Pig stratum comeum

To date, Monteiro-Riviere and co-workers [60] have published the only studies using electron microscopy to examine the mechanism of iontophoretic transport. They applied mercuric chloride (7.4%) in vivo in pigs for 1 hr (current density 200 p,A/cm ) and subsequently exposed the biopsies to ammonium sulfide vapor to precipitate and localize the mercury, similar to earlier passive transport studies [28]. The micrographs revealed that mercuric chloride traverses intracellularly through the first few layers and intercellularly through the remainder of the stratum comeum. The authors concluded that the intercellular pathway is the predominant route for passive and iontophoretic drug delivery systems. However, it is difficult to eliminate follicular transport as a possible pathway, since only small areas can be examined at a time ( 1 mm ) and the low density of hair follicles (11/cm ) makes it difficult to study them with the electron microscope. 

To determine sonophoretic transport route(s) at the ultrastructural level, Bom-mannan and co-workers [72] visualized guinea pig skin with transmission electron microscopy (TEM) after the application of colloidal lanthanum hydroxide (an electron dense tracer) and treatment in vivo with 10- or 16-MHz ultrasound (0.2 W/cm ). In control samples, the tracer did not appear to penetrate into the stratum comeum. But in samples exposed to sonophoresis, the tracer was found in localized areas of the intercellular space, which may correspond to the lacunae (polar head-group domains) described by Hou and co-workers [73] as polar or head-group domains. In addition, sonophoresis for 5 minutes (at both 10 and 16 MHz) did not appear to alter the morphology of the epidermal cells, whereas damage to cells was observed after 20 minutes of treatment (16 MHz). Thus, sonophoresis appears to cause the permeation of the tracers via the intercellular route within the stratum comeum. 

Although the basic structure of skin is similar in most terrestrial mammals, between- and within-species differences exist in the thickness of the epidermis and dermis in various regions of the body (Monteiro-Riviere, 1991 Chapter 1, this volume). For example, in the pig, epidermal and stratum comeum thickness is almost twice that in cattle and horses. Stratum comeum thickness in sheep is similar to that in cattle, yet the epidermis in sheep is only half as thick. Other investigators have speculated that transappendagial transport of dmgs across skin in cattle and sheep... 

Table 1 A Comparison Between the Lipid Organization in Pig, Human, and Mouse Stratum Comeum...

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