CELL CONTACTS IN T H E MOUSE MAMMARY GLAND I. Normal Gland in Postnatal Development and the Secretory Cycle

The nature and distribution of cell contacts have been examined in thin sections and freeze-fracture replicas of mammary gland samples from female C3H/Crgl mice at stages from birth through pregnancy, lactation, and postweaning involution. Epithelial cells of major mammary ducts at all stages examined are linked at their luminal borders by junctional complexes consisting of tight junctions, variable intermediate junctions, occasional small gap junctions, and one or more series of desmosomes. Scattered desmosomes and gap junctions link ductal epithelial and myoepithelial cells in all combinations; hemidesmosomes attach myoepithelial cells to the basal lamina. Freeze-fracture replicas confirm the erratic distribution of gap junctions and reveal a loose, irregular network of ridges comprising the continuous tight-junctional belts. Alveoli develop early in gestation and initially resemble ducts. Later, as alveoli and small ducts become actively secretory, they lose all desmosomes and most intermediate junctions, whereas tight and gap junctions persist, The tight-junctional network becomes compact and orderly, its undulating ridges oriented predominantly parallel to the luminal surface. It is suggested that these changes in junctional morphology, occurring in secretory ce~s around parturition, may" be related to the greatly enhanced rate of movement of milk precursors and products tbrough the lactating epithelium, or to the profound and recurrent changes in shape of secretory cells that occur in relation to myoepithelial cell contraction, or to both. I N T R O D U C T I O N The mammary gland in lactation is a massively productive organ of secretion. Because its cycle of differentiation, lactation, regression, and inactivity is under the control of hormones that can be experimentally manipulated in laboratory animals, and because the amplitude of this cycle is great, the gland is a particularly useful model of development in secretory epithelia (40, 65). In common with other epithelia, mammary parenchyma shows strong morphological and functional polarity, but the particular expression and significance of polarity differ with the behavior of cells occupying different positions in the gland (secretory alveolus vs. excretory duct) or in the developmental cycle (resting state vs. lactation). The occurrence of characteristic cell junctions serving to maintain a layer of cells as a coherent sheet and a permeability barrier is essential to the polarized function of epithelia in general, alTHE JOURNAL OF CELL BIOLOGY • VOLUME 56, 1973 • pages 797-818 797 on A uust 9, 2017 jcb.rress. org D ow nladed fom though both the ultrastructure of the junct ional complex (15) and the degree of "leakiness" of the transepithelial permeability barrier (18) may differ in different organs. Changes in properties of cell junctions during functional differentiation of an epithelial tissue might be expected, and could be important in a strongly cyclic one. To determine whether the distribution and morphology of cell contacts in the mammary gland vary with the natural sequence of hormonal environments and cellular responses, we have examined thin sections and freeze-fracture replicas of mammary gland samples from female mice at stages from birth through pregnancy, lactation, postweaning involution, and the resting state. The observations to be reported indicate that significant changes do occur, particularly in relation to active secretion and excretion of milk by alveoli. The growth pattern of mouse mammary gland is well known from light microscope studies (1 I, 16, 43, 68, 69). Each of the ten glands at birth consists of a primary duct and a system of branches extending a short distance from the nipple into the mammary fat pad. The ducts elongate and branch until, at maturi ty (around 9 wk after birth), the duct system has ramified through the fat pad, the branches ending in slender or club-shaped twigs; little additional growth occurs in the adult virgin mouse (but there is variation among strains and individuals). Further ductal branching occurs in early pregnancy and alveolar buds grow out from the sides and ends of the ducts. By about 12 days of gestation, clusters of small alveoli cover the duct system, and during the remainder of gestation the alveoli enlarge and become distended with secretion, while the adipose stroma of the gland begins to release its fat stores. Some cell proliferation within alveoli continues (66, 67) both before and after parturit ion (at about 21 days), and full secretory development of the gland is reached several days post partum. Nursing usually stops at about 3 wk post partum, and imiolution of the gland sets in. In this process almost all of the alveoli and presumably some ductules disintegrate and the remains are removed by phagocytic cells. Thus there appear to be two kinds of tissue, distinct in time and space, in the mammary gland: ducts, which ramify through the fat pad before puberty, proliferate further under the influence of hormones in early pregnancy, and for the most part persist when lactational hormone levels fall at weaning; and alveoli, which originate from ductal tissue specifically in response to the hormones of pregnancy, produce milk in response to lactogenic hormones, and generally do not survive the withdrawal of these hormones. In addition to the typical epithelial cells, myoepithelial cells lie at the periphery of both ducts and alveoli, within the basement membrane. During suckling, milk ejection is facilitated by myoepithelial cell contraction triggered by the pituitary hormone oxytocin (12, 29, 56). Recent accounts of mouse or rat mammary ultrastructure (2, 9, 21, 23, 24, 28, 38, 42, 51, 55, 59, 70) provide details of cell morphology at various stages. The classic description of cell junctions in vertebrate epithelia by Farquhar and Palade (15) has been amplified by a number of recent authors (3, 7, 10, 17, 22, 27, 39, 45, 61), who have identified a new entity, the gap junction, and have applied the technique of freezefracturing to junct ional morphology in several kinds of tissues. M A T E R I A L S A N D M E T H O D S