Mammary gland patterning in the AXB/BXA recombinant inbred strains of mouse

Prenatal Mammary Gland Development in the Mouse: Research Models and Techniques for Its Study from Past to Present

Mammary gland development starts during prenatal life, when at designated positions along the ventrolateral boundary of the embryonic or fetal trunk, surface ectodermal cells coalesce to form primordia for mammary glands, instead of differentiating into epidermis. With the wealth of genetically engineered mice available as research models, our understanding of the prenatal phase of mammary development has recently greatly advanced. This understanding includes the recognition of molecular and mechanistic parallels between prenatal and postnatal mammary morphogenesis and even tumorigenesis, much of which can moreover be extrapolated to human. This makes the murine embryonic mammary gland a useful model for a myriad of questions pertaining to normal and pathological breast development. Hence, unless indicated otherwise, this review describes embryonic mammary gland development in mouse only, and lists mouse models that have been examined for defects in embryonic mammary development. Techniques that originated in the field of developmental biology, such as explant culture and tissue recombination, were adapted specifically to research on the embryonic mammary gland. Detailed protocols for these techniques have recently been published elsewhere. This review describes how the development and adaptation of these techniques moved the field forward from insights on (comparative) morphogenesis of the embryonic mammary gland to the understanding of tissue and molecular interactions and their regulation of morphogenesis and functional development of the embryonic mammary gland. It is here furthermore illustrated how generic molecular biology and biochemistry techniques can be combined with these older, developmental biology techniques, to address relevant research questions. As such, this review should provide a solid starting point for those wishing to familiarize themselves with this fascinating and important subdomain of mammary gland biology, and guide them in designing a relevant research strategy.

In-silico QTL mapping of postpubertal mammary ductal development in the mouse uncovers potential human breast cancer risk loci

Genetic background plays a dominant role in mammary gland development and breast cancer (BrCa). Despite this, the role of genetics is only partially understood. This study used strain-dependent variation in an inbred mouse mapping panel, to identify quantitative trait loci (QTL) underlying structural variation in mammary ductal development, and determined if these QTL correlated with genomic intervals conferring BrCa susceptibility in humans. For about half of the traits, developmental variation among the complete set of strains in this study was greater (P < 0.05) than that of previously studied strains, or strains in current common use for mammary gland biology. Correlations were also detected with previously reported variation in mammary tumor latency and metastasis. In-silico genome-wide association identified 20 mammary development QTL (Mdq). Of these, five were syntenic with previously reported human BrCa loci. The most significant (P = 1 × 10(-11)) association of the study was on MMU6 and contained the genes Plxna4, Plxna4os1, and Chchd3. On MMU5, a QTL was detected (P = 8 × 10(-7)) that was syntenic to a human BrCa locus on h12q24.5 containing the genes Tbx3 and Tbx5. Intersection of linked SNP (r(2) > 0.8) with genomic and epigenomic features, and intersection of candidate genes with gene expression and survival data from human BrCa highlighted several for further study. These results support the conclusion that mammary tumorigenesis and normal ductal development are influenced by common genetic factors and that further studies of genetically diverse mice can improve our understanding of BrCa in humans.

Cooperation of signalling pathways in embryonic mammary gland development

Mammary glands become functional only in adult life but their development starts in the embryo. Initiation of the epithelial bud and ductal outgrowth are coordinated through short-range signals between epithelium and mesenchyme. Studies of natural and induced mouse mutants in which early mammary development is perturbed have identified genetic networks that regulate specific steps in these processes. Some of these signals contribute to aberrant mammary development in humans and are deregulated in cancer.

