Plate 1 illustrates the basic functional unit of the human breast, the TDLU. The illustrations show the structure of the TDLU in various stages of the life cycle, early menarche (Figure 1D), fully developed nulliparous (Figure 1E), fully developed parous (Figure 1F), early menopausal (Figure 1G), and elderly (Figure 1H).

A. (Figure 17.) Diagram of terminal ductal lobular unit, or TDLU. The dotted line is the interface between the loose intralobular connective tissue, and the dense extralobular connective tissue. ETD is extralobular terminal duct, ITD is intralobular terminal duct (the axial space of the lobule) and T is the "target zone". T is the site of approximately one half of the smallest, and presumed earliest, epithelial proliferations hypothesized as precancerous. Other epithelial proliferations of presumed precancerous significance arise within the lobule itself.

B. (Figure 18.) Histology of a small normal human mammary lobule in the "resting state". Note that at least two layers of cells are visible: myoepithelial cells (dense small nuclei at the periphery of the acini) and epithelial cells, most of which are luminal in location.

C. Image showing details of acinar organization.

D. (Figure 11.) Subgross preparation of portion of mammary gland of a 15 year old human female. Menarche at age 13. Note duct system with end buds, but no lobules or acini.

E. (Figure 12.) Subgross preparation of portion of mammary gland of a 22 year old nulliparous woman. Note well developed lobules with acini.

F. (Figure 13.) Subgross preparation of portion of mammary gland of a 30 year old parous woman. Note well developed lobules with acini.

G. (Figure 14.) Subgross preparation of portion of mammary gland of a 55 year old parous postmenopausal woman. Note duct dilatation and atrophy of lobules.

H. (Figure 15.) Subgross preparation of portion of mammary gland of 80 year old woman. Note marked atrophy of lobules, and the presence of trapped secretion within the dilated duct system.