Lipoidal permeation pathway

For a number of years the existence of a porous or polar pathway through the stratum comeum, in parallel with the lipoidal pathway, has been hypothesized. Although there has been some criticism of this concept, it is our belief that the root of the lack of a common consensus among scientists in the field can be attributed largely to the limited number of systematic studies in the literature that directly address the issue of the diffusion of polar and ionic permeants across skin. Based upon recent studies that have focused upon this aspect of transdermal diffusion, the existence of a porous permeation pathway through HEM is clear (Hatanaka et al., 1993, 1994 Peck et al., 1993, 1994, 1995). At this point, we have made no attempt to correlate the findings from our studies with specific structural properties of the HEM. In some cases, authors have implicated shunt routes such as hair follicles and sweat ducts to account for permeation data not consistent with the concept of lipoidal membrane permeation (Cornwell and Barry, 1993 Scheuplein and Blank, 1971). Under ionto-phoretic conditions, such shunt routes have been shown to contribute to current conduction (Cullander and Guy, 1991 Scott et al., 1993). When efforts have been made to estimate the effective Rp of skin samples under iontophoretic conditions (Ruddy and Hadzija, 1992), osmotic conditions (Hatanaka et al.,... 

There are two routes potentially involved in drug absorption across the nasal epithelial barrier the transcellular and paracellular routes [20], Several experimental evidences dealing with the mechanism of transnasal permeation support the existence of both lipoidal pathyway (i.e., transcellular route) and an aqueous pore pathway (i.e., paracellular route). 

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