Identification of Cell Types in the Developing Goat Mammary Gland

The goat was chosen as the model system for investigating mammary gland development in the ruminant. Histological and immunocytochemical staining of goat mammary tissue at key stages of development was performed to characterize the histogenesis of the ruminant mammary gland. The mammary gland of the virgin adult goat consisted of a ductal system terminating in lobules of ductules. Lobuloalveolar development of ductules occurred during pregnancy and lactation which was followed by the regression of secretory alveoli at involution. The ductal system was separated from the surrounding stroma by a basement membrane which was defined by antisera raised against laminin and Type IV collagen. Vimentin, smooth-muscle actin and myosin monoclonal antisera as well as antisera to cytokeratin 18 and multiple cytokeratins stained a layer of myoepithelial cells which surround the ductal epithelium. Staining of luminal epithelial cells by monoclonal antibodies to cytokeratins was dependent on their location along the ductal system, from intense staining in ducts to variable staining in ductules. The staining of epithelial cells by monoclonals to cytokeratins also varied according to the developmental status of the goat, being maximal in virgin and involuting glands, lowest at lactation and intermediate during gestation. In addition, cuboidal cells, situated perpendicular to myoepithelial cells and adjacent to alveolar cells in secretory alveoli, were also stained by cytokeratin monoclonal antibodies and antisera to the receptor protein, erbB-2, in similar fashion to luminal epithelial cells. These results demonstrate that caprine mammary epithelial cell differentiation along the alveolar pathway is associated with the loss of certain types of cytokeratins and that undifferentiated and secretory alveolar epithelial cells are present within lactating goat mammary alveoli.     

The cytochemical localization of adenylate cyclase activity in mucous and serous cells of the salivary gland

The present study was undertaken to localize adenylate cyclase activity in salivary glands by cytochemical means. For the study, serous parotid glands and mixed sublingual glands of the rat were used. Pieces of the fixed glands were incubated with adenosine triphosphate (ATP) or adenylyl-imidodi-phosphate (AMP-PNP) as substrate: inorganic pyrophosphate or PNP liberated upon the action of adenylate cyclase on the substrates is precipitated by lead ions at their sites of production. In both glands, the reaction product was detected along the myoepithelial cell membranes in contact with secretory cells, indicating that a high level of adenylate cyclase activity occurs in association with these cell membranes. The association with a high level of the enzyme activity might be related to the contractile nature of myoepithelial cells which are supposed to aid secretory cells in discharging secretion products. A high level of adenylate cyclase activity was also detected associated with serous secretory cells (acinar cells of the parotid gland and demilune cells of the sublingual gland), but not with mucous secretory cells. In serous cells, deposits of reaction product were localized along the extracellular space of the apical cell membrane bordering the lumen. This is the portion of the cell membrane which fuses with the granule membranes during secretion. Since the granule membranes are not associated with a detectable level of adenylate cyclase activity, it appears that the enzyme activity becomes activated or associated with the granule membranes as they become part of the cell membrane by fusion. The association with a high level of adenylate cyclase activity appears to be related to the ability of the membrane to fuse with other membranes. It is likely, since the luminal membrane of mucous cells which does not fuse with mucous granule membranes during secretion is not associated with a detectable enzyme activity.     

Localization of vimentin in myoepithelial cells of the rat mammary gland

Summary Myoepithelial cells in the virgin rat mammary gland have been shown to contain vimentin, using a polyclonal antiserum to vimentin purified from hamster fibroblasts. This antiserum has been shown to be specific for vimentin by immunoblotting and ELISA techniques. Similar results were obtained with a monoclonal antibody to vimentin. In the mammary glands of pregnant rats, the staining with vimentin antibodies is much weaker in the myoepithelial cells of the developing alveolar buds than in the main ducts. Similarly, in lactating glands, the staining of myoepithelial cells is much weaker in the secretory alveoli than in lactiferous sinuses. In each case, staining with antivimentin co-localizes with staining with polyclonal antisera to callous keratin (which specifically stain myoepithelial cells in the rat mammary gland).     

