclusively explain the biology of these distinct tumours. This is mainly supported by the significantly higher incidence of vimentin-expressing breast cancers compared to any other myoepithelial breast tumours and the fact that vimentin is already observed in ductal in situ components. We therefore propose the alternative hypothesis that vimentin-expressing breast carcinomas may derive from breast progenitor cells with bilinear (glandular and myoepithelial) differentiation potential.

Do myoepithelial cells hold the key for breast tumor progression?

Mammary myoepithelial cells have been a neglected facet of breast cancer biology, largely ignored since they have been considered to be less important for tumorigenesis than luminal epithelial cells from which most of breast carcinomas are thought to arise. In recent years as our knowledge of stem cell biology and the cellular microenvironment has been increasing, myoepithelial cells are slowly starting to gain more attention. Emerging data raise the hypothesis whether myoepithelial cells play a key role in breast tumor progression by regulating the in situ to invasive carcinoma transition and that myoepithelial cells are part of the mammary stem cell niche. Paracrine interactions between myoepithelial and luminal epithelial cells are known to be important for regulation of cell cycle progression, establishing epithelial cell polarity, and inhibiting cell migration and invasion. Based on these functions, normal mammary myoepithelial cells have been called "natural tumor suppressors." However, during tumor progression myoepithelial cells seem to loose these properties, and eventually this cell population diminishes as tumors become invasive. Better understanding of myoepithelial cell function and their role in tumor progression may lead to their exploitation for cancer therapeutic and preventative measures.

Intrauterine breast development and the mammary myoepithelial lineage

As their name implies, the myoepithelial cells found at the epithelial-mesenchymal interface of the human mammary gland disclose features suggestive of a dual epithelial-like and muscle-like differentiation, i.e. they co-express various keratins and vimentin intermediate filaments, as well as smooth muscle-related antigens. This article provides an overview of the literature on intrauterine breast development with special emphasis on the myoepithelial component of the fetal human mammary gland epithelium. It discusses original and recently published immunohistochemical data on myoepithelial precursors and reasserts the relevance of developmental, morphological fetal tissue-based studies to the understanding and the clinical management of adult diseases.

Oncogenic pathways in hereditary and sporadic breast cancer

Cancer is a genetic disease. Breast cancer tumorigenesis can be described as a multi-step process in which each step is thought to correlate with one or more distinct mutations in major regulatory genes. The question addressed is how far a multi-step progression model for sporadic breast cancer would differ from that for hereditary breast cancer. Hereditary breast cancer is characterized by an inherited susceptibility to breast cancer on basis of an identified germline mutation in one allele of a high penetrance susceptibility gene (such as BRCA1, BRCA2, CHEK 2, TP53 or PTEN). Inactivation of the second allele of these tumour suppressor genes would be an early event in this oncogenic pathway (Knudson's "two-hit" model). Sporadic breast cancers result from a serial stepwise accumulation of acquired and uncorrected mutations in somatic genes, without any germline mutation playing a role. Mutational activation of oncogenes, often coupled with non-mutational inactivation of tumour suppressor genes, is probably an early event in sporadic tumours, followed by more, independent mutations in at least four or five other genes, the chronological order of which is likely less important. Oncogenes that have been reported to play an early role in sporadic breast cancer are MYC, CCND1 (Cyclin D1) and ERBB2 (HER2/neu). In sporadic breast cancer, mutational inactivation of BRCA1/2 is rare, as inactivation requires both gene copies to be mutated or totally deleted. However, non-mutational functional suppression could result from various mechanisms, such as hypermethylation of the BRCA1 promoter or binding of BRCA2 by EMSY. In sporadic breast tumorigenesis, at least three different pathway-specific mechanisms of tumour progression are recognizable, with breast carcinogenesis being different in ductal versus lobular carcinoma, and in well differentiated versus poorly differentiated ductal cancers. Thus, different breast cancer pathways emerge early in the process of carcinogenesis, ultimately leading to clinically different tumour types. As mutations acquired early during tumorigenesis will be present in all later stages, large-scale gene expression profiling using DNA microarray analysis techniques can help to classify breast cancers into clinically relevant subtypes.

Basal cells of second trimester fetal breasts: immunohistochemical study of myoepithelial precursors

The molecular characterization of human mammary myoepithelial cells is incomplete, hindering our understanding of its importance in breast physiology and pathology. Because data on the precursors of this cell lineage remain scarce and often contradictory, basal epithelial cells of second trimester fetal breasts were studied by light microscopy (LM) and immunohistochemistry (IHC). Up to 20 wk of gestational age, the mammary rudiments only comprised roundish primary outgrowths, "primary buds," more likely to represent immature nipples than true mammary tissue. At 21 wk secondary outgrowths, "projections," extended from enlarged primary buds into well-vascularized layers of dense mesenchyme. Basal projection cells had a partial myoepithelial-like phenotype: they reacted with CD29, CD49f, CD104, keratin 14, vimentin, S100beta protein, and p63; furthermore, many became positive for keratin 17, alpha-smooth muscle actin, and CD10 (but not for keratin 19) between wk 21 and 25. The continuous basement membrane associated with the fetal mammary rudiments was strongly positive for collagens type IV and VII, and for laminin 5. Consistently strong and basally polarized staining for hemidesmosomal components suggested that although incompletely differentiated, most second trimester myoepithelial precursors might already mediate local epithelial-mesenchymal interactions, i.e., complex signaling pathways which are crucial for both orderly growth during development and maintenance of homeostasis during adult life. Because they are likely implicated in the phenomenon of menstrual cycle-related growth spurts in the adult resting breast, the strategically positioned cells of the myoepithelial lineage might constitute critical protagonists in defective epithelial-mesenchymal signaling associated with cancer progression.

Establishing a framework for the functional mammary gland: from endocrinology to morphology

From its embryonic origins, the mammary gland in females undergoes a course of ductal development that supports the establishment of alveolar structures during pregnancy prior to the onset of lactogenesis. This development includes multiple stages of proliferation and morphogenesis that are largely directed by concurrent alterations in key hormones and growth factors across various reproductive states. Ductal elongation is directed by estrogen, growth hormone, insulin-like growth factor-I, and epidermal growth factor, whereas ductal branching and alveolar budding is influenced by additional factors such as progesterone, prolactin, and thyroid hormone. The response by the ductal epithelium to various hormones and growth factors is influenced by epithelial-stromal interactions that differ between species, possibly directing species-specific morphogenesis. Evolving technologies continue to provide the opportunity to further delineate the regulation of ductal development. Defining the hormonal control of ductal development should facilitate a better understanding of the mechanisms underlying mammary gland tumorigenesis.

Human breast development

This review presents an atlas of the histology of the normal physiological states of the human breast including prenatal, prepubertal, and pubertal development, adult resting gland, pregnancy, lactation, and postinvolution. The aim is to produce a pictorial overview of the main stages in development and the common findings in the adult that are considered to be within the range of normality. Unlike inbred strains of animals, in humans it is clear that the chronology of ductal and lobular development is not predictable, either in the fetus, the infant, the peripubertal breast, or the adult. This is probably due to the individual variation in hormone levels both in utero and after birth. For many of the developmental time points there are very little data available. In this review we indicate the current state of knowledge of human breast development and some of the main similarities and differences with the rodent, the main animal model. The major phases of growth and development are described and accompanied by photographs that are representative of each stage. Stress is placed on terminology as there is confusion in the literature. This article is written as an accessory to the companion review on breast cancer.