Development of the Mammary Gland
- a Whole Mount Analysis -

Introduction

Unlike most mammalian organs, the mammary gland undergoes the majority of its development in the adult organism. Such a characteristic provides a fertile ground for direct in vivo studies and analyses. This distinction also allows scientists to avoid some of the technical difficulties encountered during developmental studies with fragile fetal specimens. The mammary gland consists of two main components: the parenchyma and the surrounding stroma. The parenchyma refers to the system of branching ducts and alveoli within the gland. The branching ducts are connected to the nipple by a single primary duct . The structure of the mammary ducts embody two main cell types, inner mammary epithelial ductal cells and an outer layer of myoepithelial cells. The adipose stroma within the gland provides a frame of support as well as a substrate within which the parenchyma can grow and function.

The proliferation and differentiation of the mammary gland involves an ample variety of hormones and growth factors such as estrogen, progesterone, and prolactin. The functional and structural development of the gland itself can be divided into seven stages: embryonic, postnatal, juvenile, puberty, pregnancy, lactation, and involution. These categories are based on the mouse model.

Embryonic: The earliest indication of mammary gland formation can be detected in embryos as early as 10-11 days of age. Around day 12 a small epithelial bud becomes visible which slowly increases in size until day 15.5. By day 16 a primary sprout starts to grow into the fat pad. Several mammary ducts are present at birth. During the embryonic period two forms of mesenchymes can be noted. The first is a dense mammary mesenchyme which surrounds the epidermal bud and consists of several layers of fibroblasts directly attached to the epithelium. The second is a tissue designated as the future mammary fat pad. This feature is required for normal epithelial morphogenesis . Both the mesenchyme and fat pad support the development of the mammary epithelium into ductal and lobular units unique to the mammary gland.

Postnatal: During the first three weeks of life, the mammary ducts elongate and branch slowly. The gland displays isometric growth which keeps pace with, but does not exceed the rate at which the animal is growing. (see newborn image 1.3)

Juvenile: Starting at about the fourth week, growth of the mammary ducts increases significantly. The gland's growth rate now exceeds the isometric rate it followed previously. The system of ducts begins to elongate around the lymph node. Terminal end buds, highly proliferative and active structures found at the tips of the ductal branches, expand greatly. These end buds are influenced by systemic steroid hormones and aid the ducts in linear growth as well as the regulation of branching patterns. Actually, the ductal pattern is created by the penetration of these terminal end buds through the stromal fat pad . The terminal end bud is composed of two distinct cell types. The first are cap cells which make up the outermost layer of the end bud. Cap cells interact with the surrounding stroma through a thin basal lamina. The second are body cells (about 6-10 layers) which fill the interior of the end bud. It is during this early developmental period (4-7 weeks of age) when the terminal end buds are most prominent. Also, a tremendous amount of apoptosis is observed in the body cells of the end buds during this stage. It has been demonstrated that apoptosis is an important mechanism in ductal morphogenesis.

Puberty: During puberty the ductal system proliferates within the adipose stroma. Mitotic activity remains very high until the ducts have reached the periphery of the mammary fat pad. At this point the terminal end buds form terminal ductal structures with low mitotic activity. Ductal development decreases with the acquirement of sexual maturity. Estrogen, along with several other hormones, plays a critical role during this period by enhancing development of the ductal system and the stroma. (see 6 and 10 week images 1.5 and 1.6)

Pregnancy: About 50% of the total growth of the mammary gland occurs during pregnancy from day 12 until term. Thus, during this period extensive and rapid proliferation occurs in the form of branching of the mammary ducts. The mammary epithelium expands vastly as well and in such a way that it fills in the stroma between the ducts. During pregnancy the mammary gland comes under the influences of estrogen, progesterone, and other placental hormones. Pregnancy is usually 18-21 days in the mouse. (see pregnancy images 1.7 and 1.8)

Lactation: Lactation involves the manufacturing and secretion of milk. The initiation of lactation appears to be induced by a decrease in estrogen and progesterone. About 20% of total mammary growth occurs during the first 14 days of lactation. Mammary epithelial proliferation continues into early lactation. Several hormones are also involved in the maintenance of lactation such as prolactin, insulin, and glucocorticoid.

Involution: When lactation ceases after weaning, the mammary gland involutes. The process of involution involves the suspension of milk protein gene expression, collapse of the mammary alveolar structures and removal of the secretory epithelial cells through programmed cell death and phagocytosis. There is also degradation of the basement membrane and replacement of most of the epithelial cells by adipose tissue. Mammary involution comprises of two distinct phases. The first is apoptosis among the secretory epithelial cells. The second phase is characterized by the degradation of the lobular alveolar structures and the mammary basement membrane and extracellular matrix. The pressure of apoptosis induced signals and the loss of survival factors may have significant control over mammary gland involution.
And with each pregnancy, the above patterns repeat themselves as described.

It is animal models such as the mouse which enable us to study the functions and developmental characteristics of the mammary gland to such amazing depth. There are profound similarities between the functions of the mammary gland in humans and the mouse model. Mammary cancer in rodents show a pathogenesis very much similar to that in humans. The mouse is indeed an ideal research model. Mice are relatively easy to feed and raise. They have a short gestation term and a long reproductive period during which they can produce over 10 litters and 100 offspring. Thanks to such models, we are able to make great strides towards the future. Breast cancer has seized the lives of countless women and continues to threaten and devastate the lives of so many more. Presently, this disease still runs rampant in our societies. With animal models such as the mouse, we as scientists have an extremely valuable tool with which to aid in the fight against breast cancer.

Here at the National Institute of Health's Section of Mammary Gland Biology, we are dedicated and focused on obtaining and providing a greater understanding of the workings of the mammary gland. Our scientists are currently working on different areas of mammary biology. However, there are other laboratories at the NIH as well as in the surrounding vicinity which are focusing on the biology of the mammary gland as well. We all need to correspond with one another and keep an open line of communication. This Web site was established in 1995 in order to promote such interactions. We want to encourage discussions and provide a channel for exchanging ideas. There are indeed many different tracts and aspects of mammary gland biology. We are hoping this Web site can serve as a type of intersection where the different areas of mammary gland biology can come together to share and converse.

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