Distribution of myoepithelial cells and basement membrane proteins in the resting, pregnant, lactating, and involuting rat mammary gland

Using antisera to specific proteins, the localization of the rat mammary parenchymal cells (both epithelial and myoepithelial), the basement membrane, and connective tissue components has been studied during the four physiological stages of the adult rat mammary gland, viz. resting, pregnant, lactating, and involuting glands. Antisera to myosin and prekeratin were used to localize myoepithelial cells, antisera to rat milk fat globule membrane for epithelial cells, antisera to laminin and type IV collagen to delineate the basement membrane and antisera to type I collagen and fibronectin as markers for connective tissue. In the resting, virgin mammary gland, myoepithelial cells appear to form a continuous layer around the epithelial cells and are in turn surrounded by a continuous basement membrane. Antiserum to fibronectin does not delineate the basement membrane in the resting gland. The ductal system is surrounded by connective tissue. Only the basal or myoepithelial cells in the terminal end buds of neonatal animals demonstrate cytoplasmic staining for basement membrane proteins, indicating active synthesis of these proteins during this period. In the secretory alveoli of the lactating rat, the myoepithelial cells no longer appear to form a continuous layer beneath the epithelial cells and in many areas the epithelial cells appear to be in contact with the basement membrane. The basement membrane in the lactating gland is still continuous around the ducts and alveoli. In the lactating gland, fibronectin appears to be located in the basement membrane region in addition to being a component of the stroma. During involution, the alveoli collapse, and appear to be in a state of dissolution. The basement membrane is thicker and is occasionally incomplete, as also are the basket-like myoepithelial structures. Basement membrane components can also be demonstrated throughout the collapsed alveoli.     

Control of mammary myoepithelial cell contractile function by α3β1 integrin signalling

In the functionally differentiated mammary gland, basal myoepithelial cells contract to eject the milk produced by luminal epithelial cells from the body. We report that conditional deletion of a laminin receptor, α3β1 integrin, from myoepithelial cells leads to low rates of milk ejection due to a contractility defect but does not interfere with the integrity or functional differentiation of the mammary epithelium. In lactating mammary gland, in the absence of α3β1, focal adhesion kinase phosphorylation is impaired, the Rho/Rac balance is altered and myosin light-chain (MLC) phosphorylation is sustained. Cultured mammary myoepithelial cells depleted of α3β1 contract in response to oxytocin, but are unable to maintain the state of post-contractile relaxation. The expression of constitutively active Rac or its effector p21-activated kinase (PAK), or treatment with MLC kinase (MLCK) inhibitor, rescues the relaxation capacity of mutant cells, strongly suggesting that α3β1-mediated stimulation of the Rac/PAK pathway is required for the inhibition of MLCK activity, permitting completion of the myoepithelial cell contraction/relaxation cycle and successful lactation. This is the first study highlighting the impact of α3β1 integrin signalling on mammary gland function.     

The role of calcium and potassium ions in the response of the mouse mammary secretory and myoepithelial cells to treatment with oxytocin

Experiments were carried out in lactating white mice. Removal of Ca(2+)-ions from the perfusion solution reduced the amplitude and duration of the membrane potential changes in the secretory cells in the mammary gland alveolus. The extra- and intracellular Ca(2+)-ions participate in development of contraction responses of myoepithelial cells. Removal of K(+)-ions from perfusion solution and increase of K(+)-ions concentration in the medium to 20 mmol/l inhibit the development of response of secretory cells to oxytocin action. These changes in K(+)-ions concentration do not affect the contractile response of the myoepithelial cells. The findings suggest that there are essential differences in the participation of Ca(2+)- and K(+)-ions in mechanisms of the mammary secretory and myoepithelial cells responses to oxytocin action.     

Localization of choline acetyltransferase in the alveolar portion of the mammary gland of the white mouse

The light optic investigation has been performed on the 10th--15th days of lactation. The enzymatic activity has been determined by Burt method (1971). The product of histochemical reaction is revealed in the secretory cell and in the basal parts of the alveoli, that corresponds to location of myoepithelial cells. Localization of the reaction product in the secretory cells of the neighbouring alveoli is heterogeneous. The enzyme of acetylcholine synthesis is revealed along the course of single neural fibers included into the composition of the ileo-hypogastric nerve of the mammary gland. The results obtained, together with the data of certain physiological investigations make it possible to substantiate participation of the cholinergic mechanism in the alveolar cells activity and consider participation of the cholinergic process in intercellular interactions, that ensure functional conjugation of myoepithelial and secretory cells.     

CELL CONTACTS IN THE 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 cells around parturition, may be related to the greatly enhanced rate of movement of milk precursors and products through 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.     

