Oral Delivery of Proteins and Peptides: Concepts and Applications - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 482

associated structures of the lamina propria do not play an important role immunolog-
ically. Small lymphoid nodules are present in the upper small intestine, whereas large
organized aggregates of lymphoid tissue (Peyer's patches) are present in the ileum.
The basolateral membrane differs from the apical membrane. The protein-to-lipid
ratio is very low in the basolateral membrane; thus it is thinner and more permeable
than the apical membrane. These membranes differ in their enzymatic composition.
Tight junction, responsible for paracellular transport, is present between the cells
( Fig. 10.2 ).
Aqueous Boundary Layer
Besides providing cellular barriers, the aqueous boundary layer presents an impor-
tant hydrodynamic barrier that a drug must traverse before reaching the surface of
the mucosal membrane [33] . This barrier layer, often referred to as the dormant,
unstirred, or aqueous diffusion cover, is located at the intestinal lumen and membrane
interface that a drug molecule has to pass before it passes through the membrane.
The thickness of the aqueous boundary layer varies from the tips of the villi to the
crypts. It is thinner at the tips and thicker and less moved in the crypts. This aque-
ous boundary layer offers great resistance to the hydrophobic molecules and is the
rate-limiting step. But with hydrophilic drugs, including peptides, diffusion across
the aqueous boundary layer is much faster because the layer has less resistance to
hydrophilic molecules [34] .
Figure 10.2 Intestinal
epithelial absorptive
cells.