Signalling pathways implicated in early mammary gland morphogenesis and breast cancer

Specification of mammary epithelial cell fate occurs during embryogenesis as cells aggregate to form the mammary anlage. Within the embryonic mammary bud, a population of epithelial cells exists that will subsequently proliferate to form a ductal tree filling the stromal compartment, and which can produce milk upon terminal differentiation after birth. Subsequently, these structures can be remodelled and returned to a basal state after weaning before regenerating in future pregnancies. The plasticity of the mammary epithelial cell, and its responsiveness to hormone receptors, facilitates this amazing biological feat, but aberrant signalling may also result in unintended consequences in the form of frequent malignancies. Reflecting this intimate connection, a considerable number of signalling pathways have been implicated in both mammary gland morphogenesis and carcinogenesis.

Identification of the scaramanga gene implicates Neuregulin3 in mammary gland specification

The mouse scaramanga (ska) mutation impairs mammary gland development such that both abrogation and stimulation of gland formation occurs. We used positional cloning to narrow the interval containing scaramanga (ska) to a 75.6-kb interval containing the distal part of the Neuregulin3 (Nrg3) gene. Within this region the only sequence difference between ska and wild-type mice is in a microsatellite repeat within intron 7. This alteration correlates with variations in Nrg3 expression profiles both at the whole embryo level and locally in the presumptive mammary region in ska mice. Localized expression of Nrg3 and its receptor, Erbb4, in the presumptive mammary region around the future bud site prior to morphological appearance of buds and the expression of bud epithelial markers further support an inductive role. Finally, Neuregulin3 (Nrg3)-soaked beads can induce expression of the early bud marker Lef1 in mouse embryo explant cultures, and epithelial bud formation can be observed histologically, suggesting that initiation of mammary bud development occurs. Taken together, these results indicate that a Neuregulin signaling pathway is involved in specification of mammary gland morphogenesis and support the long-held view that mesenchymal signal(s) are responsible for mammary gland inductive/initiating events.

Identification of the mammary line in mouse by Wnt10b expression

Mammogenesis in rabbit, rat, and human begins with the formation of an elevated ectodermal ridge in the embryo. Attempts to demonstrate a morphologically or histologically equivalent mammary line in the mouse have yielded controversial results. We show here that a mammary line exists in the mouse embryo at embryonic day (E) 11.25 as a concise line of Wnt10b expression and a broader band of Wnt6 expression in the surface ectoderm, between the subaxillary and suprainguinal region of each flank. Additional streaks of Wnt10b expression in the axillary and inguinal region join the mammary line on the flank slightly later. Expression of Wnt10b and Wnt6 becomes restricted to the placodes within 1.5 days. The ectoderm of the mammary line is organized as a pseudostratified epithelium connecting the developing mammary placodes at around E11.5, whereas all other surface ectoderm is single-layered. Thus, the mammary line expressing Wnt10b defines a distinct ectodermal region that we consider the equivalent of the ectodermal ridge in, for example, rabbit. To date, the formation of the mammary line expressing Wnt10b is the earliest discernible ectodermal event in murine embryonic mammary gland development.

Mouse embryonic mammogenesis as a model for the molecular regulation of pattern formation

In this review we describe how mouse embryonic mammogenesis depends on a continuous communication between the epithelial and mesenchymal compartment of the mammary rudiment. Although the functions of only a few genes in the regulation of these epithelio-mesenchymal interactions during mouse mammary development are known so far, key roles are suggested for WNT, FGF and PTHrP signaling. However, the exact mechanism of action of these signaling pathways and their possible cross-talk in the induction of mammary development are not clear, nor does our current knowledge suffice to explain how the number and positions of the mammary rudiments are so well defined. Nonetheless, by the description of aberrant induction and/or maintenance of the mammary rudiments in a variety of inbred mouse strains and mutants, we have accumulated data demonstrating that the mammary rudiments develop independently of each other at these positions. In addition, each rudiment pair responds differently to altered levels of gene expression. This not only clarifies the unique identity of each placode, but the different molecular requirement of each placode also suggests that different molecular mechanisms may underlie the formation of such identical structures. For future investigations in the field, such a unique molecular identity of each mammary rudiment should be of critical concern.