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.
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Alveolar
epithelial cells :
The images that follow are electron micrographs of mammary tissue, including
epithelial cells and their organelles.
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| Electron microscopy (EM) at high magnification allows us to see the
organelles in a mammary secretory cell: |
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| 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. |
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| 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. |
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The
next sections provide more images and information about mammary epithelial
cells and the organelles responsible for milk synthesis.
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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.
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| Milk proteins are synthesized at the level of the rough endoplasmic
reticulum (RER). Some RER in mammary cell is shown here. |
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| 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). |
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| 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. |
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| 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. |
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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.
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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.
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| 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. |
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| 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. |
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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.
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