Alterations in the expression and localization of protein kinase C isoforms during mammary gland differentiation.

Protein kinase C (PKC) is involved in signaling that modulates the proliferation and differentiation of many cell types, including mammary epithelial cells. In addition, changes in PKC expression or activity have been observed during mammary carcinogenesis. In order to examine the involvement of specific PKC isoforms during normal mammary gland development, the expression and localization of PKCs alpha, delta, epsilon and zeta were examined during puberty, pregnancy, lactation, and involution. By immunoblot analysis, expression of PKC alpha, delta, epsilon and zeta proteins was increased in mammary epithelial organoids during the transition from puberty to pregnancy. In mammary gland frozen sections, PKCs alpha, delta, epsilon and zeta were stained in the luminal epithelium and myoepithelium, in varying isoform-and developmental stage-specific locations. PKC alpha was found in a punctate apical localization in the luminal epithelium during pregnancy. During lactation, PKC epsilon was present in the nucleus, and PKC zeta was concentrated in the subapical region of the luminal epithelium. Additionally, marked staining for PKCs alpha, delta, epsilon, and zeta was observed in the myoepithelial cells at the base of ducts and alveoli. This basal ductal and alveolar staining differed in intensity in a developmentally-specific fashion. During most time points (virgin, pregnant, lactating, and early involution), myoepithelial cells of the duct were more intensely stained than those lining the alveoli for PKCs alpha, delta, epsilon and zeta. During late involution (days 9-12), the preferential staining of ducts was lost or reversed, and the myoepithelial cells lining the regressing alveolar structures stained equally (PKCs epsilon and zeta) or more intensely (PKCs alpha and delta), coincident with the thickening of the myoepithelial cells surrounding the regressing alveoli. The increased PKC isoform staining at the base of alveoli during involution suggests that alveolar regression may be influenced by alterations in signaling in the alveolar myoepithelium.     

Localization of cholinesterases in structures of the alveolar portion of the mammary gland

Cholinesterase (ChE) localization in the alveolar compartment of the mammary gland has been studied by light microscopy and ultrahistochemical methods. The product of reaction to ChE is associated with micropinocytotic vesicles of the capillar endothelium and myoepithelial cells, separate nerve terminals in interalveolar connective tissue and perivascular nerve terminals. In secretory cells, the ChE activity is revealed in the sites of fat droplet and on the membranes of endoplasmatic reticulum.     

Identification of alpha transforming growth factor as a possible local trophic agent for the mammary gland

Biologically active alpha-transforming growth factor (alpha-TGF) has been identified in medium conditioned by rat mammary myoepithelial and, to a lesser extent, by epithelial cell lines in culture and in the rat mammary gland. The alpha-TGF has been identified by its wide spectrum of activity in promoting growth of mammary-derived cells in vitro, by its chromatographic behaviour on reversed-phase high-performance liquid chromatography (HPLC), by its competition with epidermal growth factor (EGF) for the EGF receptor, and by the presence of messenger RNA for alpha-TGF in the secreting cells. In vivo the amount of alpha-TGF isolated is sixfold greater from the mammary glands of lactating than from those of virgin female rats. It is proposed that alpha-TGF is produced by the myoepithelial cells of the mammary gland, as a local trophic agent that stimulates growth of the various cell types of the gland.     

Identification and Characterization of Epithelial Cells in Mammalian Tissues by Immunofluorescence Microscopy Using Antibodies to Prekeratin

The occurrence of intermediate-sized filaments containing prekeratin-like proteins ('cytokeratins') has been examined in various organs of rat and cow by electron microscopy and by immunofluorescence microscopy on frozen sections using antibodies to defined constitutive proteins of various types of intermediate-sized filaments (prekeratin, vimentin, desmin). Positive cytokeratin reaction and tonofilament-like structures have been observed in the following epithelia: epidermis; ductal, secretory, and myoepithelial cells of sweat glands; mammary gland duct; myoepithelial cells of lactating mammary gland; milk secreting cells of cow; ductal, secretory, and myoepithelial cells of various salivary glands; tongue mucosa; bile duct; excretory duct of pancreas; intestinal mucosa; urothelium; trachea; bronchi; thymus reticulum, including Hassall corpuscles; mesothelium; uterus; and ciliated cells of oviduct. None of the epithelial cells mentioned has shown significant reaction with antibodies to vimentin, the major component of the type of intermediate-sized filaments predominant in mesenchymal cells. The widespread, if not general occurrence of cytokeratin filaments in epithelial cells is emphasized, and it is proposed to use this specific structure as a criterion for true epithelial character or origin.     

