Abstract
IntroductionThe mouse mammary gland provides a powerful model system for studying processes involved in epithelial tissue development. Although markers that enrich for mammary stem cells and progenitors have been identified, our understanding of the mammary developmental hierarchy remains incomplete.MethodsWe used the MMTV promoter linked to the reverse tetracycline transactivator to induce H2BGFP expression in the mouse mammary gland. Mammary epithelial cells (MECs) from virgin mice were sorted by flow cytometry for expression of the mammary stem cell/progenitor markers CD24 and CD29, and H2BGFP. Sorted populations were analyzed for in vivo repopulation ability, expression of mammary lineage markers, and differential gene expression.ResultsThe reconstituting activity of CD24+/CD29+ cells in cleared fat pad transplantation assays was not distinguished in GFP+ compared to GFP- subpopulations. However, within the CD24+/CD29lo luminal progenitor-enriched population, H2BGFP+, but not H2BGFP-, MECs formed mammary structures in transplantation assays; moreover, this activity was dramatically enhanced in pregnant recipients. These outgrowths contained luminal and myoepithelial mammary lineages and produced milk, but lacked the capacity for serial transplantation. Transcriptional microarray analysis revealed that H2BGFP+/CD24+/CD29lo MECs are distinct from H2BGFP-/CD24+/CD29lo MECs and enriched for gene expression signatures with both the stem cell (CD24+/CD29+) and luminal progenitor (CD24+/CD29lo/CD61+) compartments.ConclusionsWe have identified a population of MECs containing pregnancy-activated multipotent progenitors that are present in the virgin mammary gland and contribute to the expansion of the mammary gland during pregnancy.
Highlights
The mouse mammary gland provides a powerful model system for studying processes involved in epithelial tissue development
Mammary glands were harvested from double transgenic (MMTVrtTA/histone 2b fused to eGFP (H2BGFP)) virgin females at four weeks post-partum and analyzed by flow cytometry and immunostaining
In contrast to a previous report that this MMTVrtTA transgene drives ubiquitous expression in mammary epithelial cell (MEC) [21], H2BGFP expression was detected in only a fraction (18 ± 3.1%) of MECs from double transgenic mice (Figure 1a; see Discussion)
Summary
The mouse mammary gland provides a powerful model system for studying processes involved in epithelial tissue development. Pregnancy features dramatic proliferation and alveolar differentiation, which are critical for lactation These processes are governed by different signals and hormones, and could involve the activity of different cell types [2,3]. The Visvader and Eaves laboratories reported that subpopulations of mammary epithelial cells (MECs) within the CD24+/CD29+ or CD24+/CD49f+ compartments are capable of forming a fully functional mammary gland that can be serially transplanted [4,5]. These groundbreaking studies were followed by the discovery of additional markers for the purification and characterization of several MEC subtypes [6,7,8,9]
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