FIGURE 1
At birth the female mouse mammary gland parenchyma consists of a single primary main lactiferous duct that branches into three to five secondary ducts. From the second to the fifth weeks of life, during the "ductal stage", the mammary tree grows by continuous branching and sprouting of new, ending in club shaped terminal end buds (TEBs) (Figure 1a). The "alveolar bud stage" is characterized by budding of "lateral buds" that end in terminal ducts (TDs), or cleavage of TEBs into two smaller buds, the "alveolar buds (ABs)". (Figures 1a,c). The ABs sprout new buds, referred to as "ductules" that cluster around the duct, forming a primitive lobule type 1(Lob 1) signaling the beginning of the "lobular stage". The primitive Lob 1 is composed of 6.26+4.10 ductules per cross section (Figures 1a,d) and with increasing stimuli form more complex lobular structures such as the lobules type 2 (Lob 2) and type 3 (Lob 3). Lob 2 contain 22-32 ductules per cross section (mean 26.64+5.22), (Figures 1a,e). Lob 3 contain 53-90 ductules per cross section (mean 69.50+ 13.59) (Figures 1,f). All the lobular ductules are associated with a duct, the intralobular terminal duct. As the ductules present in Lob 2 and Lob 3 start accumulating secretory material within the lumen they are called acini or alveoli. The mouse mammary lobular structures are surrounded mainly by fat and very small amount of connective tissue (Figures b-f) (drawings and images provided by Dr. Jose Russo). Go to smaller image.