Immunocytochemical identification of proliferating cell types in mouse mammary gland

To study cell proliferation in different cell types and segments of the mammary gland, we devised a dual staining procedure, combining nuclear labeling by 5-bromo-2'-deoxy-uridine (BrdU) uptake (revealed by a dark-brown precipitate) and an alternative (red or blue) cytoplasmic labeling by antibodies specific for the differentiation proteins of epithelial, myoepithelial, and secretory cell types. The following markers, revealed by APAAP or beta-galactosidase procedure, were selected: alpha-smooth muscle actin for the myoepithelial cells, keratin (detected by AE1 monoclonal) for the luminal epithelial cells, alpha-lactalbumin and beta-casein for the secretory cells. To follow the full process of organogenesis, the study was conducted in mouse mammary glands from virgin, primed, and lactating animals and from glands cultured in vitro under specific hormone stimulation. Cell proliferation was localized mainly in focal areas (end buds), and mostly corresponded to "null" undifferentiated cells. Estrogen and progestin stimulation induced a relative increase of proliferating differentiated cells of either epithelial or myoepithelial type, localized in ducts and alveolar structures. Prolactin stimulation induced proliferation in secretory cells.     

Lactation affects expression of intermediate filaments in human breast epithelium

Abstract The human breast contains two epithelial lineages, luminal epithelial and myoepithelial. Specific patterns of expression of intermediate filaments have previously been demonstrated in the resting breast. To determine how terminal differentiation and lactation influenced expression of intermediate filaments in breast epithelial cells, we used Western blot analysis to measure the levels of vimentin, α-smooth muscle actin, keratin 14, and keratin 18 in the resting and lactating breast. Confocal immunofluorescence was used to determine the subcellular site of localization of the intermediate filaments. Vimentin was localised to myoepithelial cells in both the resting and lactating gland. There was a four-fold increase in vimentin protein levels in lactating tissue relative to resting tissue, and this may be related to increased cellular activity of the myoepithelial cells which surround secretory alveoli. Alpha-smooth muscle actin and keratin 14 were detected in myoepithelial cells, and similar levels of expression were found in lactating and resting tissue. In the resting breast, keratin 18 and keratin 8 were detected in luminal epithelial cells in a filamentous form, whereas in lactating tissue it was present in a punctate form in luminal cells and also seen as granules in the lumen of alveoli. Our results indicate that intermediate filament expression patterns are altered in the lactating human breast, and this may reflect their role in the fully functional gland.     

Lactation affects expression of intermediate filaments in human breast epithelium

The human breast contains two epithelial lineages, luminal epithelial and myoepithelial. Specific patterns of expression of intermediate filaments have previously been demonstrated in the resting breast. To determine how terminal differentiation and lactation influenced expression of intermediate filaments in breast epithelial cells, we used Western blot analysis to measure the levels of vimentin, alpha-smooth muscle actin, keratin 14, and keratin 18 in the resting and lactating breast. Confocal immunofluorescence was used to determine the subcellular site of localization of the intermediate filaments. Vimentin was localised to myoepithelial cells in both the resting and lactating gland. There was a four-fold increase in vimentin protein levels in lactating tissue relative to resting tissue, and this may be related to increased cellular activity of the myoepithelial cells which surround secretory alveoli. Alpha-smooth muscle actin and keratin 14 were detected in myoepithelial cells, and similar levels of expression were found in lactating and resting tissue. In the resting breast, keratin 18 and keratin 8 were detected in luminal epithelial cells in a filamentous form, whereas in lactating tissue it was present in a punctate form in luminal cells and also seen as granules in the lumen of alveoli. Our results indicate that intermediate filament expression patterns are altered in the lactating human breast, and this may reflect their role in the fully functional gland.     

