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Milk Composition & Synthesis
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Milk Synthesis
Alveolar Epithelial Cells


Note: This section is replicated from the same section that you read in the Mammary Micro-Structure Module. This material also is relevant to the Milk Synthesis sections.


Alveolar epithelial cells : The single layer of epithelial cells lining the alveolar lumen are essential to production of milk. These cells are polar in nature. Here polar means that the function of the basal membrane (on the blood side) of epithelial cells having dramatically different functions compared with the apical membrane (membrane side of the cell exposed to the alveolar lumen). Contents of the cells are also polarized, with the nucleus tending to be located in the basal half of the cell, the Golgi apparatus usually just apical to the nucleus and most of the secretory structures (secretory vesicles, fat droplets) generally on the apical half of the cell.

The basal membrane is in contact with and attached to the basement membrane; a thin layer of structural proteins which provide structural support for the epithelial cells. The epithelial cells are joined together by junctional complexes (tight junctions on the basolateral membranes, between cells) which generally are not permeable to molecules. Anchored to the basement membrane and tightly associated with each other, the single layer of epithelial cells forms a barrier to passage of substances directly from the blood side of the cells to the milk side (lumen). The qualities of this barrier (called a membrane, as distinguished from the lipid bilayer membrane surrounding the cell itself) and the polarized nature of the cells are at the heart of how and why the mammary epithelial cells secrete milk.


Alveolar epithelial cells : The images that follow are electron micrographs of mammary tissue, including epithelial cells and their organelles.

Electron microscopy (EM) at high magnification allows us to see the organelles in a mammary secretory cell: Electron micrograph of lactating mammary epithelial cell.
Another view of secretory cells in the mammary tissue by electron microscopy. The lumen is in the upper left corner of this picture.

CM = casein micelles inside of a secretory vesicle; G = Golgi apparatus; RER = rough endoplastic reticulum; SV = secretory vesicles.

Electron micrograph of lactating mammary epithelial cell.
Another EM image of secretory cells. The lumen is at the top of the image. The arrows point to areas in which the section has cut through many microvilli. The nucleus is in the bottom middle of the image.

LD = lipid droplet prior to being secreted; SV several secretory vesicles.

Electron micrograph of lactating mammary epithelial cell showing organelles.

The next sections provide more images and information about mammary epithelial cells and the organelles responsible for milk synthesis.


Alveolar epithelial cells : Below are images illustrating specific organelles of mammary alveolar epithelial cells. These organelles are involved in milk component synthesis. All of these images are from electron micrographs.

Milk proteins are synthesized at the level of the rough endoplasmic reticulum (RER). Some RER in mammary cell is shown here.
Electron micrograph of lactating mammary epithelial cell showing RER.
The Golgi apparatus is where proteins are processed (including processing of oligosaccharides of glycoproteins), casein micelles are assembled, and lactose is synthesized. This image illustrates the Gogi apparatus in a mammary cell. The Golgi apparatus is typically found at the apical side of the nucleus (the nucleus is in the lower left corner of the image).
Electron micrograph of lactating mammary epithelial cell showing Golgi apparatus.
Another Golgi apparatus is shown in this image. Note the secretory vesicle (SV) in the lower right corner of the image. This contains a casein micelle (arrow); note that this micelle is not as electron dense as the ones below and it is probably in the process of forming, while the ones below are completely formed, densely packed and are being secreted.
Electron micrograph of lactating mammary epithelial cell showing Golgi apparatus.
Casein micelles are usually found in the secretory vesicles or in the lumen (after secretion). Sometimes large casein micelles are found in very small secretory vesicles, especially in nonlactating tissue. Just because a gland is said to be nonlactating does not necessarily mean it does not synthesize some limited amount of milk components; this is tissue from a dry cow's mammary gland.
Electron micrograph of lactating mammary epithelial cell showing casein micelles.

Note that the casein micelles (the large round structures indicated by the arrows above) are very dark compared with the one in the image above, indicating that they are electron dense. This is consistent with the tight granular packaging of the casein molecules along with the inorganic calcium and phosphorous material within the micelle. Even so, there are at least two electron densities of the micelles in this image. Note that the largest micelle in the middle is inside of a small vesicle; these single micelles are still contained within a membrane-bounded secretory vesicle, just very small compared to those shown below. The dark ovoid-shaped structure just above the magnification line (lower right) is not a casein micelle; it may be a lysozome or peroxisome.

Alveolar epithelial cells : Below are more images illustrating secretory vesicles and tight junctions of alveolar epithelial cells. All of these images are from electron micrographs.

This image shows secretory vesicles in lactating mammary cells. Note the casein micelles. The secretory vesicles contain most milk components except fat. Note the electron dense casein micelles.
Electron micrograph of lactating mammary epithelial cell showing secretory vesicles.
This image shows secretory vesicles in a cell, casein micelles (arrows), and microvilli at the cell's lumenal surface (arrowhead). The lumen is the region in the upper left. These secretory vesicles are all near the apical surface of the cell and are about to have their contents secreted.
Electron micrograph of lactating mammary epithelial cell showing secretory vesicles.

Note that the secretory vesicles are fusing within the cell above. The lipid bilayer membrane surrounding the secretory vesicles closest to the apical membrane will fuse with the lipid bilayer membrane of the cell (plasma membrane) and open a hole to the lumen. The contents of the fused secretory vesicles then are open to the lumen and the contents diffuse into the lumen. One secretory vesicle (center of image) is just in the process of fusing with the plasma membrane.


Alveolar Epithelial Cells
Milk Composition & Synthesis
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