Additional details are evident at higher magnification. In distinguishing between the loose and dense connective tissue, recall that both extracellular and cellular features show differences that are evident in both the figure and the inset. Note the thicker collagenous fibers in the dense connective tissue in contrast to the much thinner fibers of the loose connective tissue. The loose connective tissue contains far more cells per unit area and a greater variety of cell types. This figure shows a cluster of lymphocytes (L) and, at still higher magnification (inset), plasma cells (P) and individual lymphocytes (L). Both plasma cells and lymphocytes are cells with a rounded shape, but plasma cells are larger and show more  cytoplasm. In addition, regions of plasma cell cytoplasm display basophilia. Elongate nuclei in spindle-shaped cells belong to fibroblasts. In con- connective tissue (see CT(L), in figure below, and together, the ducts and surrounding connective tissue constitute a lobule. Two terminal duct lobular units (TDLU) are bracketed in this figure. Beyond the lobular unit, the connective tissue is more dense (CT(D)). The two types of connective tissues can be distinguished at the low magnification of this figure.
trast, although the cell types in the dense connective tissue may also be diverse, a simple examination of equal areas of loose and dense connective tissue will, by far, show fewer cells in the dense connective tissue. Characteristically, the dense connective tissue contains numerous aggregates of adipocytes (A).
The epithelial cells within the resting lobular units are regarded as being chiefly duct elements. Usually, alveoli are not found; their precursors, however, are represented as cellular thickenings of the duct wall. The epithelium of the resting lobule is cuboidal; in addition, myoepithelial cells are present. Reexamination of the inset shows a thickening of the epithelium in one location, presumably the precursor of an alveolus, and myoepithelial cells (M) at the base of the epithelium. As elsewhere, the myoepithelial cells are on the epithelial side of the basement membrane. During pregnancy, the glands begin to proliferate. This can be thought of as a dual process in which ducts proliferate and alveoli grow from the ducts.
MAM MARY G LAN D I NACTIVE STAG E A, adipocytes CT(D), dense connective tissue CT(L), loose connective tissue D, ducts L, lymphocytes M, myoepithelial cells P, plasma cells TDLU, terminal duct lobular unit KEY
PLATE 103 Mammary Gland, Late Proliferative and Lactating  Stages