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Mammary Micro-structure
The Alveolus

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An alveolus is composed of a single layer of epithelial cells surrounding a lumen, and surrounded by a basket network of myoepithelial cells (their cell bodies are in green in this diagram). This is surrounded by a basement membrane (made of connective tissue proteins). Blood vessels and capillaries run through the stroma (inter-alveolar space). The stroma also contains fibroblasts, leukocytes (plasma cells) and other connective tissue cells.

The series of images below will help better define an alveolus.

Remember that the alveolus is cut in cross-section in these histological sections. Under the microscope the tissue would look like this: Histological view of lactating mammary tissue.
Another section of lactating mammary tissue from a cow: Histological view of lactating mammary tissue.
A single alveolus is outlined in the image to the right. Histological view of lactating mammary tissue with alveolus outlined.

A single alveolus is shown in the image to the right.

Note the nucleus in each cell. Also, note the stippling (holes) in each cell. These are the secretory vesicles and fat droplets inside of the cells. On the lumenal surface on the right side, about half way down the line of cells is a large fat droplet just about to be secreted from the cell.

Histological illustrating a single alveolus.

Alveolus : The single layer of epithelial cells lining the alveolar lumen are essential to production of milk. All precursors and substrates of milk components must pass into the epithelial cells before they can be converted into milk lactose, fat, or proteins.

Here is a histological section of lactating pig mammary tissue showing the classic alveolar structure of the mammary tissue. The arrows point to some leukocytes which have entered the lumen. The small darker staining things in the center of the picture are probably blood vessels. Histological view of lactating sow mammary tissue.
Here are a few alveoli from lactating pig mammary tissue. Note the stained material in the lumen; this is the milk contents. Also note near the bottom of the right-hand alveolus that there are several round clear areas. These are fat droplets almost ready to be secreted from the cell. Lipid is extracted from the tissue slices in the histological processing, so they appear as clear area. Histological view of lactating sow mammary tissue.

The next section of this page provides more images of alveoli visualized with various types of stains.


The images below further illustrate various aspects of mammary alveoli.

This black and white image to the right is from human lactating tissue. It shows the typical alveolar structure. Can you follow the connective tissue sheath around the lobule? Histological view of lactating mammary tissue.
This histological section of lactating pig mammary tissue was immunostained for casein. Casein is identified by an antibody specific for porcine casein. The antibody is bound to the casein in the tissue section. Antibody that does not bind to the tissue (to the casein) is washed off. This is followed by binding of a second antibody to the anti-casein antibody; the sections antibody is covalently attached (conjugated) to an enzyme. Second antibody which is not bound to the first antibody is washed off. A color reagent is then put on the slide and the enzyme reacts with the color to form the insoluble brown colored deposit where casein is located. The dark brown color represents the presence of casein in the apical portion of the epithelial cells and in the lumen. There is little staining in the stroma. The blue stain of the cells is a counter-stain used to visualize the alveolar structure. Immunohistological staining of lactating mammary tissue for casein.
This histological tissue section has been stained with a fluorescent material (called DAPI) which binds to chromatin in the nuclei. The blue circles are nuclei of cells in the alveoli and stroma. The nuclei of one alveolus have been made red (by computer imaging) to demonstrate that there is a single layer of epithelial cells lining the alveolus. Note that there is relatively little stromal tissue between the alveoli in this lactating tissue. Fluorescent staining of nuclei of lactating mammary tissue.
The two images to the right are of the same tissue section. The section was stained for both chromatin (with DAPI) and for cytokeratin (a cytoskeletal protein found in epithelial cells). The cytokeratin is localized by binding an antibody specific for cytokeratin, then binding the second antibody which is conjugated to a fluorescent dye. The presence of the cytokeratin is seen by the red color. Note that only the epithelial cells are red and not the lumen or stroma. This would be expected if cytokeratin is only found in the epithelail cells. Also, note the black holes inside the cells where the nuclei do not stain; cytokeratin would not be found in the nucleus. Fluorescent staining of nuclei of lactating mammary tissue.
Fluorescent staining of cytokeratin of lactating mammary tissue.

The images below further illustrate various aspects of mammary alveoli.

This tissue section has been stained with a lectin (binds oligosaccharides; mostly found on glycoproteins). The lectin is conjugated to a fluorescent dye (FITC) which gives the green color. This lectin is called Con-A (Concanavalin-A; from the Jack Bean), and it binds to high-mannose oligosaccharides. Note that most of the tissue is green, so this type of oligosaccharide is present in abundance it he tissue and distributed throughout the tissue. One alveolus has been circled in red (by computer imaging) to illustrate the alveolus. Staining of lactating mammary tissue with lectin (Concanavalin A).
This tissue section has been stained with another lectin called Wheat Germ Agglutinin (WGA; from, you guessed it, wheat). The WGA-FITC binds to a more select group of oligosaccharides and is localized primarily in the apical portion of the cells (see the alveoli on the right side) and in the lumen. Some milk components are glycoslyated with oligosaccharides that bind WGA. A limited amount of staining is also seen in the stroma and basement membrane area, indicating that these contain glycoprotiens which have WGA-binding oligosaccharides. The red arrows indicate the lack of staining where the nuclei are located in the epithelial cells. Staining of lactating mammary tissue with lectin (Wheat Germ Agglutinin).
This section illustrates the classic alveolar structure of lactating tissue (this is from a cow). Note that in the lactating tissue there is little connective tissue area between alveoli. Most of the area of the tissue is the lumen of the alveoli. Now compare this image with the one from a dry cow below. Histological view of lactating mammary tissue.
This section is from a nonlactating cow (about 30 days after drying off). The image is a little lower magnification than the image of the lactating tissue above. Nevertheless, it is clear that the alveoli have shrunk and that there is a great deal more stromal area between alveoli and ducts. Most of the larger structures with a lumen would be ducts which do not regress as significantly as the alveoli during mammary gland involution. Histological view of nonlactating mammary tissue.

Mammary Structure