Proteomics - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 401

occludens, or tight junctions; zonula adherens, or intermediate junctions; and macula
adherens, or spot desmosomes. The zonula adherens, or intermediate junctions, are
located immediately below the tight junctions and are composed of significant inter-
cellular spaces and occupied by a fine fibrillar material. This junction is the chief
site for membrane insertion for filaments of terminal web. Desmosomes of epithelial
cells are composed of two straight patches of dense material to the inner leaflet of
the corresponding plasma membrane and are separated by a clear space. Their main
function is to stabilize the cytoskeletal network of epithelial cells.
�.4.�.1.6 Undifferentiated Crypt Cells
These cells are mainly linked with proliferation and secretion and have a negligi-
ble role in absorption. Following their formation in the crypt, many undifferentiated
daughter cells differentiate into villus and migrate from crypt to villus. The undiffer-
entiated cells differentiate into goblet cells, Paneth cells, and endocrine cells with a
secretory nature. Electron microscopy reveals relatively a lesser abundance of glyco-
proteins. Similarly, tight junctions of undifferentiated cells are shallow and relatively
less complex. The undifferentiated crypt cells contain less alkaline phosphatase, dis-
sacharidase, and dipeptidase.
�.4.�.1.7 M (Microfold) Cells
Peyer's patches are clusters of subepithelial, lymphoid follicles found in the intes-
tine. They are oval or rectangular in shape and found on the antimesentric wall of
the intestine. They are more prominent in ileum and are characterized by specialized
epithelial cells called M cells.
The M cells are compactly arranged with adjacent absorptive cells and create
incavations that enfold lymphocytes and other similar defensive components. Along
with these, Golgi bodies, endoplasmic reticulum, mitochondria, and so on, are pres-
ent. These cells share tight junctions with adjacent absorptive cells. They also play a
key role in initiation and triggering of immunological response.
�.4.�.1.� Goblet Cells
The small intestinal epithelial microvilli are covered by a mucus layer secreted by
specialized cells called goblet cells, found in varying proportions throughout the
intestine with the most in the ileum. The presence of mucus-secreting glands imparts
them a goblet-like appearance. The mucus secreted by goblet cells is basically glyco-
protein in nature. The role of goblet cells in transepithelial transport is not very clear.
8.4.4 Physical Barriers
8.4.4.1 Transcellular Transport
Transcellular transport is divided into simple passive transport, passive diffusion
with an efflux pump, active transport, and endocytosis. Downhill transport of mol-
ecules following the concentration gradient is called simple passive transport. Active
transport, in contrast, is uphill and transports the molecules against the concentration