The mammary glands form from a thickening of the epidermis called the milk lines. The milk lines develop in all mammals and extend from the axilla to the groin. Later in development, milk buds form as discrete areas epidermal buds in most mammals. In the mouse, ten nipples and ten glands form, five on each side. The number 1 fat pad and gland is in the neck region, adjacent to the salivary glands. Tumors of the salivary glands are frequently mistaken for mammary tumors and visa versa. The number 2 and 3 mammary glands are on the chest wall and overlap but are divided by the pectoralis muscle of the chest . The number 4 gland is on the abdomen and is the easiest to identify and dissect. Since the number 4 gland and fat pad is the most accessible, it is most frequently used for experimental transplantation. The number 5 gland is in the groin region.
The mammary gland of the mouse originates at the milk bud, forming the primitive nipple that extend into the mammary fat pad as a series of branched ducts. The mammary fat pads form as local areas of adipose tissue between the subcutaneous tissue and the body wall. They are ensheathed between two fascial plains. The growth rate is slow in the first three postpartum weeks, not reaching the lymph node of the number 4 fat pad. Thereafter, the female gland is subject to the growth stimulus of puberty and grows at a more rapid rate to fill the mammary fat pad between 9 and 12 weeks. The growth is almost exclusively at the terminal end bud that has a cap of undifferentiated "stem" cells.
The mammary gland of the virginal female forms a series of ducts with a limited number of branches. Further, the ducts in wild type mice without the Mouse Mammary Tumor Virus (MMTV) do not form side buds or tertiary branches until stimulated by hormones during pregnancy. With pregnancy, the gland begins forming side buds which either branch as tertiary ducts or form alveoli. The process results in the progressive acquisition of alveoli culminating in the filling of the entire fat pad with alveolar structures. After the weaning of the pups, the maternal gland begins to regress at different rates. The early phases of regression are characterized by fragmentation of nuclei (programmed cell death or apoptosis) and infiltrates of macrophages. The fully regressed mammary gland retains many of the branches, distinguishing it from a nulliparous mammary gland resulting in short pointed terminal ducts with a spiculated appearance resembling thorns. The areas around the terminal ducts often have brown and yellow pigment that stain for iron.
The mammary gland of the retired breeder female will often regress to a point that it is difficult to distinguish from the straight mammary tree of the virgin animal but, most of the time, it retains its peculiar spiculated appearance.
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