Stratum germinativum

The stratum corneum consists of separated, nonviable, cornified, almost nonpermeable corneocytes embedded into a continuous lipid bilayer made of various classes of lipids, for example, ceramides, cholesterol, cholesterol esters, free fatty acids, and triglycerides [6], Structurally, this epidermis layer is best described by the so-called brick-and-mortar model [7], The stratum corneum is crucial for the barrier function of the skin, controlling percutaneous absorption of dermally applied substances and regulating fluid homeostasis. The thickness of the stratum corneum is usually 10-25 /an, with exceptions at the soles of the feet and the palms, and swells several-fold when hydrated. All components of the stratum corneum originate from the basal layer of the epidermis, the stratum germinativum. 

The epidermis consists of several types of cells. The epidermal cell type apposed to the dermis is the stratum germinativum (basal cell layer), over which are the stramm spinosum, stratum granulosum, stratum ludicum, and the outermost layer or stratum corneum. The basal cell layer consists of one layer of columnar epithelial cells. On division, the basal cells are pushed up and become the stratum spinosum, which consists of several layers of cells. As these cells approach the surface of the skin they become larger and form the stratum granulosum. 

In the initial studies of trace elements of human skin cross sections, a Ca profile evolved which was increasing from the basal, germinative level of the epidermis toward the horny layer. The drastic concentration drop of Ca concentration down to threshold values at the border of the stratum germinativum and the stratum corneum was a particularly interesting feature.15-17 Only a few years later this finding could be correlated to the fact that a Ca concentration >0.1 mmol was essential if a fully cornified stratum corneum was to be obtained in cell culture. This relationship between Ca and terminal differentiation of the epidermal cells was verified in a PIXE study of epidermal cell cultures.18... 

Stratum Granulosum Stratum Spinosum Stratum Germinativum... 

The epidermis contains five histologically distinct layers which, from the inside to the outside, are the stratum germinativum, stratum spinosum, stratum granulosum, stratum lucidum and the stratum comeum (Figure 2). The stratum corneum, comprising anucleate (dead) cells, provides the main barrier to topical drug delivery and hence is considered in further detail here. 

The epidermis has four layers the basal layer (stratum germinativum), which is the deepest layer covering the dermis, the spinous layer (stratus spinosum) the outer layer and the comified layer (stratum comeum). 

Underlying the stratum corneum is the viable epidermis, which has a thickness of about 50 to 100 pm. The viable epidermis, which makes a flat interface with the dead, horny layer, consists of, from top to bottom, the translucent layer (stratum lucidum), the granular layer (stratum granulosum), the spinous or prickle layer (stratum spinosum), and the basal layer (stratum germinativum) [1,2]. The lipid compositions of the stratum granulosum and stratum germinativum are shown in Fig. 1. 

The epidermis is composed of various consecutive complex layers, including the stratum comeum (horny layer), stratum ludicum (dear layer), stratum granulo-sum (granular layer), and stratum germinativum (ger-minative layer), which itself is subdivided into two parts stratum spinosum (spinous or prickle layer) and stratum basale (basal layer) (Marquardt et al., 1999). For a general overview of percutaneous poison uptake, we will now restrict the introduction to the homy and ger-minative layers, which are in this sense the most significant epidermal strata. 

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