Mitosis in Milk Secreting Epithelial Cells of Mammary Gland An Ultrastructural Study

In all stages of lactation mitotic configurations were observed in mammary gland epithelial cells of rats. An electron microscopic study is presented which shows that ultrastructure of such mitotic stages is normal and that mitotic cells contain typical products of milk secreting cells such as casein micelle-containing vesicles and milk fat droplets. Such secretory products can even be observed in the immediate vicinity of the chromosomes and microtubules of the spindle apparatus. The endoplasmic reticulum of mitotic cells appeared altered in that it did not show typical cisternal stacks characteristic of interphase cells. While the numbers of such mitotic cells were very low, especially from the second week of lactation on (always less than 0.1% of the milk secreting epithelial cells encountered), the observations clearly demonstrate that differentiation for milk secretory activity and cells division are not mutually exclusive. We conclude that postpartum growth of mammary gland epithelium and replacement of epithelial cells lost during desquamation into the milk liquids can occur by division of existing differentiated milk secreting cells and does not require mitotic activity of non-lactating 'stem cells' which are not observed in lactating alveoli.     

Developmental regulation of calcium-binding proteins (calelectrins and calpactin I) in mammary glands

We recently showed that mammary glands contain a novel class of calcium-binding proteins (CBPs) that bind to membranes in a calcium-dependent manner. We have also established that these mammary CBPs are equivalent to the calelectrins and calpactin I/p36. Since it has been suggested that these proteins might be involved in exocytosis, we examined mammary glands for these CBPs during secretory differentiation. Immunohistochemical examination showed glands from virgin animals to be rich in calelectrins and calpactin I/p36, while glands from lactating animals contained little immunoreactive material. In addition, silver-staining and immunoblot estimation of the CBPs in lysates from collagenase harvested secretory epithelia showed these proteins to be significantly reduced compared to nonsecretory epithelia. Close examination of the CBP immunoreactive cells of the mammary gland shows that ductal cells are prominent in their staining and that the immunoreactive material is associated with the cell surface. Also, in juvenile glands the myoepithelial stem cells (cap cells) of the elongating end bud are devoid of the CBPs. In contrast to the in vivo data, epithelia cultivated on collagen gels demonstrate comparable levels of the CBPs in both nonsecretory and secretory monolayers. The in vivo data indicate that the CBPs are developmentally regulated during mammary gland differentiation such that secretory epithelia are essentially devoid of these novel proteins. Furthermore, a role for calelectrin and calpactin I/p36 in exocytotic casein secretion is questioned.     

Effects of serum on mammary epithelial proliferation in vitro during mammary gland development

The proliferative response of mammary gland epithelium from nonpregnant, pregnant, and lactating mice to mammary serum factor and insulin was studied in vitro. Mammary gland epiithelium from nonpregnant and lactating animals has a delayed proliferative response to mammary serum factor and insulin when compared to the response of epithelium from pregnant animals. The results show that as the animals go through pregnancy into lactation the mammary gland epithelium becomes less responsive to mammary serum factor while it retains its responsiveness to insulin. The concentration of mammary serum factor in sera from animals at various physiological stages is constant. Sera from hypophysectomized rats, on the other hand, show a 50% drop in mammary serum factor activity. This loss of activity cannot be reversed by injecting prolactin, 17-beta-estradiol, or growth hormone into the hypophysectomized animals. A hypothesis that the mammary gland is composed of two proliferative epithelial populations is developed, and the possible role of prolactin in stimulating DNA synthesis is discussed.     

Developmental expression and assembly of connexins into homomeric and heteromeric gap junction hemichannels in the mouse mammary gland

During the development of the mammary gland, duct-lining epithelial cells progress through a program of expansive proliferation, followed by a terminal differentiation that allows for the biosynthesis and secretion of milk during lactation. The role of gap junction proteins, connexins, in the development and function of this secretory epithelium was investigated. Connexins, Cx26 and Cx32, were differentially expressed throughout pregnancy and lactation in alveolar cells. Cx26 poly-(A)(+) RNA and protein levels increased from early pregnancy, whereas Cx32 was detectable only during lactation. At this time, immunolocalization of connexins by confocal microscopy and immunogold labeling of high-pressure frozen freeze-substituted tissue showed that both connexins colocalized to the same junctional plaque. Analysis of gap junction hemichannels (connexons) isolated from lactating mammary gland plasma membranes by a rate-density centrifugation procedure, followed by immunoprecipitation and by size-exclusion chromatography, showed that Cx26 and Cx32 were organized as homomeric and heteromeric connexons. Structural diversity in the assembly of gap junction hemichannels demonstrated between pregnant and lactating mammary gland may account for differences in ionic and molecular signaling that may physiologically influence the onset and/or maintenance of the secretory phenotype of alveolar epithelial cells.