Stratum lucidum

Regional distribution studies indicate that the highest density of kainate receptor occurs in the stratum lucidum of the hippocampus (mossy fiber system) and in the inner and outer layers of the neocortex. The highest density of NMDA receptors is found in the hippocampus, stratum radiatum, and in the striatum, thalamus, and cerebral cortex. The distribution of AMPA receptors is similar to that of NMDA receptors, but in the cerebellum AMPA receptors predominate in the molecular layer... 

The epidermis consists of five principal layers and is an area of both intense biochemical activity and differentiation. These layers are the stratum comeum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The stratum corneum (horny layer) is the uppermost layer of the epidermis and the skin. The stratum corneum is composed of dead keratinocytes, which are called corneocytes, and has an abundance of keratin and lipid structures [8], The stratum comeum is considered the rate-limiting barrier for the diffusion of chemical compounds across the skin. The stratum lucidum (clear layer) is composed of two to three layers of dead flattened keratinocytes which appear translucent under a microscope and are present only in thick glabrous skin. 

Before the discussion of transdermal transport, it is essential to understand the anatomy and barrier properties of skin. Briefly, the skin can be viewed as two main layers the epidermis and the dermis. The thickness of epidermis varies from approximately 0.07 to 0.12mm (except on the palms and the soles), and is composed of the stratum corneum, stratum lucidum, stratum... 

Meissner s corpuscle Melanocyte Merkel cells Langerhans ce Stratum corneum Stratum lucidum ... 

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. 

Watanabe M, Fukaya M, Sakimura K, Manabe T, Mishina M, Inoue Y (1998) Selective scarcity of NMDA receptor channel subunits in the stratum lucidum (mossy fibre-recipient layer) of the mouse hippocampal CA3 subfield. Eur J Neurosci 70 478-487. 

Stratum corneumy Stratum lucidum Stratum granulosum-" ... 

The stratum lucidum is a thin, clear layer only found in specific areas of exceptionally thick skin, and it lacks hair (e g., plantar and palmar surfaces). This layer appears as a translucent, homogeneous line between the stratum granulosum and stratum comeum and consists of several layers of fully keratinized, closely compacted, dense cells devoid of nuclei and cytoplasmic organelles. The cytoplasm contains eleidin, a protein that is similar to keratin but has a different staining affinity and protein-bound phospholipids. 

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 main cells of the viable epidermis are keratinocytes. They get their growth factors and nutrients by passive diffusion via the interstitial fluid, which is estimated to represent about 15% of the total volume of the epidermis and drains into the lymphatic system. The viable epidermis also contains melanocytes, Langerhans cells, migrant macrophages, and lymphocytes. The top two layers of the viable epidermis, the stratum lucidum and the stratum granulosum, are physiologically very important. Removal of these three epidermal layers results in water loss and an enhancement of skin permeability [8],... 

The epidermis is an epithelium consisting of inner viable epidermis, a living hydrophilic layer, and outer nonviable epidermis, a hydrophobic layer made from dead cells. It is differentiated into stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale In the direction of dermis (Fig. 16.2). The viable epidermal layer has a thickness of about 0.02 to 0.2 mm. It is composed of many layers of keratinocytes, a widespread distribution of melanocytes, Langerhans cells, dendritic T cells, epidermotropic lymphocytes and Merkel cells, and a number of catabolic enzymes such as esterases, proteases, phosphatases, nucleotidases and lipases [Walters and Roberts, 2002 Barry, 2001). The outer and nonviable epidermis, namely, stratum corneum, is about 10 to 20 pm thick [Gregor and Ulrich, 2010). The stratum corneum Is deemed to be the major obstacle of drug permeation. 

Kageyama and Wong-Riley (1982) showed that cytochrome oxidase staining in the somata of CA3 pyramidal cells and various interneurons was more intense than in CAl pyramidal and dentate granule cells, while very low levels of cytochrome oxidase activity was observed in the stratum lucidum of CA3. 

Fig. 2.3 One observes the usual progressive transformation of cuboidal basal cells into polygonal cells and then flattened cells with the characteristic obliteration of the nucleus. The granular layer is formed by pale cells, almost always with visible spots, covered by a translucent zone of stratum lucidum, and covered by keratin. Hematoxylin-phloxine-saffron stain. Obj x40...

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