Mammary development in the embryo and adult: a journey of morphogenesis and commitment

Christine J. Watson 0 Walid T. Khaled 0 0 Department of Pathology, University of Cambridge , Tennis Court Road, Cambridge CB2 1QP, UK Mammary gland development occurs through distinctive stages throughout embryonic and pubertal development and reproductive life. At each stage, different signals are required to induce changes in both the epithelium and the surrounding mesenchyme/stroma. Recent studies have provided new insights into the origin, specification and fate of mammary stem and progenitor cells and into how the differentiated lineages that comprise the functional mammary gland are determined. The development of new tools and culture techniques has also enabled the factors that influence branching morphogenesis in the embryonic and pubertal gland to be identified. A surprising recent discovery has been that mammary epithelial cells commit to differentiated lineages using the same signalling pathways that regulate lineage determination in T helper cells. - Introduction Mammary glands are epidermal appendages that possibly evolved from ancient apocrine glands that were associated with the skin (Oftedal, 2002). The primary function of the mammary gland is to provide nutrition for the young in the form of milk protein and fat. However, there are other benefits that are provided by lactation, such as the provision of immune factors that are secreted into the milk, which provide protection from infection, and also the close contact that occurs between mother and infant during nursing, which might have developmental benefits (Peaker et al., 2002). The mammary gland is a complex secretory organ that consists of a number of different cell types: epithelial cells that form the ductal network of the gland; adipocytes, which constitute the fat pad and in which the ductal network is embedded; vascular endothelial cells, which make up the blood vessels; stromal cells, including fibroblasts; and a variety of immune cells. There are two main types of epithelium in the mammary gland: luminal and basal. The luminal epithelium forms the ducts and the secretory alveoli, whereas the basal epithelium consists essentially of myoepithelial cells. These two types of epithelium form a bi-layered structure of simple epithelium that is embedded within the fatty stroma. There are three main stages of mammary gland development both in rodents and humans: embryonic, pubertal and adult. Hormones and growth factors play a role in these different stages of mammary development and are also implicated in breast cancer. The mammary gland is an ideal tissue in which to study a range of developmental processes, as discussed below. In the embryo, the signals that induce the formation of mammary placodes from the skin are beginning to be elucidated. Similar processes are involved in the formation of other appendages, such as teeth and feathers (Wu et al., 2004). After birth, mammary development is arrested until puberty, when extensive elongation of the ducts, accompanied by secondary branching, takes place, thus providing a readily accessible system in which to study branching morphogenesis. The hallmarks of development during pregnancy are the formation of tertiary branches, which terminate in alveolar buds, and the rapid proliferation of the luminal epithelium accompanied by differentiation and commitment to the secretory alveolar lineage. A lactogenic switch occurs during late pregnancy that is accompanied by the expression of the milk proteins, whey acidic protein (WAP) and -lactalbumin, and by the formation of lipid droplets. Finally, following lactation, removal of the now surplus alveolar cells is accomplished by cell death (apoptosis). Post-lactational regression, or involution, is the most dramatic example of physiologically regulated apoptosis in an adult tissue. In a tightly coordinated series of events, ~80% of the epithelium is removed within a few days. The mouse mammary gland provides, therefore, a model that can be genetically manipulated to provide insights into a variety of normal developmental processes. Mouse models have also been used extensively to study the development of breast cancer. In this review, we discuss recent studies in mice on the morphogenesis and lineage commitment events that occur during all three stages of mouse mammary gland development. Important new insights have been obtained from these studies, including the unanticipated involvement of signalling pathways previously associated with T lymphocyte lineage decisions, in mammary epithelial lineage choice. Tools have been developed that allow the enrichment of mammary stem cells from the adult gland, and it can be only a matter of time before we can prospectively identify mammary stem cells and the factors required for their self renewal. Importantly, this will allow the hierarchy of progenitors and their inter-relationship to be determined. This will be a major step forward, not just for developmental biology, but also for breast cancer research. The ability to genetically modify the mouse has made it the model of choice and this review therefore focuses on studies in the mouse. Although there are some differences in the architecture and hormonal control of mammary glands between mice and other rodents and between mice and humans, similar developmental processes are shared between them. Embryonic mammary gland development Mammary development is not evident in the mouse until midgestation. The first distinct feature is the formation of the milk lines from overlying ectoderm (as discussed in more detail below), followed by the formation of five pairs of placodes that invaginate to form buds (see Fig. 1A,B). These induce the formation of the mammary mesenchyme. The buds then sprout and branch to form a rudimentary structure that has approximately five ductules that embed in the subdermal fat pad (see Fig. 1C). Development is arrested from embryonic day (E) 18 until puberty. Mammary development in the male differs between mouse and man with regression of the rudimentary tissue in mice being induced in response to androgens, whereas human males retain a connection to the nipple. Fig. 1. Embryonic mammary gland development. (A,B) Diagram of an E10.5 mouse embryo (A) showing the position of the milk line (dashed line between limbs), and of an E12.5 mouse embryo (B) showing the positions of the five pairs of mammary placodes, which become mammary buds (MB1-5) along the anteroposterior axis (MB1 and MB5 are hidden by the limb buds and only one flank is shown). (C) Overview of mouse embryonic mammary gland development. Placodes, which are visible at E11.5, transform into bulbs of epithelial cells, which sink into the underlying mesenchyme at E13.5 to become the mammary buds. The mesenchymal cells (orange) that surround the buds condense to become the mammary mesenchyme (grey). By E15.5, these buds elongate to form sprouts, which develop a lumen with an opening to the skin, marked by the formation of the nipple s (...truncated)