Page 75 - Atlas of Histology with Functional Correlations

Except for mature red blood cells, erythrocytes, all mammalian cells contain a

nucleus. In addition, all cells are surrounded by a cell or a plasma membrane,
which forms an important barrier or boundary between the internal and the
external environments. Internal to the cell membrane is the cytoplasm, a dense,
fluidlike medium that contains numerous organelles, microtubules,
microfilaments, and membrane-bound secretory granules or ingested material.

The membrane that surrounds the cell consists of a phospholipid bilayer, a
double layer of phospholipid molecules. Interspersed within and embedded in
the phospholipid bilayer of the cell membrane are the integral membrane
proteins and peripheral membrane proteins, which make up almost half of the

total mass of the membrane. The integral membrane proteins are incorporated
within the lipid bilayer of the cell membrane. Some of the integral proteins span
the entire thickness of the cell membrane. These are the transmembrane
proteins, and they are exposed on the outer and the inner surface of the cell

membrane. The membrane proteins participate in transporting molecules across
the lipid bilayer, serve as membrane receptors for different hormones, attach to
and support the internal cytoskeleton of the cell membrane, and possess specific
enzyme activity. The peripheral proteins do not protrude into the phospholipid

bilayer and are not embedded within the cell membrane. Instead, they are
associated with the cell membrane on both its extracellular (outer) and
intracellular (inner) surfaces. Some of the peripheral proteins are anchored to the
network of tiny microfilaments of the cytoskeleton of the cell and are held

firmly in place. Also present within the plasma membrane is the lipid molecule
cholesterol. Cholesterol stabilizes the cell membrane, makes it more rigid, and
regulates the fluidity of the phospholipid bilayer.

Located on the external surface of the cell membrane in certain specialized

cells is a delicate, fuzzy cell coat called the glycocalyx, composed of
carbohydrate molecules that are attached to the integral proteins of the cell
membrane and that project from the external cell surface. The glycocalyx is seen
on the microvilli of the absorptive cells in the small intestine and proximal
convoluted tubules in the kidney. Glycocalyx is not seen with routine

histological stain unless the sections are stained with periodic acid–Schiff or
viewed with the electron microscope. The glycocalyx performs an important role
in cell recognition, in cell-to-cell attachments or adhesions, and as a receptor or

binding sites for different bloodborne hormones.

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